[Figures of Chapter 15]

Key to the Eleven Figures and Characters of the Fifteenth Chapter
The first figure of the present chapter shows part of the occipital bone free from other bones, and is seen here in the outer surface of the base of the skull.

The second sets forth the first vertebra [atlas] of the neck, shown in its anterior and superior aspect.

The fourth exhibits the first cervical vertebra drawn in its inferior and posterior aspect.

The fifth demonstrates the anterior face of the second cervical vertebra [axis]. ¹

The seventh offers to view the second cervical vertebra in its inferior face.

The eighth illustrates the anterior and superior face of the third cervical vertebra.

The ninth delineates the posterior and superior face of the third cervical vertebra. Because the lower surface of this cervical vertebra resembles the lower surface of the second vertebra, we do not include it here, to avoid setting out too great a mass of illustrations before the present chapter. For this reason as well we have not individually depicted the remaining cervical vertebrae after the third, especially as their description can be readily inferred from the figures of the three upper vertebrae.

In the tenth is seen the structure of the first three cervical vertebrae depicted in the anterior part.

In the eleventh is shown the assembly of the first three cervical vertebrae from a posterior view.

Man was given a neck for the sake of the lungs
That a neck was given mankind for the sake of the lungs is best proven by the fact that it is always missing when there are no lungs. For example, this is the reason fish (which also have no lungs) have no neck. Those animals to which Nature gave lungs are also endowed with a neck. ²² For it was necessary that the rough artery [trachea] (shown in fig. 1, ch. 38; vessels running through the neck and lungs are seen in the figure at the end of Bk. 3 and the one attached to the end of Bk. 4), through which we bring air into the lungs when we inhale and blow it out again when we exhale, run from the mouth to the lungs, and for this reason that a space intervene between mouth and lungs, since it was fitting that man be endowed with adequate exhalation ²³ (which is the material of the voice). For the voice is impossible without benefit of the rough artery. Therefore we do not wonder that no animal without a neck emits a voice. ²⁴

The seven vertebrae of the neck ²⁶
There are, then, seven vertebrae in the human neck (A to B in the fig. illustrating ch. 14, I to K in the three skeletons), whose nature and use I shall now attempt to explain, beginning my account with the motions of the head and neck. It is not my intention here to give an account of the neck’s use in birds and quadrupeds that have long legs, since no one fails to understand that Nature provided them, because they have no hands, with an extended neck and jaws for taking in food, and for this reason fashioned them a neck with more vertebrae. ²⁷

Galen’s opinion about the motions of the head ²⁸
So Galen, Nature’s rare miracle and the most perfect interpreter of her works, nowhere demands so learned, intelligent, and hard-working a reader in considering a part’s construction as when he describes the motions of the head and its joints with the two highest vertebrae. What more need I say? So few, he says, will understand what he says in the twelfth book of De usu partium ²⁹ about the motions of the head, and as it were so deters the reader by the difficulty of the subject that it should seem strange to no one that I too was very much in need of a preceptor’s aid in this part; I read through his remarks with all possible care and a mind thoroughly trained from childhood for the study of anatomy, and I carefully compared the things themselves to Galen’s text in order to understand his meaning. You will finally know whether my efforts were in vain when you learn that I not only grasped what Galen was trying to explain, but I also at last understood that Nature’s artifice in these joints and the motions of the head are incorrectly explained by him. Galen ascribes two motions to the head, explaining that one consists of nodding up and down, the other from side to side. ³⁰ By the former, he means a forward bending or inclination of the head in a nod; when we bend the head backward or recline it, that is just the motion by which Thracians and most of the Cretans are seen to this day to move their head in a negative way. ³¹ Those who raise their head in refusal move their head straight backward and raise it; they do not rotate or shake it as we do when refusing. By the other motion, which Galen says is made to the sides, he understands what we do when we incline the head to the side and move it as if to the shoulders or scapuli. You will learn from the fourth book of De anatomicis administrationibus what this opinion of Galen’s is about sidewise motion and about nodding the head up and down, if you pay careful attention to the place where he says the capitula of the occipital bone (B in fig. 1) which are articulated to the first vertebra (N in fig. 2) are situated, rise above it, and press into it when the head is moved to this side or that, and when according to Galen we nod the head up or down. Without a doubt, his doctrine about the movements of the head is of this sort: he affirms that the first of the motions, which is nodding the head up and down, is accomplished by means of the second neck vertebra or over it; the latter motion, by which the head is moved to the sides, he says is performed over the first neck vertebra, as may readily be gathered from several places in Galen, particularly from the fourth book of De anatomicis administrationibus. ³² Because he wrote that work last of all, ³³ he explained his opinion more succinctly and clearly there than elsewhere. You may now inquire with me whether his view there is consonant with the truth, and (restraining your feelings for a while) decide the motions of the head differently from Galen.

A different opinion from Galen’s about the motions of the head
In the first place, we move the head either with a primary motion in which the neck holds still, or with a secondary motion, where the head follows the motion of the neck even if you try to move it in a different direction from the neck. There are two movements peculiar to the head: in one, we bend it forward and incline it backward, or extend it. You perform this motion with a rigid, unmoved neck or with a neck moved simultaneously, whether you do it with a motion equal with the head, or a contrary motion. For although you move the neck forward and the head necessarily follows the neck, still you can bend the head backward with its own motion and easily observe that the head and neck have their separate and distinct motions in bending and unbending. The other peculiar movement of the head should be counted that by which we rotate it more or less as if we were spinning it as a wheel is turned on an axle. You will undergo this motion without moving the neck, whether you have tried to look left or right, or rotated the head. This motion is completely confined to the head: the neck does not have it. Therefore, however you move the neck, this rotation of the head is easily accomplished. For when the neck is bent and the head is inclined

Joints made for movements of the head
The head is therefore endowed with its own motions, of which the first, which consists of flexion and return, is accomplished by means of the joint of the head with the first vertebra of the neck, for the head is flexed and returned above the first vertebra. ³⁴ The second motion, by which the head is rotated, is accomplished above the second vertebra of the neck. For the head is rotated together with the first vertebra of the neck above the second, as if on an axle. ³⁵ This is my opinion regarding the motions of the head and the joints, and you will very quickly learn how much it differs from the views of Galen. Whether it approaches closer to the truth you should determine from the construction of the bones, from the ligaments joining them, and the muscles moving them. I will describe the ligaments and muscles in the second book. In the present I shall address only the bones; perhaps in my discussion of them some mention of the ligaments as well may come in.

