Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 23 On the Humerus or Arm Bone

Description of the lower part of the humerus

The lower part [pars distalis], which is articulated to the two bones in the forearm (the first four figures of the next chapter), to wit the ulna and the radius, is much more complicated in the number and appearance of depressions as well as its heads and protrusions. First, there is seen in the middle of the lower part of the humerus a depression (K, L, M in figs. 1 and 2) with its high points [condylus humeri, trochlea humeri] very much resembling the little wheel in a pulley on which ropes turn. As the wheel of a pulley is round and orbicular in circumference but smooth and wide on the sides; so on this end [pars distalis] of the humerus the round, orbicular part is seen which you will say resembles nothing so much as a little wheel. In its perimeter like a little wheel on which a cord is turned, it is carved out with a depression [sulcus] (so to speak) that is rounded, smooth, and slick; sides protrude on both sides of this depression as if to hold a rope and prevent it from slipping off: in such a way this part of the humerus is depressed in the middle and protrudes on each side. But it differs in this respect from the projections of a perfect pulley wheel, that the wheel’s sides are nearly always evenly raised above its depression. But this part of the humerus projects quite significantly on its inner side [facies medialis] (L in figs. 1, 2) and exactly resembles a pulley wheel, while on its outer side [facies lateralis] (M in figs. 1, 2) it is much less extended. 15 This is not to say that Nature was more inattentive than the makers of pulley wheels or less clever, but that it was not necessary for the depression of the humerus or the sides of its groove to be evenly extended everywhere. For as a pulley wheel that faces a frame 16 on one side can protrude

