The nature and use of bone
Bone is the hardest and driest of all parts of the human body, the most earthy ¹ and cold, and, with the sole exception of the teeth, ² most lacking in sensation. God, the supreme maker of things, rightly made its substance of this temperament so as to supply the entire body with a kind of foundation. For what walls and beams provide in houses, poles in tents, and keels and ribs in ships, the substance of bones provides in the fabric of man. ³

Differentiation of bones by use
Some bones, because of their strength, are made to be as it were the supports of the body; among these are the bones of the tibia and femur, the dorsal vertebrae, and nearly the entire structure of bones. Others ⁴ are like bulwarks for the other parts, thrown up by nature as the safest walls and fortifications, as for example the skull, the spines of the vertebrae and their transverse processes, the sternum, the ribs. Others are put in charge of the joints of other bones, to keep them from being moved too loosely, or bent in angles that are too sharp. ⁵ For this function ossicles are formed, compared in size by professors of anatomy to sesame seeds [ossa sesamoidea]; among these are certain bones articulated to the second joint of the thumb, the first joints of the other four fingers, and the first joints of the five toes. ⁶ The teeth are variously suited to cutting, breaking, crushing, and grinding food, and likewise two ossicles [malleus et incus] ⁷ of the hearing mechanism serve a particular function in the work of hearing. The chief function of each bone will be shown more fully in the chapters on individual bones; for the present it suffices to enumerate the use of bones in general; how (in a word) they support the entire mass of the body like a prop, and everything is attached to them, strengthened by them, and hung from them to such a degree that, as we have just now said, the type of bones can be chosen from their use and function.

Size and shape
Bones vary in size, since several are large, like the femur, the tibia, the humerus, and bones attached to each side of the sacrum; there are several small bones, like the bones of the carpus, the teeth, and the ossicles that are like sesame seeds. Others again are broad, like the bones [os ilii] joined to the sides of the sacrum, the scapulae, the sacrum, the bones of the vertex [os parietale], frontal bone [os frontale], and occiput [os occipitale]. Others are narrow, slender, and long, like the fibula, the radius, the ulna, and the ribs, among many others. It will be better to reserve all the varieties that can be derived from shape (since they are innumerable) to the individual descriptions of bones. It would in fact be difficult, before the bones have been described, to explain which of these are rough, as we shall call those in the base of the skull stony because they look like a broken rock [os temporale, pars petrosa]. Likewise those that are smooth, like the bones of the vertex [os parietale], the frontal bone, and the sternum. Then there are those that resemble a triangle like the scapula, quadrangular like the bones of the vertex, those that have the look of a wedge, like the bone in the head named sfhnoeide/j [os sphenoidale] after a wedge; ⁸ and those that look like yokes, called zugw/mata [os zygomaticum] by the Greeks ⁹ and iugalia, yokelike, by us; those that imitate our figure ∫, like the clavicles, and show the effigy of a sword, like the sternum; also those that recall the letter u, like the bone called the u(oeide\j, hyoid; ¹⁰ the bones that resemble a shuttle with which wider threads are woven, like the bone of the forearm that is given the name of a shuttle, radius. Also those that we compare to the cube or tessera in a mosaic, ¹¹ like the foot bone named kuboeide/j ¹² [os cuboideum] because of its cube shape, and those that approach the look of a skiff, like the foot bone named after a skiff by the Greeks, skafoeide/j ¹³ [os naviculare, formerly os scaphoideum]; those said to be like a millstone, a shield, or a little plate, patella, like the bone at the front of the knee joint; those that slightly suggest the outline of all Italy, like the femur; and which resemble a pin, fibula, like the thin bone in the shin called the os fibula: those that resemble the beak of the cuckoo (Lat. coccyx or cuculus), like the bone at the bottom of the sacrum, which is called the coccyx; those that somehow have the look of an anvil or a molar, like the smaller ossicle [os incus] of the organ of hearing; or a hammer, like the larger ossicle [os malleus]

Inset text with illustration, page 2 Since the varieties of bone are not as obvious in the illustrations preceding individual chapters describing particular bones as they are in the bones discussed in this chapter, we have illustrated here a part of the arm bone, or (as we should say with Celsus) 17 the humerus, sectioned longitudinally. It shows at its head, which is articulated to the scapula, small cavities shaped like those in pumice, marked A 18 ; scale covering those cavities, marked B. 19 The exterior portion of the bone is marked C [cartilago articularis], clearly visible here. 20 D marks the large hollow space [cavitas medullaris], surrounded by the extremely solid and thick part of the bone, marked E and F [osseus compactus], and extending along the longitude [shaft] of the humerus. At G, near the top of the hollow, and at the bottom, where H is seen, bony lines [trabeculae] occur, woven in the marrow enclosed in this cavity. In addition, under the humerus we have shown the bone resembling a small boat [os naviculare], which will be represented in the eleventh figure of Chapter 33; here it is dissected through the middle, marked I and K, so that the substance of the bone may appear, constructed like pumice. On each side, L marks the porous or spongy material [osseus spongiosus] of this bone. M is the scale [osseus compactus] that makes up the bone’s surface and everywhere surrounds its spongy substance. Beneath this bone we have added one of the ossicles [ossa sesamoidea] that is attached to the first joint of the toe [os hallicis] and is marked w and y in the second figure of the thirty-third chapter. This, marked N, we have divided all the way through the middle, so that a completely solid bone, totally without small cavities, could somehow be seen.