Description of the occipital bone where it is articulated to the first vertebra ³⁶
The occipital bone, near the sides of the foramen [f. magnum] that provides a path for the dorsal medulla (A in fig. 1), puts forth on each side a single capitulum [condylus occipitalis] (the right one is shown as B in fig. 1), tending more to the anterior part of the foramen than the posterior. If you inspect the anterior (C in fig. 1) and posterior (D in fig. one) ends of these capitula, they are oblong and narrow, and constructed very much as if some bony head such as the femur’s (A in figs. 1 and 2, ch. 30), which is articulated with the hip bone, had been cut into two pieces and one piece of that head had been relocated at either side of the foramen carved in the occipital bone. For if you imagined each of the capitula of the occiput joined together on their inner sides, you would see that a round (though in man rather flat) head is made of the two of them. Since these things are so, it is clear that each capitulum is, as we said, oblong and protrudes ³⁷ more on the inside (E in fig. 1) than on the outside (F in fig. 1), in the same way that in the round head of a bone the middle and more prominent part in the whole surface bulges farther than the parts located on the circumference. These are the capitula ³⁸ which I shall presently write are articulated into the first cervical vertebra [atlas, facies articularis superior]. ³⁹ But for now attention must be paid to a roughness and thickness (G in figure 1) [os occipitale, pars basilaris] in the occipital bone between the two capitula just mentioned, prepared so that a stronger insertion may be effected of the smoothly rounded ligament ⁴⁰ (I in the fig. later to be added to the margin) attaching the dens of the second vertebra (H in the same fig.) to the occipital bone. This roughness sometimes appears so prominently as to justify calling it a kind of tubercle, projecting for the sake of the ligament mentioned as well as for the insertion of the muscles [m. longus capitis] that flex the neck (A and B in the 8th table of muscles). The area behind the foramen transmitting the dorsal medulla is also rough but not so thick as the anterior. Also, there is seen at the posterior region of either capitulum a slight depression [fossa condylaris] (H in figure 1), prepared so that a foramen [f. intervertebrale] will be made on each side with the other depression [facies articularis superior] of the first vertebra, through which the first pair of nerves of the dorsal medulla (F in fig. 3, ch. 11, Bk. 4) is to pass.

Description of the first cervical vertebra
The first cervical vertebra is more solid and dense than all the other bones of the spine, but thinner along its longitude, and most unlike all the others in shape. It has the largest foramen (I in figs. 2, 3, 4) cut in it for the dorsal medulla, because the medulla is thickest here. Its body is carved out in its inner portion [arcus anterior atlantis] (K in figs. 2 and 3), and it shows a pocket [fovea dentis] (L in figs. 3 and 4) lightly scooped out in the posterior part of its body, which is covered with cartilage and receives the dens (g, n in fig. 5, G in figs. 6 [10] and 11) of the second vertebra. The body of the first vertebra is hollowed out with the intention that the tooth should be able to nest there into the first vertebra. ⁴¹ Lest the first vertebra, its body carved out in this manner and nearly lacking a body, ⁴² be rendered unduly weak and feeble, it is augmented with a certain process and tubercle [t. anterius] (M in figures 2 and 10) in its anterior part, by means of which it is thickened and made stronger; in addition, the tubercle itself is suited for receiving an insertion, that of the first pair of muscles ⁴³ that move the spine. Such is the usefulness of this tubercle; but it is not extended so far that it prevents excessive flexion of the head or supports the head in its flexed position; or that it causes the bent head to be bent more quickly back upward and to the rear, as Galen incorrectly stated in the fourth book of De anatomicis administrandis. ⁴⁴ For (to say nothing here about the motion of the head except to name the vertebra over which it occurs) the head can in no way be inclined to the point that the occipital bone touches this tubercle: so far is the mass of the head from being supported by it. ⁴⁵ The first vertebra is strongest and thickest at the sides of the cavity [arcus interior atlantis] (K in figure 3) where it admits the tooth of the second, and on each side it shows a depression [massa lateralis atlantis, facies articularis superior] (N marks the right one in figures 2, 3, and 10) where the capitulum [condylus] of its own side of the occipital bone is received. These depressions, one on each side, are oblong to match those capitula perfectly (if you inspect their anterior and posterior portions, O and P respectively in figs. 2 and 3) ⁴⁶ ; they turn upward at their outer sides (R in figs. 2 and 3) and are deep at the inner sides (Q in figs. 2 and 3), just as if both depressions had been cut away from a single deep, round cavity. For if you divided the socket of the hip or another bone possessed of a round socket and placed one part of it by each side of the first cervical vertebra where the cavities now are, they would look just like the depressions of this first vertebra. ⁴⁷ Or

The head is flexed forward and back over the first vertebra
The capitula of the occiput are received in these depressions of the first vertebra and are flexed and extended in them or drawn forward and back in an extremely straight motion. In no way is the head moved sideways by this double joint [articulatio atlanto-occipitalis], though this has been stated in so many places by Galen, the prince of anatomists. In the first place, when the neck is not moved we are unable to move the head sidewise even a little bit. Also, if the head were moved sidewise over the first vertebra, who doubts that in this motion the first vertebra must be kept unmoved, since Aristotle has rightly shown in his book On the Common Movement of Animals that in a motion the second bone of a joint must always be immobile? ⁴⁸ But I prefer not to dwell on the impiety with which Galen disgracefully imputes a crude negligence to the Maker of things while he believes that he is fully celebrating the Maker and in such a wordy introduction asks for a reader who is attentive and trained in anatomical technique. ⁴⁹

The worthiest joint in the whole body
What aspersion more disgraceful (if one may tell the truth) could have been unfairly cast upon Nature, who is far dearer to us than Galen, than the utmost inattention to the worthiest joint in the entire body? Everybody knows that this joint is the one that will not endure a small inclination even for the smallest moment of time, much less a disjointing or dislocation which would soon make a person incapable of breathing, mute, and without motion and sense as if the root of his nerves had been affected. ⁵⁰ What joint is there in the whole body, I ask you, where the head of a bone is so moved in another bone’s socket that the head no longer touches the socket? None, of course, except, if we believe Galen, the joint of the head with the first vertebra of the neck. Galen himself, believing that the head moves sideways by means of this joint, was compelled to claim in the fourth book of De anatomicis administrationibus that when the head is moved to the right side the right head of the occipital bone is pulled into the right socket of the vertebra while the left rises out of the left socket, and the opposite when the head is inclined to the left side. ⁵¹ We therefore do not attribute sidewise motion to this twin joint [articulatio atlanto-occipitalis] of the head with the first vertebra, but forward and backward flexion. Let us not so swear by the words of Galen that we impute such negligence to the infinite Author of things in the principal joint of the body; nor should we be eager in defending Galen to imagine that he understood a sidewise motion to be the one that I say is accomplished in a circle and by rotation. We also do not agree with Julius Pollux ⁵² that rotation takes place over the first vertebra. For besides the fact that Galen did not understand that motion, who could have been so stupid as to declare that the two oblong heads of the occipital bone, which are some distance apart and enter two similarly elongated high or deep sockets of the first vertebra, could be moved in a circle? Who ever saw a compass attached to a post by both legs move in a circle? It is impossible that someone hoping to defend Galen could imagine the heads of the occiput are low and argue that the head is for that reason able to be rotated above the first vertebra, when Galen never considered any other osseous heads worthy of the name korw/nh, “beak,” and took it only for these heads of the occiput which he calls by this name throughout (but quite incorrectly) and only rarely introduces the point which occurs in dogs rather than humans by that name. ⁵³ Such, then, is the articulation of the head with the first vertebra, and a partial description of this vertebra. We shall now proceed with the rest of it, coming gradually to the other motion of the head.

