The shaft of the humerus is prismatic in its upper and middle parts, and flattened from front to back at its lower extremity. Covered over by thick muscles, its outward shape has for us but little interest; it is necessary only to note with regard to its direction that, when the arm is hanging down, the humerus is not altogether vertical, but is tilted slightly downwards and inwards. We shall see that the axis of the bones of the forearm is oblique in the opposite direction; for this reason the arm and forearm form at the elbow a very obtuse angle, looking outwards (Figs. 24, 26).

Among the details to be noted upon the shaft of the humerus, the bicipital groove is worthy of remark. This vertical groove, which separates the great from the small tuberosity on the front of the humerus at its upper end (Fig. 18), is prolonged along the shaft of the bone, and presents an internal lip slightly prominent, and an external or anterior lip which is much more marked, which gives insertion to the broad tendon of the great pectoral muscle. At the line of junction of the superior with the middle third of the bone, this lip forms the anterior margin of a rough surface shaped like the letter (7, Fig. 18), of which the angle looks downwards, and which, giving insertion to the deltoid muscle, has received the name of the deltoid impression. On the back of the humerus is another groove, broad and shallow, called the spiral groove of the humerus, which passes downwards and outwards along the shaft of the bone behind and below the deltoid impression.

The inferior extremity of the humerus deserves to be studied in detail, as its shape gives the key to the movements of the elbow-joint, and explains at the same time several features of the region to be seen in the living model. This extremity is flattened from front to back, and is enlarged into a broad surface which is partly articular, partly non-articular. In the middle are two important articular prominences, smooth and covered with cartilage. Of these the internal portion is called the trochlea (trochlea, pulley). It possesses a groove and two unequal marginal ridges. The internal ridge is more prominent and descends lower than the external one. The other projection (11, Fig. 18), which is situated on the outer side of the trochlea, is rounded in shape, and receives the name of the radial head, or capitellum. It is only obvious when the humerus is looked at from the front or from below: in other words, it does not appear (like the trochlear surface) on the posterior aspect of the lower end of the bone. This surface articulates with the upper end of the radius. There are three depressions, or fossæ, to be seen in relation to the articular surfaces of the lower end of the humerus. On the front of the bone, just above the trochlea and capitellum, are two, which receive the bones of the forearm during flexion of the limb: the coronoid fossa (for the coronoid process of the ulna), above the trochlear surface, and the radial depression for the head of the radius, much shallower, and placed above the capitellum. On the back, above the trochlear surface, is the olecranon fossa, into which the olecranon process is received during extension of the forearm.

The lateral portions of the inferior extremity of the humerus are formed by rough, non-articular projections which give attachment to muscles and ligaments, and are known respectively as the external condyle and the internal condyle of the humerus (14, 14, Fig. 18). Above each condyle is a well-marked bony ridge, which is called the supra-condyloid ridge (external or internal) (13, 13, Fig. 18).

The lower end of the humerus articulates with the upper extremities of the bones of the forearm; and we will next proceed to study the formation of these extremities in order to understand the movements of the elbow-joint and the form of the region.

The forearm is formed of two bones (Fig. 21), which, when the upper limb is hanging beside the body, the palmar surface of the hand being turned forwards, are placed parallel to each other—one on the outer, one on the inner side. The inner bone (1, Fig. 21) is called the ulna, or cubitus, and it is that which by its upper extremity (olecranon) forms the bony prominence of the elbow; the outer bone (10, Fig. 21) is called the radius (from the Latin radius, a spoke of a wheel), and it is by this bone chiefly that the bones of the wrist and hand are carried. For the present we will describe only the upper extremities of these two bones (Figs. 21 and 22).

The superior extremity of the ulna presents two processes and two articular cavities. The cavities are the greater and lesser sigmoid cavities; the processes are the coronoid and olecranon processes. The bone articulates with the pulley or trochlea of the humerus by means of the great sigmoid cavity, and with the margin of the head of the radius by the lesser sigmoid cavity. The greater sigmoid cavity (2, Fig. 21) presents in its centre a ridge prominent from before backwards, which corresponds to the groove in the trochlea of the humerus. Below and in front, the great sigmoid cavity is formed (5, Fig. 21) by a bony prominence called the coronoid process (compared to the beak of a crow: κορώνη, a crow; εἶδος, form), which is lodged in the coronoid fossa of the humerus in flexion of the forearm (17, Fig. 18). Above and behind, the great sigmoid cavity is formed by the olecranon process (ὠλενη, the elbow; κάρηνον, the head), a large square projection (4, Fig. 21), which constitutes the prominent point of the elbow, and which accentuates in a high degree the form of the forearm during flexion. During extension of the forearm the olecranon process is lodged partly in the olecranon fossa of the humerus (4, Fig. 23). The lesser sigmoid cavity of the ulna is a small concave surface placed on the outer side of the coronoid process for articulation with the margin of the head of the radius.

