xx.] ANTERIOR AND POSTERIOR EXTREMITY. 363 



axis of the trunk and parallel to each other, as in Fig. 133, 

 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 pre-axial 

 and post-axial (in reference to the axis of the limb itself) to 

 avoid the confusion with anterior and posterior in the modi- 

 fied positions they assume in Man and various animals. In 

 the figures the pre-axial side is left light, and the post-axial 

 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 pre-axial border of the 

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

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

 osity. The pre-axial border of the posterior extremity in- 

 cludes the hallux, the tibia, the condyle commonly called 

 " internal," and the lesser trochanter. All these parts, then, 

 should be regarded as serially homologous. 



Leaving for the present the shoulder and pelvic girdles 

 out of consideration, we will next consider the adaptive 

 changes which take place in the segments of the limb 

 proper in various animals. These will be best understood 

 by dividing them into stages (all of which are represented 

 in the diagram), though it is not meant to imply that the 

 limbs actually go through so many distinct phases in the 

 course of development, as all the various modifications from 

 the primitive to the most adaptive positions may take place 

 gradually and even simultaneously. 



the Amphibia, and is in harmony with Gegenbaur's derivation of the 

 limbs from the archipterygium. (H. G.) 