The remaining description of the first vertebra. ⁵⁴
Near to the outer sides of the depressions [massa lateralis atlantis, facies articularis superior] that receive the capitula of the occipital bone, the first cervical vetrebra puts forth a single transverse process on each side (S in figs. 2, 3, 4, 10, 11), much longer than the other transverse processes of the neck vertebrae, extended and a bit wider, but not as wide in humans as in dogs and simians with tails, where it is like a kind of wing. The transverse processes of the first vertebra are extended more prominently so that each may receive a more convenient insertion of two muscles [m. obliquus capitis superior] than the transverse processes of the other vertebrae of the neck. Into these processes are separately inserted the fifth (I and H in the 14th table of muscles) [m. obliquus capitis superior] and sixth (L and K in the same table) [m. obliquus capitis inferior] pairs of muscles that move the head. ⁵⁵ These processes are perforated by a large foramen (T in figure 2) [foramen transversarium] through which a vein and an artery (s in the last fig. of Bk. 3) [arteria et vena vertebralis] going to the skull are borne. Near the posterior part of the depressions in which the head is articulated, a depression (V in figures 3 and 11) [sulcus arteriae vertebralis] is carved in the first vertebra on either side, corresponding to the one (H in fig. 1) [fossa condylaris] that I said was hollowed in the occipital bone near the posterior part of the capitula [condylus]. Both depressions, in the occipital bone and in the first vertebra, together provide a path to the first pair of nerves [nn. cervicales I] of the dorsal medulla. In dogs, not a depression but a special foramen is cut in the first cervical vertebra for this emerging nerve, because their first vertebra is quite wide and deep. This depression [sulcus a. vertebralis] in the human vertebra extends even to the posterior part of the foramen [f. transversarium] by which the transverse process is pierced; thus, this depression makes room not only for a nerve but also for a vein and an artery. It was explained a little earlier that these vessels enter the skull through the foramen (f in ch. 12, fig. 2) [canalis condylaris] carved out near the base of the capitula of the occipital bone. The first vertebra of the neck is the only one that lacks a posterior process or spine, because like a thorn

Depressions of the first vertebra that receive the protuberances of the second
In the lower region of the first cervical vertebra, beneath the two depressions [massa lateralis et facies articularis superior] in which the head is received, two others [massa lateralis et facies articularis inferior] (the right is Y, the left a and b in figure 4) are lightly and superficially incised; having a concave surface, they are for this reason termed glhnoeidei=j. ⁵⁸ Their outline is round, having in their center (between a and b in figure 4) a scarcely distinguishable hollow; but the outer brows of the depressions (b in figure 4) descend much deeper than the inner ones (a in figure 4), ⁵⁹ if you imagine this vertebra pressing into the second in an erect body. ⁶⁰

The protuberances of the second vertebra
These depressions are coated with cartilage and receive two tubercles ⁶¹ (the right one is c in figures 5 and 6; the left is d, e, f) of the second vertebra of the neck, which are located near the side of the vertebra’s body and protrude very slightly, just as the depressions of the first vertebra are lightly hollowed out. ⁶² The outline of the tubercles is also round like that of the depressions, though at the same time it is seen to stretch out a little in the posterior part (f in figures 5, 6) beyond the circumference of a circle. Also, the inner side of the protuberances (d in figures 5 and 6) rises higher than the outer side (e in figures 5 and 6), but this height and depression of the sides is not as conspicuous as that of the brows on the depressions of the first cervical vertebra, and the swellings in the middle or center are not as high as the depressions between the brows of the first vertebra are deep, exactly as if the sides of the swellings rose higher than their middle. Consequently, in bare bones fitted together the depressions do not exactly match the tuberosities, and the space in the middle of those depressions does not appear quite filled. This occurs only in humans: in dogs and horses and other animals that turn their head a great deal further, there is a single continuous tubercle on each side of the second vertebra, and the depression also appears continuous in the first cervical vertebra, closely matching the tubercle. Nature often corrects this inequality in humans with a special cartilage ⁶³ that, in addition to the cartilage [hyaline] attached to the bones like a crust, comes between the depression and the tuberosity and grows only on the ligaments ⁶⁴ containing the joint ⁶⁵ in the circle; it is coated above and below with a rather viscous humor [synovia] like an ointment. Such then is the double attachment (one on each side) of the depressions of the first vertebra with the tuberosities of the second, created for the sake of the motion which I shall presently relate.

Here we have drawn the special cartilage on each side which we have sometimes observed.

Dens of the second vertebra.
A third attachment of the second vertebra to the first must now be added to our account; this is effected with the first vertebra by the dens (g and n in figure 5, h, i, k, l, n in figure 6, G in figure 10 and 11) of the second vertebra of the neck. ⁶⁶ Between the two tuberosities [processus articularis superior] of the second vertebra, from the upper and middle region of its body, grows a certain conspicuous, tall process, quite hard and solid, which because it resembles a tooth — particularly a human canine tooth — the Greeks called o)dontoeide/ij ⁶⁷ and o)dou/j; Hippocrates called the second vertebra by this name, ⁶⁸ but not the first (as Julius Pollux ⁶⁹ and Caelius, ⁷⁰ ignorant of dissections, wrongly count it). Still others have named this process from its resemblance to a spinning top or pinecone kwnoeidh=; ⁷¹ others, because it swells like a spear-point or pyramid pu/rinon or purinoeidh=. ⁷² This process enters the cavity of the first vertebra (K and L in figure 3) which we have written is located where the body [arcus anterior atlantis] of the first vertebra should be, and where we have written is carved the depression [fovea dentis] which is lined with cartilage and receives the anterior part [facies articularis anterior] of the dens that projects slightly with a smooth and slippery bulge (g in figure 5). The back of the dens, placed opposite this swelling, bulges out more and is thicker than where it first comes out of the second vertebra. For here the dens makes a single depression on each side (k and l in figure 5 and 6) and a third on the posterior side (i in figure 6).

