THE POSITION OF THE LIMBS. 33 



altered tlian this, the radius in the antebrachium, and the 

 tibia in the crus, are turned forwards, or towards the head ; 

 the ulna and the fibula backwards, or towards the caudal ex- 

 tremity. On looking at these parts with respect to the axis 

 of the limb itself, the radius and the tibia axe pre-axial, or 

 in front of the axis ; while the ulna and fibiUa 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 poUex, 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 back- 

 wards and the thigh forwards, 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 perpendi- 

 cular to it. The proper ventral surface of the brachium 

 looks forwards, and that of the thigh backwards, while the 

 dorsal surface of the latter looks forwards. The dorsal sur- 

 face of the antebrachium looks outwards and backwards, 

 that of the leg directly f oi-wards. The dorsal surface of the 

 manus is external, that of the pes, superior. Thus, speak- 

 ing broadly, the back of the arm corresponds with the fi'ont 

 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 is 

 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 backwards ; the proper 

 dorsal surface of the thigh looking upwards and forwards, 



B 