Varieties based upon epiphyses, processes, heads, etc.
For the same reason, the appearance of processes, epiphyses, heads, brows, cavities, and tubercles are not taken into account for the present; nor can any variety of bones be derived from these features, since I have so far not explained what I should name a process, an epiphysis, or other features of this kind, being necessarily on the point of doing so in the second of the chapters that follow. Similarly, varieties derived from the way bones are joined together cannot be understood unless the joints have been carefully described The fourth chapter will attest that these are quite numerous, and not very easy to learn.

Cartilage
Likewise, one will not readily ascertain the types [e.g., os costale] that can hang from cartilage, unless the nature of cartilage has been explained. So long as cartilage is unknown, it is hard to understand which bones are altogether without cartilage, such as the bones of the vertex; which are covered with it everywhere, like some bones of the carpus, or only in some part, like the femur; which degenerate into cartilage, like the bones of the nose, the ribs, and the sternum. ¹⁶

Varieties of substance and structure
We shall reserve these differences for their own place and add those which are inferred from the substance and structure of the bones. Certain bones are completely solid, and no matter how they are broken open, no cavities or hollows are observed inside; such, among others, are the two bones of the nose [ossa nasalia] and the bone that will be numbered the second [os lacrimale] of the bones of the upper maxilla, the smallest in the area [orbita] of the eyes. Also in this category are those that are compared to the size of a sesame seed, and the two ossicles [os malleus, os incus] peculiar to the organ of hearing. All of these, unless completely dried up by the passage of time, reveal no cavity at all inside. Many appear outwardly solid, as if covered by a continuous casing or layer [osseus compactus], but when broken open some of them are seen within to be filled entirely with small pores and spaces closely resembling the cavities of extremely compact sponge [os spongiosum], or very smooth pumice, and look like dried-up mushroom, ²¹ as in a number of lesser bones, the bones of the carpus, and those of the tarsus; among the greater bones, the sacrum, the bodies of the vertebrae, the sternum, the heel bone [calcaneus], the talus, and the bones of the vertex. Others have, besides the small cavities placed without arrangement or number, a large and noticeably hollow pocket [cavitas medullaris], which is surrounded by very solid and strong osseous tissue [osseus compactus] in the farthest corners and marked as if with bony lines [trabeculae]. Bones that have such a cavity generally have only one. Those belonging to this class include, among the lesser bones, the metatarsal, metacarpal, and digital bones, especially the tips of the first and second digits, though it seemed otherwise to Galen, the chief of the professors of anatomy, who declared the bones of the digits solid. ²² The larger bones are equipped with this kind of cavity: the femur, tibia, humerus, lower maxilla [mandibula], the fourth bone of the upper maxilla [corpus maxillae], the frontal bone, the temporal bones, and generally the cuneiform bone [os sphenoidale]. The teeth, which are easily the hardest of all the bones, also have this type of hollow [cavitas dentis], but at the same time they are entirely without the small cavities and little pumice-like holes recently mentioned. The greater cavities [marrow] and the structure [trabecular] not unlike pumice or dried mushroom occur in bones not only to make them

In what part of the bones marrow is located
Marrow is located not only in these larger hollows [cavitas medullaris] of the bones, as one would think; the small cavities built like sponges are also packed with marrow no less than the more conspicuous spaces. In my account of the individual bones I will carefully explain that these hollows and bone density are most justly provided to each of the bones in proportion to its need of greater or lesser lightness, strength, and hardness, or proportionately to its control of this or that organ of sense. ²⁵

Differences in foramina
Besides the pores and cavities, with which the bones abound inside, certain bones are traversed by large foramina, as in the occipital bone [foramen magnum] and the vertebrae [foramen vertebrale], by which they transmit the dorsal medulla. Also the bones of the pubis have a foramen [foramen obturatum] larger than all the others, to make them lighter. Others are perforated with small foramina, like many bones of the head and maxillae, providing a passage for veins, nerves, and arteries. Also, the bone that will be numbered eighth [os ethmoidale] among the bones of the head is believed to be perforated like a seive with tiny foramina [lamina et foramina cribrosa] for the sake of odors. Other bones reveal no foramen on the outer surface that is discerned by sense, like the carpal bones, the teeth, and many bones of the fingers. In several bones small [nutrient] foramina occur which are visible on the outer surface, but they do not penetrate the entire bone; they are provided only for veins and arteries entering the thicker bones. Foramina of this sort occur throughout the bone of the calcaneus, the talus, the sacrum, ²⁶ and the bodies of the great vertebrae. This is evidently because these are too thick to take in sufficient nourishment only by vessels coming through their surface.

Variety based upon sensation
A quite minor difference depends on sensation in bones. Teeth are endowed with feeling, but we believe the other bones lack this capacity. Yet we should not too rashly deny bones all sense of feeling, since even the most outstanding physicians affirm that in frequent surgical procedures we encounter bones that sometimes feel pain. Such authorities are not unaware at the same time that virtually all ²⁷ the bones are covered by a certain membrane, which the Greeks therefore called the perio/steon [periosteum], ²⁸ because of which it seemed to some that bones (if they feel anything) have the power of feeling.

Differentiation by the membrane enclosing the bones
Only the teeth are bare — in that portion which protrudes above the gums. The other bones are entirely covered and hidden before dissection. At the point where they protrude, the teeth lack the covering membrane, like the interior surface of the skull where the cerebrum is contained; a hard membrane [dura mater] clothes the cerebrum and closely surrounds it; elsewhere, the previously named membrane [periosteum] completely encloses the bones except in those places where they are articulated with each other [articulationes cartilagineae, a. synoviales] or are otherwise joined [articulationes fibrosae]. ²⁹