Though this figure belongs to the second book, I have placed it here because it is no small aid to understanding what is said here. The first neck vertebra is marked A, B, C. The second is D and D. G is the body of the second, H the dens, I the smooth ligament [apical ligament of the dens] inserted from the dens to the occipital bone. K is the transverse ligament securing the dens to the first vertebra.

The joining of the dens with the first vertebra
The depressions that are seen on the sides are between the base of the dens and the tuberosities of the second vertebra, ⁷³ and together with the depressions of the first neck vertebra, which match them, provide a foramen on each side (m in figure 11) suitable for transmitting small nerves (N in figure 2, chapter 11, book 4) ⁷⁴ which we sometimes see proceeding into the muscles that flex the neck and which we consider twigs of the second pair of the nerves of the dorsal medulla. The depression seen on the posterior surface of the dens is carved so that the ligament [l. transversum atlantis] ⁷⁵ may be bound more firmly to it which proceeds transversely from one side of the first vertebra to the other and is conspicuously wrapped transversely (as I was saying) across the posterior surface of the dens, ⁷⁶ precisely holding the dens into the cavity of the first vertebra, also preventing it from wandering from its place, and from pinching the dorsal medulla here. ⁷⁷ That this ligament may have a better origin, the first vertebra is rough and uneven where the ligament takes its origin or is inserted (o, o in figures 3 and 11), ⁷⁸ and it also makes a foramen that does not penetrate deeply, ⁷⁹ which allows the ligament to be more firmly grown out or inserted. Nowhere in my account of the ligaments

The head with the first vertebra above the second
This is how the structure of the first vertebra with the second is accomplished, ⁸² using two broad tuberosities on the second vertebra and two depressions on the first, and again another depression on the first to which the tuberculum of the dens is joined. I should prefer that this construction be learned rather from an actual examination of the things than from my less elegant than truthful description, so that it may be more quickly judged whether Galen stated accurately that we flex and extend the head over the second vertebra, or nod and indicate refusal in the manner of Thracians with these joints, ⁸³ or whether I have wrongly asserted that the head is swiveled by these structures together with the first vertebra as if on an axle [i.e., horizontally] [articulatio atlanto-axialis mediana], and that there is absolutely no flexion and extension at this point, contrary to the opinion of Galen solemnly repeated in so many places. For if you scrutinize individual features in the construction of the bones, the second vertebra of the neck will remind you of a beam set in the earth to which we have attached an axle. The dens will be this axle; and the first vertebra, to which the head is joined when swiveled in this motion as if they were a single body, resembles another beam turned on the axle.

A is one pivot, B the other.

Why Nature did not wish the head to be simultaneously rotated and inclined to the sides over the first vertebra.
The reason why Nature did not provide, in addition to forward and backward flexion, still two other motions, one toward the sides and the other in circular rotation, ⁹⁰ above the first vertebra [articulatio atlanto-occipitalis], in the same way she wished the arm bone and the femur to be moved with three types of motion ⁹¹ you may infer is due especially to the fact that it was worthwhile for the first vertebra, being greater and wider than all the rest for the sake of the dorsal medulla, to be pierced (I in figs. 2 and 3) [foramen vertebrale], and that for that reason a single large, round socket could not be formed in it, nor indeed because of its own foramen (A in fig. 1) [f. magnum] was the occipital bone able to swell out into a single head so great that it could be moved in that socket [facies articularis superior] of the first vertebra with every type of motion as the femur is moved in the socket of the bone of the hip. And so because the head had to be attached with a double joint, on the right and the left, and virtually the whole double joint [articulationes zygapophysiales] performed only a simple motion, Nature justly wished that the head be flexed forward and back over the first vertebra [articulatio atlanto-occipitalis]. For it was unable to be moved to the side above this vertebra unless Nature were forgetful of herself here and had desired a single head of the occipital bone to be held in a single depression of the vertebra and another head raised and quite dislocated from the first vertebra, which she attempted in no joint, being ever mindful that the bones of the joint be in contact with each other and never separate or create some kind of unoccupied place invented by Galen. ⁹² Next, the head would not have been able to be rotated over the first vertebra unless Nature had put forth from the first vertebra an acute process into the head cavity where the brain is contained, a process on which the head would be turned as on an axis, and unless, finally, she had intended to vitiate many other things now quite rightly shaped. And so the head is rotated

The head cannot be flexed forward and back over the second vertebra, nor be inclined to the side.
For if the first vertebra were moved forward and raised above its posterior seat without movement of the dens, the dorsal medulla would then be altogether squeezed and crushed in the foramen of the first vertebra, and would be torn away. Also the first vertebra could in no way be flexed backward over the second unless it were itself deeply carved out on its anterior surface (K, L in figures 3 and 4) where it holds the dens, and the first vertebra, not obstructed by the dens, would freely be moved backward. Moreover, the head could in no way be moved toward the side above the second vertebra for the same reason we have proved that lateral motion is impossible over the first. Otherwise it would always be necessary for the first vertebra to be lifted from the second on one side, a danger that does not now threaten from rotation.

On what vertebrae lateral motion occurs
Nature imparted lateral motion with remarkable diligence to all the vertebrae of the neck; these cannot be moved to the side separately and individually, but they are all inclined in that direction at the same time in successive motions so harmoniously that the head is borne suitably to the side in a consequent motion without any harm. But let us now return to the second vertebra of the neck, and so in turn to the five remaining, explaining only the true construction of each, lest by chance the present chapter grow immense with some new dispute, and I seem too anxiously to uproot the opinions of Galen (to whose diligence elsewhere I have, like all students of medicine, the greatest debt), and zealously to mention all his anatomical lapses.

The foramen that transmits the second pair of nerves of the dorsal medulla to the posterior. ⁹³
The second vertebra of the neck is lightly carved out on the posterior surface [lamina arcus vertebrae] (p in fig. 6) of the swellings ⁹⁴ by which it is joined to the first vertebra so as together with the first vertebra to make a foramen [f. intervertebrale] (r in figure 11) on each side by which the second pair ⁹⁵ of nerves of the dorsal medulla escapes to the posterior. But the first vertebra forms the greater portion of this foramen, being quite noticeably hollowed out (q in fig. 4) [massa lateralis atlantis] near the posterior part of the depressions by which it is articulated to the second vertebra.