The superior extremity of the radius forms a small discoidal head (11, Fig. 21) attached by a narrow neck to the body of the bone; this head is flattened at the top and hollowed out to articulate with the capitellum of the humerus (Fig. 22). The margin of the head of the radius revolves in the lesser sigmoid cavity of the ulna.

We see, then, that the articular surfaces of the elbow are formed on the humerus (6, 7, 8, Fig. 22) by a transverse series of projections (the trochlea and capitellum), and on the bones of the forearm by a series of depressions moulded on these projections, so that the whole describes a movement similar to that between two cog-wheels (Fig. 22), a species of transverse hinge. Thus it is easy to understand, à priori, how this disposition of the parts does not permit of any lateral displacement of the bones, or transverse movement; the movements forward and backward are, in fact, the only kind possible in the elbow-joint. The forward movement—that is to say, that by which the anterior surface of the forearm is brought near the anterior surface of the arm—constitutes flexion of the forearm. The movement in the opposite direction constitutes extension.

The disposition of the ligaments—that is, of the fibrous bands or articular capsule which fasten the bones together—modifies very slightly the mechanism we have just deduced from the shape of the articular surfaces; in fact, this capsule is formed on the inner and outer surfaces by ligamentous fibres, very dense and short, called the lateral ligaments, which prevent all lateral movement. On the other hand, the anterior and posterior portions of the capsule are loose, so as not to offer any opposition to the movements of flexion and extension. The only limit to these movements is that resulting from the bony projections of the ulna coming in contact with the humerus. Thus, the movement of flexion can be prolonged until the coronoid process arrives at the coronoid fossa and touches the bottom of that cavity; then the fleshy masses of the forearm come into contact with the anterior surface of the arm, especially if the model is muscular, and flexion is no longer possible. The movement of extension, on the contrary, has a limit which it is important to state precisely, resulting from the beak of the olecranon touching the bottom of the olecranon fossa (Fig. 23); this is produced when the forearm has attained, in the movement of extension, that situation which brings its own axis into direct line with that of the arm. The extension of the elbow cannot therefore exceed the degree which brings the humerus and forearm into the same plane; that is to say, the forearm can never make with the arm an angle facing backwards.

By comparing the particulars of the mechanism of the elbow with that which we have previously seen of the mechanism of the scapulo-humeral articulation, it will be easy to understand how we may, from the study of the articular surfaces and ligaments, learn the laws of the mechanism of joints. For example, the head of the humerus received into a single concavity may allow to the arm every kind of movement; in the same way the arrangement of a hinge-joint, with a series of projections and depressions, fitted one to the other in a transverse line, renders possible in the elbow-joint only the movements of flexion and extension.

In regard to external form, we learn the following facts from a knowledge of the bones which form the elbow-joint.

1. With respect to the angle which the forearm makes with the arm, if we examine it either upon the skeleton or upon the living subject, the upper limb hanging beside the body, with the palm of the hand turned forward, it is seen that the humerus (page 67) is slightly oblique from above downwards and inwards, while the two bones of the forearm are directed obliquely in the opposite direction—that is, from above downwards and outwards. In other words, the bones of the arm and forearm form at their point of junction—that is, at the level of the elbow—an angle the base of which looks outwards and the apex inwards. This angle only appears in extension of the limb, and is due to the twisted and tilted form of the trochlear surface of the humerus.

2. Concerning the bony prominences which are seen beneath the skin at the elbow (Fig. 24), after studying these osseous structures, we should be able to recognise upon the living model those details of the figure which correspond to the four bony points—namely, the coronoid process in front, the olecranon behind, the external condyle on the outer side, and the internal condyle on the inner side. The coronoid process, covered by muscles, is so buried in the surrounding structure that it does not show externally. It is much the same also with the external condyle, as this projection, not very prominent in the skeleton, disappears completely in the living subject, since this external condyle is situated at the bottom of the angle facing outwards which the forearm makes with the arm, and the mouth of this angle is filled up by the external muscles of the forearm (especially the brachio-radialis, or supinator longus), which take their origin from the external border of the humerus. The external condyle and the external supra-condyloid ridge can be felt beneath the skin; and the former occupies a depression at the back of the elbow on its outer side in extension of the forearm.

On the other hand, the internal condyle, supra-condyloid ridge, and the olecranon process always show clearly beneath the skin, and the olecranon forms that projection, commonly called the point of the elbow, which is so prominent behind during flexion of the forearm, and which follows the movements of the forearm, seeming to rise towards the arm during the extension of the forearm, and to descend during flexion. The internal condyle projects as the apex of the angle formed by the axis of the forearm with that of the arm (Fig. 26, page 81); this is a fixed point placed a little above the line of the elbow-joint, which is useful in measurements.

There is one detail which we must not forget at this stage; in comparing the length of the forearm with that of the arm the olecranon must be excluded, as it projects above the lower end of the humerus; on the other hand, the internal condyle should rather be chosen as a fixed point from which measurements may be taken.