CHAP. XX.] THE CORRESPONDENCE, <^c. 329 



In the first place, it is perfectly obvious that the fore and 

 liind limbs have each a ^similar division into four main seg- 

 ments ; the shoulder girdle, the arm, the fore-arm, and the 

 manus of the one representing respectively the pelvic girdle, 

 the thigh, the leg, and the pes of the other. 



To proceed to further details, it is necessary to place the 

 limbs (at least in imagination) in an exactly corresponding 

 position — one, in fact, which is often impossible in the adult 

 animal on account of the modifications of the articular sur- 

 faces to suit the posture best adapted for the habits and 

 mode of life of the individual, but which is the position of 

 all limbs when they first appear as bud-like processes from 

 the side of the body of the embryo. In this position the 

 limbs are extended at right angles to the axis of the trunk 

 and parallel to each other, as in Fig. 125, a and b. There 

 is then to each limb a superior or dorsal surface (turned 

 towards the observer in the figure), an inferior or ventral 

 surface, and an anterior and a posterior edge. These last 

 are called by Professor Huxley preaxial and postaxial (in 

 reference to the axis of the limb itself) to avoid the con- 

 fusion with anterior and posteiior in the modified positions 

 they assume in Man and various animals. In the figures 

 the preaxial side is left light, and the postaxial side is 

 shaded. 



The dorsal surface of the anterior extremity includes the 

 back of the hand and the extensor surface of the fore-arm 

 and arm. The dorsal surface of the posterior extremity 

 includes the dorsum of the foot, front of the leg, and the 

 extensor surface of the thigh. The preaxial border of the 

 anterior extremity has in it the pollex, the radius, the 

 condyle commonly called " external," and the greater tuber- 

 osity. The preaxial border of the posterior extremity in- 

 ckides the hallux, the dbia, the condyle commonly called 