The spine of the six lower vertebrae of the neck
The remaining items to be mentioned in the second vertebra of the neck are to a great degree common with the other five lower vertebrae. The second (∫ in figs. 5, 6, 7), like the others (t in fig. 9 and no. 11 in fig. 8), has a posterior process ⁹⁶ [p. spinosus]; which has a special feature in the cervical vertebrae: it is bifurcated [bifidus] at its tip (c and ϖ in figure 8), particularly that of the second vertebra and the three ⁹⁷ beneath it. The sixth vertebra shows the division of its process rather obscurely, and the seventh much less still, being like the sixth in that its process usually has an epiphysis and is extended longer [vertebra prominens]; it somewhat resembles the processes [p. spinosus] of the upper vertebrae of the thorax [vertebra thoracica I] (N, O, P in fig. 1, ch. 16) except that it is left less pointed and broader than they are (these will be obvious if you compare the spine in figure 7 with those of the other figures, or examine N, O, P in figure 2, chapter 16). ⁹⁸ The processes [pp. spinosi] of the other neck vertebrae lack epiphyses; they are wide, uneven, and rough, like branches (so to speak), concave on their lower surface, gibbous above, and slightly depressed on both sides. ⁹⁹ The swelling line which extends along the longitude of the process effects this shape, just as in the inferior part of the process (which is concave) a certain linear channel is seen to run along the longitude of the process. A line stands out on the convex surface so that a ligament [l. interspinale] may be put forth from it to the middle of the concave surface; we shall write in the second book that this ligament extends along the longitude of the process or spine, binding one spine to another and separating the right muscles [mm. dorsi] from the left. But these lines of the posterior processes will be more conspicuous in the thoracic and lumbar vertebrae both because the processes of those vertebrae are more prominent and because they are bound to each other by a stronger and harder ligament [l. interspinale] (since they need to move less). The muscles located in the posterior part of the neck are thought to be the chief cause of the division into small processes: to provide the muscles a readier point of origin and a stronger insertion into the spines of the vertebrae, these spines are split into small processes [bifidus]. Nature devised it expressly in these vertebrae of the neck because it would not have been as safe to put out as long spines from those small vertebrae [vv. cervicales] as it is from the thoracic and lumbar vertebrae. A greater ¹⁰⁰ number of muscles is located in the back of the neck than in other parts of the back, as you shall hear at great length when I explain the muscles that move the scapulae, thorax, head, and dorsum. ¹⁰¹ Indeed, it is for the sake of the muscles that the spine of the second vertebra is thrust out higher than are several spines succeeding it, and is broader than others, because this one provides the sole origin for the third pair of muscles that move the head (A and B in the 14th table of muscles) as well as for the sixth (K and L in the same table). ¹⁰²

The second vertebra is larger than the several beneath
But lest Nature unjustly put forth too long or too wide a spine from a slender vertebra, she made the second vertebra larger than the several beneath it, not so much for the purpose of putting forth a spine as to insure that it should be fully adequate to the articulation which it constructs with the first vertebra. This is what we previously stated, that Nature has time and again created the higher vertebra greater than the lower. For the second is thicker, larger, and stronger than the third; and we have shown above that for the sake of the transverse processes the first is much wider than all the other neck vertebrae. ¹⁰³

The nature of the transverse processes of the neck vertebrae.
The transverse processes in the remaining vertebrae [cervicales] of the neck are not much extended, particularly those of the second vertebra (u in figs. 5, 6, 7), which has a short and single transverse process on each side. It is seen that it is less prominent in the second vertebra [axis] than in the others and is shorter because of the breadth

Foramen of the transverse processes.
No vertebra of the human neck has so far come to my attention that did not possess a perforated transverse process. But I have frequently discovered processes of the seventh vertebra of dogs and apes lacking a foramen. ¹⁰⁵ Only the neck vertebrae have such foramina, perforated to carry upward the vein and artery (s in the last fig. of Bk. 3) [arteria et vena vertebralis] which convey blood and spirit to the dorsal medulla and finally to the brain itself. The four additional vertebrae that immediately follow the the second have this special feature as well because their transverse processes ¹⁰⁶ (y and z in figs. 8, 10 and nos. 7, 8, 9, 10 in fig. 9) ¹⁰⁷ are wide and bifurcated; but they are not, like their spines, rough and uneven. The transverse processes resemble each other in shape, and their interior part is always wider and rises higher than the posterior. The form of the parts of the spine is different in nearly all the vertebrae, and they always protrude, and are rough, in various ways. The breadth and bifurcation of the transverse processes occurs for the same reason as it has previously been stated the spines are forked: for the origin and insertion of muscles [m. spinalis cervicis, m. semispinalis thoracis et cervicis] [mm. interspinales] [multifidus]. Besides the other muscles at this location, we shall write in the second book that two (one is marked C in the 8th table of muscles, the other N in the 14th) [m. scalenus medius] [m. scalenus posterior] are separately joined on each side to the transverse processes of the vertebrae; by their aid the neck (and afterward, by resultant motion, the head) are moved laterally. These muscles are so located on each side that one is assigned to the anterior part of the transverse processes [pp. transversi, tuberculum anterius] and the other to the posterior [pp. transversi, t. posterior]. The seventh vertebra of the neck puts out broad but quite obscurely bifurcated transverse processes; ¹⁰⁸ for since they are broad and long, no necessity presses them also to be forked, particularly when the muscles pulling the neck to the side [m. semispinalis cervicis] [m. spinalis cervicis] [m. longissimus cervicis] are not as firmly joined to the seventh as to the other vertebrae, since this one lies beneath them as a kind of base in the stated motion of the neck and ¹⁰⁹ is imperceptably moved laterally, like the vertebrae of the thorax.

Nature of the ascending and descending processes.
The ascending (d in figs. 9 and 11 and nos. 3 and 4 in fig. 8) and descending (g in figs. 6, 7, ¹¹⁰ 9, 11) processes [p. articularis superior] [p. articularis inferior], ¹¹¹ by which the vertebrae are articulated to each other, are similar in all the neck vertebrae below the second; the second also has descending processes not unlike those of the other vertebrae. Those that face downward and are joined to the vertebra beneath possess a circular depression [facies articularis inferior] not at all deeply carved, leading from the anterior surface obliquely downward to the posterior. The ascending processes, which are articulated to the vertebra above, have a rounded tubercule that bulges so lightly and superficially that you would not know whether to assign it to the number of capita or depressions. This tubercule of the ascending processes [facies articularis superior] matches the depression and is coated with cartilage [c. articularis] like the depressions, running downward from the anterior surface posteriorly and somewhat obliquely as well — something not to be considered in a cursory way. For that oblique course in the vertebrae of the neck which are nearer to the thorax always turns out smaller and straighter along the longitude of the body ¹¹² because those vertebrae should not be moved so loosely as the upper vertebrae and therefore should not be so loosely articulated. And so the ascending processes enter the depressions of the descending processes with their own tubercule, and the second vertebra of the neck is attached to the third by a double joint [articulationes zygapophysiales] as the third is to the fourth, and so all the rest in order as far as the thorax. The seventh receives the ascending processes of the first vertebra of the thorax. ¹¹³ But Nature not only gave looser joints to the neck vertebrae: she also, so that the neck can be moved more conveniently and fittingly than the remaining parts of the backbone and with a remarkable and most necessary usefulness, bestows on the vertebrae of the neck another special capacity.

