The inferior extremity of the humerus is terminated by an articular cylinder, which pro jects into a plane anterior to that of the shaft of the bone, (processes cubitalis). This cy linder is placed transversely, but in transverse extent it falls short of the widest part of the inferior third of the humerus. Various de pressions and elevations are marked upon the surface of this cylinder. I'roceeding from without inwards, we notice a convexity or rounded head, limited externally by the mar gin of the cylinder and internally by a groove, which passes in a curved direction from before backwards, the concavity of the curve corres ponding to the rounded head. This head is properly denominated the external condyle of the humerus ; it articulates with a cavity on the head of the radius ; the anatomist should notice that the axis of this head passes in a direction downwards and forwards. On the anterior surface of the humerus immediately above this head, we observe a slight and very superficial depression which receives the edge or lip of the cavity of the radius, when the forearm is in a state of complete flexion. Internal to the groove which bounds the con dyle on the inner side, we have a pulley-like surface, which is destined for articulation with the ulna. The concavity which forms the cen tral part of this pulley is deep, but deeper and wider behind than before; its anterior ex tremity terminates in communicating with an oval depression on the anterior surface of the bone (fovea anterior minor), which in flexion of the forearm receives the anterior projecting angle of the coronoid process of the ulna ; the posterior extremity terminates in a similar depression, (fovea posterior v. sinus maximus,) but a much deeper one, and of greater dimen sions generally, occupying, in short, nearly the whole posterior surface of the bone; this de pression receives the olecranon process ,of the ulna, when the elbow joint is in extension. The trochlear concavity, in passing from before backwards, takes a curved direction, so that its posterior extremity is much nearer the external part of the articular cylinder than the anterior. This has an important influence on the direc tion of the motions of the forearm. These two depressions are separated from each other by a thin osseous lamina, almost transparent. We sometimes meet with instances in which this lamina is perforated in consequence of a defect of ossification; and Meckel states that he has found this perforation more frequently in the bones of Negroes and Papuas than in those of the superior races of mankind. It is the permanent condition of many pachydermata, rodentia, carnivora, and quadrumana. On the inside the trochlear concavity is bounded by a thick and projecting lip, which, when the bone is placed at right angles with a horizontal plane surface, descends lower down than any other part, so that this part comes in contact with the plane surface, while the remaining portion of the articular cylinder is raised con sidentbly above it. This arrangement accounts for the hollow angle manifest on the outer side of the elbow-joint when the forearm is extended.
We have yet to describe two processes which are connected in great measure with the outer and inner extremities of the articular cy linder, and to which we have already referred, as being the points in which the margins of the bone terminate. The external one is trian gular and thick, rough upon its surface, and projects slightly. It is improperly called the external condyle—more correctly it should be designated epicondyle, being applied to the outer surface of what is properly the external condyle. This process affords attachment to the external lateral ligament of the elbow joint and to the principal supinator and ex tensor muscles on the forearm, whence it has been called condylus cxtensorius. The inter
nal process is very prominent, distinctly trian gular, terminating the inner edge of the hu merus and connected with the trodden; it is more correctly denominated epitrochlra. It affords insertion to the internal lateral ligament, and to the pronator and flexor muscles of the forearm. Its posterior surface is slightly hol lowed at the line of its junction with the rest of the bone; the ulnar nerve passes behind it.
The humerus is the principal lever of the pectoral extremity ; hence in all animals its strength is proportionate to the force and power which is required in the limb. In the ele phant it is a massive pillar of support; and here we may notice a variety following the same law which influences the difference in the aspect of the glenoid cavity of the scapula, already noticed ; namely, that the angle be tween the axes of the head and shaft of the humerus, is at its maximum when the arm bone is mainly an instrument of support, and diminishes as that bone is more used for pre hension and other purposes; and as this use is found for this bone chiefly in the human sub ject, we may presume that in man the angle in question is the least removed from a right angle. When this limb is used mainly for support and progression, a considerable range of motion in the shoulder joint is not required, the tuberosities at the upper extremity of the hone project and limit the motions of the joint. When, however, a considerable motion is ne cessary,these tubercles are depressed as in man, so as not to interfere with these motions. The lower extremity of the humerus likewise affords marks indicative of the mobility of the fore arm and hand ; thus, in the one case one or both of the edges of the bone which terminate in the epitrochlea and epicondyle are promi nent and strong in proportion as the muscles which arise from it are frequently called into play, as when the pronating and supinating motions of the forearm are extensive : in the other case this ridge is imperfectly developed, and the principal modification of the lower end of the bone is to be seen in the articular cy linder, where greater depth is given to the troehlea, in order to Aim] increased strength and security to the elbow-joint.
One of the most singular instances of the developement of bony processes in accordance with muscular power is in the case of the mole. In this little animal the whole anterior ex tremity is constructed entirely with reference to its burrowing habits; its short, thick, and almost square clavicle and its elongated lever like scapula tend to the same end, as its amaz ingly strong humerus. The upper extremity of this latter bone is extremely broad; it pre sents two articular surfitces, being articulated with the clavicle as well as with the scapula, and the tuberosities which give insertion to the muscles of rotation are enormously de veloped. The body of the bone is short, thick, and strong; the inferior extremity is nearly as large as the superior ; both the epicondyle and epitrochlea are very highly developed, especially the latter, which is accounted for by the fact that the muscles of pronation are those most called into action, in order to enable the animal to employ the accessory bone on the radial side of the hand, in scraping up the earth. This large size of the humerus, and great develope ment of its muscular eminences, is found in all fossorial animals, as the megatherium, the pan golins, beavers, ant-eaters, moles, and mono tremata. In the two last the development is the most remarkable.