A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 31.—Left fore-arm bones of the Tapir (Tapirus terrestris). R., radius. U., ulna. h., head of radius. h′., sigmoid notch of ulna. ol., olecranon. N.B. This figure is on a much larger scale than [Fig. 30].

The ulna is longer than the radius, its upper end being extended into a heavy process, the olecranon, or anconeal process, into which is inserted the tendon of the great triceps muscle, the contraction of which straightens the arm; this process is the bony projection at the back of the elbow-joint. Below the olecranon is a semicircular articular concavity, which embraces the humeral trochlea and its upper angle fits into the anconeal fossa of the humerus. The ulna contracts and grows more slender downwards and its lower end covers but one of the wrist-bones. While in the more primitive mammals, and in those which retain the power of rotating the hand, the ulna has nearly or quite the same thickness as the radius, it is often much more slender and in the more highly specialized of the hoofed animals, such as the horses, camels and true ruminants, the radius carries the entire weight and the ulna has become very slender, more or less of its middle portion is lost and the two ends are coössified with the radius, so that the fore-arm appears to have but a single bone. The reverse process of enlarging the ulna and reducing the radius is very rare and practically confined to the elephant tribe.

Fig. 32.—Left manus of Wolf, front side. SL., scapho-lunar. Py., pyramidal. Pis., pisiform. Tm., trapezium. Td., trapezoid. M., magnum. U., unciform. Mc. I-V, first to fifth metacarpals. Ph. 1, first phalanx. Ph. 2, second phalanx. Ung., ungual phalanx. I, first digit, or pollex. II-V, second to fifth digits.

Fig. 33.—Left manus of Man. S., scaphoid. L., lunar. Py., pyramidal (pisiform not shown). Tm., trapezium. Td., trapezoid. M., magnum. Un., unciform.

The fore foot, or hand, for which the term manus may be conveniently employed, is divisible into three parts, corresponding in ourselves to the wrist, back and palm of the hand, and the fingers. The bones of the wrist constitute the carpus, those of the back and palm the metacarpus, and those of the fingers the phalanges.

The carpus consists primitively of nine distinct bones, though one of these, as will be shown later, is not a true carpal. These bones are of a rounded, subangular shape, closely appressed together, with very little movement between them, and are arranged in two transverse rows. The upper row contains four bones, which enumerating from the inner side are the scaphoid, lunar, pyramidal (or cuneiform) and pisiform. The scaphoid and lunar support the radius, while the ulna rests upon the pyramidal. The pisiform, though very constantly present, is not a true carpal, but an ossification in the tendon of one of the flexor muscles, which close the fingers; it projects more or less prominently backward and articulates with the ulna and pyramidal. The second row is also made up of four bones, which, from within outward, are the trapezium, trapezoid, magnum and unciform. The relations of the two rows vary much in different mammals and the arrangement may be serial or alternating; thus, the scaphoid rests upon the trapezium and trapezoid and usually covers part of the magnum; the lunar may rest upon the magnum only, but very much more frequently is equally supported by the magnum and unciform and the pyramidal by the latter only. The ninth carpal is the central, which, when present and distinct, is a small bone, wedged in between the two rows. Few existing mammals have a separate central, which, though present in the embryo, has coalesced with the scaphoid in the great majority of forms. In the more advanced and differentiated mammals the number of carpals may be considerably reduced by the coössification of certain elements or the complete suppression and loss of others. In all existing Carnivora and a few other mammals the scaphoid and lunar are united in a compound element, the scapho-lunar (or, more accurately, the scapho-lunar-central); hoofed animals with a diminished number of toes generally lose the trapezium, and other combinations occur. The second row of carpals carries the metacarpals, and primitively the trapezium, trapezoid and magnum are attached each to one metacarpal and the unciform has two.

The metacarpus consists typically of five members, a number which is never exceeded in any normal terrestrial mammal; the members are numbered from the inner side, beginning with the thumb or pollex, from I to V. Many mammals have fewer than five metacarpals, which may number four, three, two or only one; the third is never lost, but any or all of the others may be suppressed, and functionless rudiments of them may long persist as splints or nodules. The metacarpals are elongate, relatively slender and of more or less cylindrical shape; but the form varies considerably in different groups, according to the way in which the hand is used. When employed for grasping, as in many arboreal animals and pre-eminently in Man, the pollex is frequently opposable to the other fingers and enjoys much freedom of motion. In the camels and true ruminants the third and fourth metacarpals are coössified to form a cannon-bone (see [Fig. 43, p. 91]), but the marrow cavities and the joints for the phalanges remain separate.

The phalanges in land mammals never exceed three in each digit, except the pollex, which, when present and fully developed, has but two. The phalanges are usually slender in proportion to their length, but in very heavy hoofed animals they are short and massive. The terminal joint is the ungual phalanx, which carries the nail, claw, or hoof, its shape varying accordingly.