Measure the oblique course from a to b.

Structure of the cervical vertebrae.
Although the bodies of the thoracic and lumbar vertebrae are joined to each other with flat and barely convex surfaces in the same way two smooth, broad beams lie against each other (compare the cervical vertebrae here to the thoracic and lumbar vertebrae in the fig. in ch. 14), Nature wished the oblong lower part of the body of the neck vertebrae (z in fig. 9 and r in fig. 8) ¹¹⁴ and of the head to swell in the manner of a joint made for evident motion, and she hollowed out the upper part of the vertebral bodies [corpus vertebrale, facies intervertebralis] (h, q, k in fig. 9 and nos. 1 and 2 in fig. 8) with a broad depression in such a way that the oblong body of the upper vertebra would enter into this depression of the vertebra beneath, and the structure of the vertebrae would rise up more fit for motion. ¹¹⁵ This type of structure ¹¹⁶ is more conspicuous in the connection of the second vertebra with the third, and in some of the vertebrae beneath. The seventh cervical vertebra rests upon the almost totally smooth surface of the first thoracic, and even its own body is not as hollowed on its upper surface as that of the vertebrae above it; this happens chiefly because it was useful for the upper vertebrae to be moved more loosely than the lower. ¹¹⁷ The muscles and ligaments of the vertebrae will show amply in the second book how safely the vertebrae are bound together by ligaments, and how zealously Nature provided for these loose connections of the bones. For the present, let us marvel supremely at the workmanship of God because He provided with such incredible craft at one time for their safety and their complex motion; and no matter how much more frequent the injuries and dislocations of the neck ¹¹⁸ we may see than in the remaining parts of the backbone, yet it should be still more striking

The number of processes.
These things being as they are, it escapes no one that not all the cervical vertebrae have the same number of processes. To avoid seeming here to have missed something, it would be necessary, if only for the sake of Galen, to count the processes of a neck vertebra in such a way that you would arrive at the number eleven: ¹²¹ this is the number of processes assigned to the neck vertebrae by Galen. ¹²² Take a third, fourth, fifth, or sixth neck vertebra (observe here fig. 8, whose numbers mark the processes in order), and count first the two processes on the superior part of the body, one on each side of the depression upon which the oblong body of the vertebra above it rests. Count as third and fourth the two ascending processes which go up to the vertebra above, and fifth and sixth the two descending by which the ascending processes of the vertebra below are received. You will then count the transverse processes seventh, eighth, ninth, and tenth; since there is one of these on each side and they look like bifurcated twins, they make up four processes. The eleventh will be the posterior process itself, or spine. ¹²³ But since the vertebrae having a double ¹²⁴ transverse process on each side also bear a bifurcated spine ¹²⁵ (c and ϖ in fig. 8), there is no reason why you should not see a twelfth here and increase Galen’s number. Likewise, because the oblong body of the vertebra (r in fig. 8 and z in fig. 9) is also extended like a process [uncus corporis] on its inferior surface, what will keep us from adding this also to the others as a thirteenth process? It is readily inferred without comment on my part that a different number of processes may be observed in the first, second, and seventh neck vertebrae, and that the number varies even in them.

The nature of foramina that transmit nerves laterally: (A) In the vertebrae of the neck.
The foramina made in the sides of the vertebrae (Q, Q in the fig. for chapter 14) [foramen intervertebrale] for transmitting nerves from the dorsal medulla and admitting veins and arteries to it, are established in a uniform set beneath the second neck vertebra all the way to the sacral bone. Between the side of the posterior portion of the vertebral body and the ascending and descending processes, a depression is carved above [incisura vertebralis superior] (j in figs. 8 and 9) and below [incisura vertebralis inferior] (t in fig. 7) on both sides, which, with the depression of the adjacent vertebra, makes a common foramen [f. intervertebrale] by which a nerve exits. These depressions vary in all the vertebrae: in the neck vertebrae, a semicircle is carved in the upper vertebra and a semicircle in the lower, with each vertebra making an equal effort toward the establishment of the foramen. But if someone scrutinized each one closely, a lower vertebra in the neck would make up a slightly larger portion of the foramen than an upper. For the depression (j in figs. 8 and 9) [pediculus arcus vertebrae] carved in the upper part of the vertebra between the ascending process [p. articularis superior] and the process which the upper part [uncus corporis] of the vertebral body puts forth (between d and q in fig. 9) is deeper than the depression (t in fig. 7) that is hollowed between the descending process [p. articularis inferior] and the vertebral body (between b and e in fig. 7).

(B) In the lumbar vertebrae.
In the lumbar vertebrae the case is quite the opposite: the upper vertebra forms the top and sides of the foramen [f. intervertebrale], while the lower vertebra makes up only the lower portion, about a third of the circle. The depression [incisura vertebralis inferior] carved in those vertebrae between the body and the descending process [p. articularis inferior] is many times larger than the one [incisura vertebralis superior] that is hollowed out in the upper part of the vertebra between its body and its ascending process [p. articularis superior].

(C) In the thoracic vertebrae.
The vertebrae of the thorax fall into a middle category: those nearer the neck resemble the neck vertebrae in this foramen of the nerves; those next to the lumbar vertebrae look like the lumbar vertebrae in the shape of the foramen. Why indeed Nature carved foramina [f. intervertebrale] for the nerves between the occipital bone and the first vertebra (H in fig. 2 and F in fig. 3 of ch. 11, Bk. 4) and again between this vertebra and the second (N in fig. 2 and I in fig. 3 of ch. 11, Bk. 4) in a different location than in the remaining vertebrae, is I believe obvious to anyone who understands from the preceding the structure of the joints between the first vertebra and the head [os occipitale], and between that vertebra and the second, and their locations. For they clearly demonstrate that those vertebrae could in no way be carved out on sides occupied by a joint unless Nature had made an altogether weak and feeble joint at this point, or wished the nerves to be broken or damaged by the rubbing of bones in constant motion. With what foresight she hollowed these foramina in the sacrum [ff. sacralia pelvica et posteriora] (nos. 25-30 in fig. 2 and E to 56 in fig. 3, ch. 11, Bk. 4) I shall explain in my account of that bone. As for what still pertains to the present chapter, the neck vertebrae have no other feature peculiar to themselves.

