With regard to the relative dimensions of the bones of the metacarpus, it is necessary to remember that, in the human being, the second metacarpal is the longest; then, in the order of decrease, come the third, fourth, fifth, and first. In quadrupeds we shall also find differences in length (see the chapter relating to the anterior limbs in certain animals), but the order of decrease is not always that which we have just mentioned.
In man the articular surface, situated at the inferior extremity of each of the metacarpals, is rounded, and is called the head. This allows the first phalanx, which is in relation with that surface, to be displaced in every direction; indeed, this phalanx can not only be flexed and extended, but it can also be moved laterally; this latter movement allows of the fingers being separated and drawn together.
In quadrupeds which can only perform the movements of flexion and extension of the digits—for example, the horse—the inferior extremity of the metacarpal has not a rounded head of a regular outline; it is marked by a prominent median crest, directed from before backwards, so that the articular surfaces, which fit more exactly, form a sort of hinge which allows of backward and forward movements only, and permits no lateral displacement. In man, at the level of the inferior extremity of the first metacarpal, in the vicinity of the articulation of this bone with the first phalanx of the thumb, we find two sesamoid bones—small bones developed in the fibrous tissue which surrounds the articulation. We also meet with such structures, but more rarely, at the level of the corresponding articulation of the index and auricular digits; and, more rarely still, at those of the middle and ring fingers. In quadrupeds, these bones are normally developed, and we shall see afterwards that in some animals, as they reach a considerable size, they are able to influence the external outlines; we shall see this, for example, in the horse.
Fig. 31.—Skeleton of the Superior Limb of a Bird (Vulture): Left Side, External Surface.
1, Humerus; 2, radius; 3, ulna; 4, radial bone of the carpus; 5, ulnar bone of the carpus; 6, first metacarpal; 7, second metacarpal; 8, third metacarpal; 9, first digit, the homologue of the thumb; 10, first phalanx of the second digit; 11, second phalanx of the second digit; 12, third digit.
The hand, in birds, is directed obliquely downwards and backwards ([Fig. 31]). For the better understanding of its position in relation to the forearm, we should remember that this latter, as we have described ([p. 44]), directed obliquely upwards and forwards, has the radius placed above the ulna; the hand being oblique in the opposite direction and placed under the forearm is, by this arrangement, inclined towards the ulnar border of the latter.