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less without harm on the side facing the frame than on the other (for there is no fear that the rope will slip off the wheel on the side that nearly touches the frame), so too the outer side of the humeral trochlea is not as endangered as the inner. For we shall soon add that there is another head [condylus humeri, capitulum humeri] (P in figs. 1 and 2) near the outer side, to which the radius is articulated and which can be considered like a frame or wall preventing the ulna from being dislocated outward from its groove. But there is another difference between the trochlea of the humerus and an actual pulley wheel: the ulna cannot be moved around to every part of the trochlea as a rope can be moved on a pulley. For although the humerus is markedly depressed in the upper and middle area of its groove (N in fig. 1, O in fig. 2) and is extremely thin and transparent like a scale, 17 nevertheless it is not open, and that is for a very good reason. For when the supreme Maker of things desired the ulna to be flexed and extended on this groove [trochlea humeri] of the humerus, he so prepared the trochlea and bestowed depressions, protrusions, and processes (C [processus coronoideus], D, E [incisura trochlearis] in figs. 1 and 5, chapter 24) on the ulna itself that we are able to flex and extend the forearm without being deprived of strength in these motions. This would have been impossible had the humerus been open on the superior part of the trochlea and there were nothing on which the ulna could be braced in its furthest extension, and the trochlea were wrapped as by a rope without a stop, base, or obstacle. And so Nature carved out two notable depressions on the top of this trochlea, one [fossa coronoidea] (N in fig. 1) in the anterior part, and the other [fossa olecrani] (O in fig. 2) in the posterior, which is wider, deeper, and in one way or another more conspicuous than the anterior. These depressions, separated only by a kind of scale, receive the processes of the ulna (which they specifically name korw/nai). The anterior depression admits the anterior [processus coronoideus] of these processes when the forearm is flexed; and when the forearm is extended, the posterior depression receives the posterior process [olecranion] of the ulna. These depressions have been made the limits of extreme flexion and extension with such foresight that they allow the forearm to be flexed to a very sharp angle and permit it to be extended not much beyond a straight line, because we clearly have no need at all of extension to an angle as we certainly do for flexion. 18 Now because the depressions of which we are speaking are like bases and obstacles in the motions of the forearm that do not let the ulna be taken too far, Hippocrates called them baqmi/dej, and the trochlea gigglimoeidh/j from its resemblance to a hinge. 19 We will correctly understand the use of these depressions when we complete the description of the ulna in the next chapter. For the present, however, we must continue our description of the lower end of the humerus. 20 At the extreme edge of the trochlea of the humerus there is a rounded and somewhat oblong head [capitulum humeri] (P in fig. 1), which like the groove is smooth and covered with cartilage; it serves the articulation of the radius with the humerus and enters the upper depression [caput radii, fovea articularis] (O in fig. 9, ch. 24) of the radius, and the radius (as I shall explain elsewhere at greater length) pronates and supinates on this head. 21 The length of this head, which I measure from its upper to its lower part along the length of the humerus, is the reason why in the flexion and extension of the forearm this head does not slip out or escape from the depression of the radius (which is round but not oblong). The entire part of the head that is covered with cartilage faces the anterior surface of the humerus and does not occur at all in the posterior, because the radius does not approach the back of the humerus in any extension of the forearm. But on the outer side of this head the humerus protrudes significantly [epicondylus lateralis] and looks like a mountain or a cliff; it is made this way so that as many muscles as possible may conveniently take their origin hence. From this part of the humerus 22 (P in fig. 2 towards Q [crista supracondylaris lateralis] in figs. 1 and 2 where the line is marked) and along a sharp line [margo lateralis] which ascends from this protrusion upward some distance along the outer side of the humerus, six muscles take their origin, 23 the first 24 of which (Q in the 12th table of muscles) [m. brachioradialis] may be counted that which is wider than the others; it is by far the longest among those that move and supinate the radius. The second (L in the 11th table of muscles) [mm. extensor carpi radialis longus et brevis], originating below it, extends the wrist by means of a two-horned tendon. The third (Z in the 9th table of muscles) [m. extensor digitorum] is the muscle by which the index, middle, and ring fingers are extended. The fourth (Q in the 9th table) [m. extensor digiti minimi] is the one which we make the chief author of extension of the little finger. The fifth (L in the 9th table) [m. extensor carpi ulnaris], extended along the ulna, is also responsible for extension of the wrist. The sixth 25 (m in the 11th table) [m. supinator] will be counted the second of the muscles that supinate the radius; a small part of it originates in its turn from the tubercle [epicondylus lateralis] of the humerus at its lowest end. Whenever mention is made of the muscles, nerves, veins, and arteries passing this point, we shall always call it the outer protrusion or tubercle of the humerus. It is, as we have stated, made for the origin of muscles, and on its posterior surface it forms a depression [sulcus] (R in fig. 2) next to the outer side (M in fig. 2) of the trochlea of the humerus where the fourth [n. radialis] (F in figs. 2 and 3, ch. 10, Bk. 4) of the nerves going to the arm is carried. 26 But just as this outer side of the humerus protrudes, so too the inner side has a conspicuous, sharp process or tubercle [epicondylus medialis] (S in figs. 1 and 2), also for the sake of muscles and nerves, but not protruding for the sake of any joint like the outer end [capitulum humeri] to which the radius is articulated. At the inner side (L in figs. 1 and 2) of the trochlea of the humerus a process [epicondylus medialis] comes out from whose tip, which is quite wide, a muscle [m. palmaris longus] (P in the 3rd table of muscles) originates which goes to the inner skin of the hand via a long tendon; next, two flexors [m. flexor carpi radialis, m. flexor carpi ulnaris] (L in the 3rd table of muscles) of the wrist. In addition, a portion of the muscle [m. pronator teres] (Q in the 7th table) that is considered the superior of the muscles that pronate the radius, also originate from the lowest part of this process (which we shall always call the inner tubercle of the humerus). Finally, an obscure portion of the beginning of certain muscles that flex the fingers (Q in the 5th table of muscles, C in the 6th) 27 hangs from this point. But

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this tubercle does not serve only for the origin of muscles; it also makes a suitable location for two nerves that go to the forearm and should be considered a bulwark for them. On its anterior surface next to the trochlea, it displays a lightly carved depression (T in fig. 1) 28 along which the third of the nerves [n. medianus] (t in fig. 2, ch. 10, Bk. 4) going to the arm is arranged. On its posterior surface, in another depresion [sulcus nervi ulnaris] (V in fig. 2), it brings down the fifth nerve [n. ulnaris] of the arm (* in fig. 2, ch. 10, Bk. 4). However large the inner tubercle [epicondylus medialis] of the humerus is (see the elbow joint in the skeletal figs.) and easily detected by touch and observed, we must not on this account believe Galen, 29 who stated it was larger than the outer tubercle [epicondylus lateralis], though the larger is much larger the inner; for because it does not present itself to touch as readily as the inner tubercle, it is not its size but the radius to which it is articulated that prevents it from being distinguished by touch. 30 In dogs, horses, sheep, and suchlike animals these protrusions of the humerus differ very little from this construction, except that the outer head of the humerus [capitulum humeri] that is articulated to the radius is less distinguishable from the outer side of the trochlea the more the radius is bound and connected in such animals with the ulna — more than in humans, whose radius is pronated and supinated quite clearly and is loosely articulated with the ulna.

Book One -- The things that sustain and support the entire body, and what braces and attaches them all. [the bones and the ligaments that interconnect them] | Chapter 23 On the Humerus or Arm Bone