The Position of the Limbs

In their primitive position, the limbs are straight, and are directed outward, at right angles to the axis of the body; but, as development proceeds, they become bent in such a manner that, in the first place, the middle division of each limb is flexed downward and toward the middle line, upon the proximal division; while the distal division takes an opposite bend upon the middle division. Thus the ventral aspects of the antebrachium and erus come to look inwardly, and the dorsal aspects outwardly; while the ventral aspects of the manus and pes look downward and their dorsal aspects look upward. When the position of the limbs has been no further altered than this, the radius in the antebrachium, and the tibia in the crus, are turned forward, or toward the head; the ulna and the fibula backward, or toward the caudal extremity. On looking at these parts with respect to the axis of the limb itself, the radius and the tibia are pre-axial, or in front of the axis; while the ulna and fibula are post-axial, or behind it. The same axis traverses the centre of the middle digit, and there are therefore two pre-axial, or radial, or tibial digits; and two post-axial, or ulnar, or fibular digits, in each limb. The most anterior of the digits (i) is called pollex, in the manus; and hallux in the pes. The second digit (ii) is the index; the third (iii) the medius; the fourth (iv) the annularis; and the fifth (v) the minimus.

In many Amphibia and Reptilia, the limbs of the adult do not greatly depart from this primitive position; but, in birds and in mammals, further changes occur. Thus, in all ordinary quadrupeds, the brachium is turned backward and the thigh forward, so that both elbow and knee lie close to the sides of the body. At the same time, the forearm is flexed upon the arm, and the leg upon the thigh. In Man a still greater change occurs. In the natural erect posture, the axes of both arm and leg are parallel with that of the body, instead of being perpendicular to it. The proper ventral surface of the brachium looks forward, and that of the thigh backward, while the dorsal surface of the latter looks forward. The dorsal surface of the antebraehium looks outward and backward, that of the leg directly forward. The dorsal surface of the manus is external, that of the pes, superior. Thus, speaking broadly, the back of the arm corresponds with the front of the leg, and the outer side of the leg with the inner side of the arm, in the erect position.

In Bats, a line drawn from the acetabulum to the foot la also, in the natural position, nearly parallel with the long axis of the body. But, in attaining this position, the leg is bent at the knee and turned backward the proper dorsal surface of the thigh looking upward and forward, while the corresponding surface of the leg looks backward and upward, and the ungual phalanges are turned backward.

The chief modifications of the manus and pes arise from the excess, or defect, in the development of particular digits, and from the manner in which the digits are connected with one another, and with the carpus or tarsus. In the Ichthyosauria and Plesiosauria, the Turtles, the Cetacea and Sirenia, and, in a less degree, in the Seals, the digits are bound together and cased in a common sheath of integument, so as to form paddles, in which the several digits have little or no motion on one another.

The fourth digit of the manus in the Pterosauria, and the four ulnar digits in the Bats, are vastly elongated, to support the web which enables these animals to fly. In existing birds the two ulnar, or post-axial, digits are aborted, the metacarpals of the second and third are anchylosed together, and the digits themselves are enclosed in a common integumentary sheath; the third invariably, and the second usually, is devoid of a claw. The metacarpal of the pollex is anchylosed with the others, but the rest of that digit is free, and frequently provided with a claw.

Among terrestrial mammals, the most striking changes of the manus and pes arise from the gradual reduction in the number of the perfect digits from the normal number of five to four (Sus), three (Rhinoceros), two (most Ruminantia), or one (Epeidae).