DISCUSSION 183 



between the anterior and posterior extremities, the reasons 

 for accepting the hypothesis of a teleological connection 

 between the two seem sufficiently trustworthy for accept- 

 ance, and it furnishes us with a very desirable basis for the 

 comparison of the functioning of the limbs. In an ideally 

 generalized vertebrate the fore leg and hind leg would 

 have equal power, and the flexors and extensors of each 

 would be respectively of the same strength as those of the 

 other; or because of the mechanical differences presented 

 by variations in the structure of the shoulder and pelvic 

 regions, it would be better to state that the limbs would be 

 capable of the same effective work. It is probable that there 

 is no living mammal so primitive as to approach such a 



Fig. 36. Schematic diagram of simplified muscular plan for the five seg- 

 ments of the right hind limb, illustrating flexors and extensors. Broken 

 line flexor action, solid line extensor action. 



generaUzed type. The genus Neotoma is, of course, far 

 more specialized; but it is nevertheless sufficiently general- 

 ized for our purpose. 



The legs, then, from a myological viewpoint are each 

 composed of five segments: the first comprises the clavicle 

 and scapula, or the pelvis; the second the humerus or femur; 

 the third the radius and ulna, or tibia and fibula; the fourth 

 the carpus and metacarpus, or tarsus and metatarsus; 

 and the fifth the digits. The metacarpals and metatarsals 

 are usually treated under the same heading as the digits, 

 but from a functional standpoint they constitute a part of 

 the carpus or tarsus, respectively. 



