156. The upper end of the ulna belonging to the elbow is large (figs. LXIX. 5, and LXXIII. 3). It sends backwards the large projection commonly named the elbow or olecranon (fig. LXXII. 3), in the centre of which there is a smooth and somewhat triangular surface (fig. LXXIII. 3) which is always covered by skin of a coarse texture, like that placed over the lower part of the knee-pan, as if nature intended this for a part on which we may occasionally lean and rest. Large at the elbow, the ulna gradually grows smaller and smaller as it descends towards the wrist, where it ends in a small round head (fig. LXXXII. 2), beyond which, on the inner side, or that corresponding to the little finger, it projects downwards a small rounded point, termed the styloid process (fig. LXXXII. 3). As the styloid process and the olecranon, the two extremities of the ulna (figs. LXXIII. 3, and LXXII. 3), are easily and distinctly felt, the length of this bone was primitively used as a measure, called a cubit, which was the ancient name of the bone.
157. The radius, the second bone of the fore-arm, placed along its outer part next the thumb, is small at its upper end (figs. LXIX. 6, and LXXIII. 4); but its body is larger than that of the ulna; while its lower end, next the wrist to which it properly belongs, is very bulky (fig. LXXXII. 1). Its upper end is formed into a small circular head, which is united by distinct joints both to the humerus and to the ulna (fig. LXIX. 6). The top of its rounded head is excavated into a shallow cup (figs. LXIX. 6, and LXXIII. 4) which receives a corresponding convexity of the humerus (fig. LXIX. 2), and its lower extremity is excavated into an oblong cavity, which receives two of the bones of the wrist (fig. LXXXIII. 1. 4).
158. The joint of the elbow is composed above of the condyles of the humerus (fig. LXIX. 3. 2), and below by the heads of the ulna and radius (fig. LXIX. 5. 6).
159. The upper surface of the ulna is so accurately adapted to the lower surface of the humerus that the one seems to be moulded on the other (figs. LXIX. 5, and LXXIII. 3), and the form of these corresponding surfaces, which are everywhere covered with cartilage, is such as to admit of free motion backwards and forwards, that is, of extension and flexion; but to prevent any degree of motion in any other direction. The joint is therefore a hinge-joint, of which the two motions of flexion and extension are the proper motions. This hinge is formed on the part of the humerus by a grooved surface, with lateral projections (fig. LXIX. 2, 3, 4), and on the part of the ulna by a middle projection with lateral depressions (fig. LXIX. 5): the middle projection of the ulna turning readily on the grooved surface of the humerus (fig. LXIX. 2).
160. The bones are held in their proper situation, first, by a ligament on the fore part of the arm, called the anterior (fig. LXXIV. 6), which arises from the lower extremity of the humerus, and is inserted into the upper part of the ulna and the coronary ligament of the radius (fig. LXXIV. 6. 8); secondly, by another ligament on the back part of the arm, called the posterior ligament (fig.LXXV. 8), placed in the cavity of the humerus that receives the olecranon of the ulna (fig. LXXV. 8); and thirdly, by two other ligaments at the sides of the ulna (fig. LXXV. 6, 7). The ulna and radius are united, first, by a ligament called the coronary, which, arising from the ulna, passes completely around the head of the radius (fig. LXXVI. 3), and the attachment of which, while sufficiently close to prevent the separation of the two bones, is yet not adherent to the radius, for a reason immediately to be assigned; secondly, by another ligament which passes in an oblique direction from one bone to the other (fig. LXXVI. 4); and thirdly, by a dense and broad ligament, termed the interosseous (figs. LXXIV. 10, and LXXVI. 5), which fills up the space between the two bones nearly in their whole extent. This ligament serves other offices besides that of forming a bond of union, affording, more especially, a greater extent of surface for the attachment of muscles, and separating the muscles on the anterior from those on the posterior part of the limb.
Anterior view of the ligaments of the elbow-joint. 1. The
lower portion of the humerus; 2. the upper portion of the
radius; 3. the upper portion of the ulna;
4. the internal condyle; 5. the external condyle; 6. the anterior
ligament; 7. portion of the internal lateral ligament;
8. portion of the coronary ligament; 9. the oblique ligament;
10. upper portion of the interosseous ligament.
Posterior view of the ligaments of the elbow-joint.
1. Lower end of the humerus; 2. internal condyle; 3. external
condyle; 4. the olecranon process of the ulna;
5. the upper portion of the radius; 6. the internal lateral ligament;
7. the external lateral ligament; 8. the posterior
ligament.
