THE POSITION OF THE LIMBS. 33 



responding bones in the carpus, the fourth and fifth tarsalia 

 have coalesced to form the cuboides. 



The Position of the Limbs. — In their primitive position, 

 the Hmbs are straight, and are directed outward, at right 

 angles to the axis of the body; but, as develojDment 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 

 di^dsion. Thus the ventral aspects of the antebracliium and 

 cms come to look inwardly, and the dorsal aspects outwardly ; 

 while the ventral aspects of the manus and j^es 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 antebracbiura, and the tibia in the crus, are turned for- 

 ward, 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 twc 

 pre-axial, or radial, or tibial digits ; and two post-axial, 01 

 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 mediiis ; 

 the fourth (iv) the annularis ; and the fifth (v) the minimus. 



In many Amphibia and Heptilia, 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 ordi- 

 nary quadrupeds, the bracLium is turned backward and the 

 thio'h 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, in- 

 stead of being perpendicular to it. The proper ventral sur- 

 face 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 antebrachium 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. 