Epiphyses of the vertebrae.
Although in younger persons the bodies of all the vertebrae after the second display epiphyses [zygapophyses] above and below, and even the second underneath, that feature too is common in all the vertebrae

The vertebra is made of several bones in children.
The first neck vertebra is made of two bones joined together like epiphyses. This union appears where the anterior part of the first (M in fig. 2, L in figs. 3 and 4) is located, and where the others sharpen into a spine (X in figs. 3 and 4). The remaining cervical, thoracic, and lumbar vertebrae (the first line would run in fig. 7 from the foramen through s; the second and third from the foramen through t and t) are constructed of three parts [lamina, pediculus arcus, corpus] besides the epiphyses. One line, filled with cartilage, ¹²⁶ runs along the length of the spine from the foramen of the dorsal medulla to the tip of the spine. The two others, brought from the same foramen, separate the body of the vertebra from the two transverse processes and the remaining posterior parts of the vertebra, as can be seen in a lamb or a kid. ¹²⁷ I should prefer, though, that it be seen in infants, because in them injuries of the bone occur more frequently than in persons strengthened by age. This large number of bones is lost in persons more advanced in age, but the epiphyses are sometimes not so hard to make out even in the prime of life.

Connections of the vertebrae in the elderly.
In the elderly, however, not only is the connection of epiphyses abolished, ¹²⁸ but the vertebral bodies, normally separated some distance by the intervention of cartilaginous ligament [discus intervertebralis], also so come together that sometimes three or four vertebrae fuse—you can see them dug up together in cemeteries—and they are impossible to separate. This fusion is effected by certain processes which the body of a single vertebra puts forth from its outer surface into another. I believe nobody doubts that this fusion is the reason why old people do not move their backs, do not straighten their backs, and are more bent over. It is particularly clear if one has occasion to examine the vertebrae of a hunchbacked person and see how curved they are and tenaciously united at their bodies. But in addition to these, difficult motion in the aged is aggravated by a curious roughness of the brows of the bones and by tubercles which prevent the heads from being moved in their sockets. Finally, a pronounced hardness of the ligaments readily prevents the vertebrae from being easily moved. ¹²⁹

Appendix: the 1555 ending to Chapter 15
The polemic against Galen which was originally in pp. 63-64, 65, and 67-68 of the 1543 edition was moved to the end of the chapter in the second edition with some additions and revisions. The new ending, covering the better part of four pages in the 1555 edition, is given here in its entirety.

Disagreements in this chapter with the writings of Galen
I would end the chapter at this point if my account of the cervical vertebrae were altogether in agreement with the writings of Galen. I will be silent about several places in which I could easily show from the space of the transverse processes of the first vertebra, ¹³⁰ from the pathways that transmit branches of the first and second pair of nerves of the dorsal medulla, ¹³¹ from the perforation [foramen transversarium] of the seventh vertebra’s transverse process, and several other such instances, that Galen had described apes and dogs rather than humans in discussing those vertebrae. But I cannot pass over the fact that Galen feels much differently than I do about the motions of the head above the first vertebra, of the first vertebra [atlas] above the second [axis], and of the remaining cervical vertebrae. This must, indeed, be more carefully considered the more earnestly Galen demands a diligent, learned, clever, and hardworking listener who can take in what he says about the movements of the head in the twelfth book of De usu partium. What more can I say? He so strains the few who will understand what is explained there, and as it were deters the reader by the difficulty of his subject, especially one who is untrained in mathematics and the method of dissection, that everyone must try to illuminate his meaning and in this way observe with me whether I am rightly in such disagreement with his views.

Galen’s opinion about the motions of the head
Galen ascribes two motions to the head, teaching that one is made in nodding the head up and down, the other side to side. ¹³² He considers the first to be the one by which we flex the head forward, incline it, and nod, and next the one with which we bend the head backward [extension] and recline it in exactly that action with which the Turks and most Cretans even to the present day signify a negative. ¹³³ When they do so they bend their head straight backward and raise it slightly; they do not rotate it from side to side or shake it as we do. By the other motion Galen means the one with which we incline the head to the side and move it as it were toward the shoulders or scapulae. You will learn his opinion about lateral and nodding motions from the fourth book of De anatomicis administrationibus if you pay close attention: what the point is where Galen states that the capitula of the occipital bone [condylus occipitalis] which are articulated to the first vertebra are located, protrude, and press when the head is moved to this side or that, and when, in Galen’s view, we nod it up and down. He declares that the motion of nodding up and down is accomplished by means of the second cervical vertebra, or above the second; and he says the other, by which the head moves to the sides, takes place above the first cervical vertebra. This may be quite readily learned from several places in Galen’s work, but especially in the fourth book of De anatomicis administrationibus. As he wrote that work last of all, he explained his view more concisely and expertly there than elsewhere. But you may now define the motions of the head quite differently from Galen and ask for yourself along with me whether that opinion of his was consonant with the truth.

A different opinion than Galen’s about the movements of the head
We move the head in the first kind of motion without moving the neck, / p. 85 / or in the second kind of motion when the head follows the motion of the neck even if you try to move it in a different direction than the neck is moved. There are two motions of the head alone. In the first, we flex it forward and incline or extend it backward; you perform this motion with the neck rigid and unmoving, or with the neck moved simultaneously whether you move it in the same motion as the head or in the opposite direction. However far you take the neck forward, the head necessarily follows the neck; but you can still bend the head backward in its own motion and easily observe at the same time that the neck and the head have their own distinct motions in bending separately forward and back. The other peculiar movement of the head should be counted the one by which we rotate it more or less as if we were spinning it as a wheel is turned on an axle. You will undergo this motion without moving the neck, whether you have tried to look left or right, or rotated the head. This motion is completely confined to the head: the neck does not have it. Therefore, however you move the neck, this rotation of the head is easily accomplished. For when the neck is bent and the head is inclined with it, you can easily move it in rotation. So even if the neck shall have been extended or moved sidewise to the shoulder, you can readily rotate the head. As the head possesses a single motion of rotation which the neck lacks, so the neck claims another motion peculiar to itself, by which it is moved to the side. The head has no motion of its own by which it is inclined to the side, but is moved to the side only by the neck and a secondary motion. Indeed, it is denied us to move the head even a little to this or that scapula except as taken that way by the neck. The head is endowed with its own motions, the first of which, consisting of flexion and reflexion, we have said is accomplished by means of the joint of the head with the first vertebra of the neck. ¹³⁴ The second motion, by which the head is rotated, is accomplished above the second vertebra of the neck, as if on an axle. ¹³⁵

