The Bones of the Thoracic Limb

The thoracic limb consists of four chief segments, viz., the shoulder girdle, the arm, the forearm, and the forefoot or manus.

The shoulder girdle (Cingulum extremitatis thoracicæ), when fully developed, consists of three bones—the scapula or shoulder-blade, the coracoid, and the clavicle or collar-bone. In the domesticated mammals only the scapula, a large, flat bone, is well developed, and the small coracoid element has fused with it, while the clavicle is either absent or is a small rudiment embedded in the mastoido-humeralis muscle. There is therefore no articulation of the shoulder with the axial skeleton.

The shoulder girdle is fully developed in birds and the lower mammals (monotremata). In the higher mammals the coracoid is reduced to the coracoid process of the scapula, and the development of the clavicle is in conformity with the function of the limb. Thus in typical quadrupeds, such as the horse and ox, in which the forelimbs are used only for support and locomotion, the clavicle is absent. Other animals which use these limbs for grasping, burrowing, climbing, etc. (e.g., man, apes, moles), have well-developed clavicles which connect the scapula with the sternum.

The arm (Brachium) contains a single long bone, the humerus or arm bone.

In the forearm (Antibrachium) are two long bones, the radius and ulna. These vary in relative size and mobility. In the horse and ox the two bones are fused, and the lower part of the limb is fixed in the position of pronation. The radius is placed in front and supports the weight. The ulna is well developed only in its upper part, which forms a lever for the extensor muscles of the elbow. In the pig the ulna is the larger and longer of the two bones, but is closely attached to the back of the radius. In the dog the ulna is also well developed and a small amount of movement is possible between the two bones.

The forefoot or hand (Manus) consists of three subdivisions, viz., the carpus, metacarpus, and digit or digits.

The carpus, popularly termed the “knee” in animals, and homologous with the wrist of man, contains a group of short bones (Ossa carpi). These are typically eight in number and are arranged in two transverse rows—a proximal or antibrachial, and a distal or metacarpal. The bones of the proximal row, named from the radial to the ulnar side (i. e., from within outward), are the radial, intermediate, ulnar, and accessory carpal bones. The bones of the distal row are designated numerically, in the same direction, as first, second, third, and fourth carpal bones.

This nomenclature, introduced by Gegenbaur, and now used largely by comparative anatomists, seems decidedly preferable to the variety of terms borrowed from human anatomy and based on the form of the bones in man. The following table of synonyms in common use is appended for comparison. The Latin terms and abbreviated notations are given in parenthesis.

Radial (Os carpi radiale, Cr) Scaphoid

Intermediate (Os carpi intermedium, Ci) Semilunar

Ulnar (Os carpi ulnare, Cu) Cuneiform

Accessory (Os carpi accessorium, Ca) Pisiform

First carpal (Os carpale primum, C1) Trapezium

Second carpal (Os carpale secundum, C2) Trapezoid

Third carpal (Os carpale tertium, C3) Os magnum

Fourth carpal (Os carpale quartum, C4) Unciform

The central carpal bone (Os carpi centrale) is omitted, since it is not a separate element in the animals under consideration here.

The metacarpus contains typically five metacarpal bones (Ossa metacarpalia I–V), one for each digit; they are long bones and are designated numerically from within outward. This arrangement occurs in the dog, although the first metacarpal is much smaller than the others, and the second and fifth are somewhat reduced. Further reduction has taken place in the other animals, resulting in the perissodactyl and artiodactyl forms. In the horse the first and fifth metacarpals are absent, the third is the large supporting metacarpal bone and carries the single digit, while the second and fourth are much reduced. In artiodactyls (e. g., ox, sheep, pig) the third and fourth are the chief metacarpals and carry the well developed digits; they are fused in the ox and sheep. The others are variously reduced or absent as noted in the special descriptions to follow.

The fossil remains of the ancestors of the existing Equidæ illustrate in a most complete manner the reduction which has occurred in this respect. The earliest known ancestor of the horse, Eohippus or Hyracotherium of the Lower Eocene, had four well developed metacarpal bones, each of which carried a digit; the first metacarpal bone was small. Eleven intermediate stages show the gradual evolution of the race from this primitive animal, which was about the size of the domestic cat. There is reason to believe that earlier forms had five digits.

The digits (Digiti manus) are homologous with the fingers of man, and are typically five in number. They are designated numerically from the radial to the ulnar side, in correspondence with the metacarpus. The full number is present in the dog. In the ox and pig the third and fourth are well developed and support the weight, while the second and fifth are reduced. The existing horse has a single digit, the third of his polydactyl ancestors. The skeleton of each fully developed digit consists of three phalanges and certain sesamoid bones. The first or proximal phalanx (Phalanx prima) articulates with the corresponding metacarpal bone above and with the second or middle phalanx (Phalanx secunda) below. The third or distal phalanx (Phalanx tertia) is inclosed in the hoof or claw, and is modified to conform to the latter. The sesamoid bones (Ossa sesamoidea) are developed along the course of the flexor tendons or in the joint capsules. Two proximal sesamoids (Ossa sesamoidea phalangis primæ) occur at the flexor side of the metacarpo-phalangeal joint and form a pulley for the flexor tendon. The distal sesamoids (Ossa sesamoidea phalangis tertiæ) are similarly placed between the deep flexor tendon and the joint between the second and third phalanx; they are absent in the dog, which has a small sesamoid on the extensor side of the metacarpo-phalangeal joints, and often at the proximal interphalangeal joint also.

Numerous cases are recorded of the occurrence of supernumerary digits (hyperdactylism) in the horse and other animals. In some pigs, on the other hand, the two chief digits are fused, and the condition appears to be inherited.