The head is flexed back and forth on the first vertebra, but not from side to side or in any other way
It is my opinion therefore that the head is flexed and extended on the two joints of the head with the first vertebra, whether it is moved forward and back in the most direct motion. But by no account is it moved to the sides by their aid, though this had been stated by Galen in ever so many places. When the neck does not move, we are unable to move the head to the sides even a little. And if the head were moved to the side over the first vertebra, who doubts that the first vertebra would have to be kept immobile in that motion, if only because Aristotle rightly showed in On the Movements of Animals that one bone of a joint must be at rest when the joint moves? ¹³⁶ Moreover, since the articulations of the first vertebra with the head are of such a kind that they cannot endure a small disarticulation or dislocation even for the smallest moment of time without a person being immediately deprived of respiration, speech, motion, and sense, as though the root of his nerves was affected, how could we join Galen and impute such negligence to Nature as to declare her so somnolent in the noblest joints of the entire body, as to require that the head of the bone which is moved needed to be raised out of the depression [facies articularis superior] by which it is articulated to another in such a way that it loses contact with the depression and is nowhere braced and planted upon it as we know all the other bones never depart or pull away from their socket in motions entrusted to them? For Galen decided that the head moves to the side over the first vertebra and was compelled to make the claim in the fourth book of De anatomicis administrationibus that when the head is moved to the right side, the right capitulum of the occipital bone is forced into the right depression of the first vertebra while the left capitulum rises out of the left depression, and the opposite whenever the head is inclined to the left side. We do not attribute a sidewise motion to this twin joint of the head with the first vertebra, but only a simple flexion and reflexion, and we are not inclined to pretend in defense of Galen that by motion to the sides he meant what I say is done in twisting and rotation. And we do not agree with Julius Pollux that rotation takes place over the first vertebra. For besides the fact that Galen did not mistake that motion for a lateral one, who is so stupid as to declare that the two separate oblong heads [condylus occipitalis] of the occipital bone fitting into the two deep, profound sockets [facies articularis superior] of the first vertebra, similarly oblong and separated from each other by a wide interval, could be rotated? Who ever saw a compass attached to a post by both legs move in a circle? It is impossible that someone hoping to defend Galen could imagine the heads of the occiput are pushed in flat upon themselves, and argue that the head is for this reason able to be rotated above the first vertebra. Among the other heads of bones Galen scarcely ever considered any worthy of the name korw/nh and took it only for these heads of the occiput which he calls by this name throughout ¹³⁷ (though not quite correctly) — the same name he gives the rare points of the capitula of the occipital bone which occur in dogs rather than humans. So far as concerns the attachment of the first vertebra with the second, I am not at all able to agree with Galen that the head is flexed in any way by this joint and reclined backward.

The head and the first vertebra are rotated over the second vertebra, but not at all flexed and extended
For if you scrutinize individual features in the construction of these bones, the second vertebra of the neck will remind you of a beam set in the earth to which we have attached an axle. The dens will be this axle; and the first vertebra (to which the head is joined when swiveled in this motion as if they were a single body) will resemble a beam turned on the axle. The ligament running transversely over the dens [l. transversum atlantis] clearly shows that the first vertebra is in no way bent above the second: only if the dens were in abeyance could the first vertebra be moved in flexion forward and backward. Who then doubts that this ligament holds the dens in the depression [arcus anterior, fovea dentis] of the first vertebra, and that no such looseness could be ascribed to it (though that is what Galen does ¹³⁸ ) that it should allow the first vertebra to be moved anteriorly so far from contact with the dens that the head would undergo true flexion? Further, two names in Julius Pollux now come to my support, one of which fits the first vertebra, the other the second. The first is named e)pistrofeu/j, the second its a)/cwn. These names undoubtedly have survived from the ancients, who trained their sons in anatomy. ¹³⁹ They meant by the latter name to imply the vertebra which remains still like an axle, on which the other is swiveled as if on an axle; by the former they meant the vertebra which is turned about as on an axle. ¹⁴⁰ Celsus (besides the poets) is my authority that these terms have survived from the ancients before Galen: though the portion of his book where he explains how the vertebra of the neck is damaged, and he himself altered things he did not understand, nevertheless he seems to have first taken that opinion which most closely approaches my own, and in fact truth itself, from some ancient source.

It is not possible for the head to be moved to the side over the first vertebra, nor even to be rotated
The reason why Nature wished only flexion and extension to be performed over the first vertebra but not two other motions, one to the sides and the other in rotation and turning (as the humerus and the femur are moved in three kinds of motion), you may infer is due especially to the fact that it was worthwhile for the first vertebra, being greater and wider than all the rest for the sake of the dorsal medulla, to be pierced [foramen vertebrale], and that for that reason a single large, round depression could not be formed in it, nor indeed because of its own foramen [f. magnum] was the occipital bone able to swell out into a single head [condylus] so great that it could be moved in that depression [facies articularis superior] of the first vertebra with every type of motion as the femur is moved in the depression [acetabulum] of the bone of the hip [os coxae]. And so because the head had to be joined with a double joint, on the right and the left, and virtually the whole double joint [articulatio atlanto-occipitalis] performed only a simple motion, Nature justly wished that the head be flexed and extended over the first vertebra. For it was unable to be moved to the side above this vertebra unless Nature were forgetful of herself here and had desired a single head / p. 87 / of the occipital bone to be held in a single depression of the vertebra and another head strangely raised and quite dislocated from the first vertebra — though she is elsewhere ever mindful that the bones of the joint be in contact with each other and never separate or create some kind of unoccupied place invented by Galen. ¹⁴¹ Next, the head would still less have been able to be rotated over the first vertebra unless Nature had put forth from the first vertebra an acute process into the head cavity [cavitas cranii] where the brain is contained, a process on which the head would be turned as on an axis, and unless, finally, she had intended to vitiate many other things now elegantly shaped. And so the head is rotated according to Nature's craft above the second vertebra [articulatio atlanto-axialis] for if it were bent there, the dens would tear the dorsal medulla.

The head cannot be flexed forward and back over the second vertebra, nor be inclined to the side.
Since in the absence of the dens the first vertebra would be moved downward, its posterior surface [arcus posterior atlantis] would be elevated and the dorsal medulla would then be squeezed and compressed in the foramen of the first vertebra, and would be torn away. Also the first vertebra could in no way be flexed backward over the second unless it were first deeply carved out on its anterior surface [fovea dentis] where it holds the dens, and the first vertebra, not obstructed by the dens, would freely be moved backward. Moreover, the head could in no way be moved toward the side above the second vertebra for the same reason we have proved that lateral motion is impossible over the first. Otherwise it would be necessary for the first vertebra to be lifted a considerable distance above the second on one side, a danger that does not now threaten in rotation.

On what vertebrae lateral motion occurs
Nature imparted lateral motion to all the vertebrae of the neck with no ordinary diligence; these cannot be moved to the side separately and individually, but they are all inclined in that direction at the same time in successive motions so neatly that by their aid the head is conveniently borne to the side in a consequent motion without any harm.