THE MUSCULAR SYSTEM 239 



lightly modified form possessed by bipedal man, are quite 

 primitive, and, together with those of the allied insectivores 

 and rodents, retain the typical five digits, a character in which 

 most of the Carnivora and nearly all of the ungulate groups 

 show a much greater specialization. Thus the distance sep- 

 arating primates from the amphibians is not very great, and it 

 is therefore not surprising that in the distal portion an homol- 

 ogy, not merely of the bones, but of the muscles as well, is still 

 quite evident. In the chapter on the endoskeleton the close 

 correspondence was pointed out between the bones of the arm 

 and hand and those of the leg and foot ; here the similar corre- 

 spondence may be considered between the muscles of these 

 parts, the subject being confined in this case, however, to the 

 distal portion, that is, the portion from the elbow or knee on. 



Take in the first place the set of four radial and ulnar 

 muscles, which in their final form in the primates become five, 

 namely, the flexor carpi ulnaris, flexor carpi radialis, extensor 

 carpi ulnaris and the two extensores carpi radiales, longus and 

 brevis. 



Their homologues in the leg have naturally become modified 

 through the difference in function and the formation of a heel, 

 and their determination is perhaps the least clear of any of the 

 distal limb muscles. There are, however, to correspond to the 

 two flexors, the tibialis posterior and the soleus-gastrochnemius 

 complex, of which the first, a tibial flexor, represents the radial 

 one, and the second, primarily a fibular flexor, the correspond- 

 ing muscle on the ulnar side. Upon the extensor aspect, the 

 tibalis anterior may be the serial homologue of both radial 

 extensors, while the ulnar extensor is represented by the two 

 peroncci, longus and brevis. In tabular form the above homol- 

 ogies appear as follows : 



ARM LEG 



Flexor carpi radialis Tibialis posterior 



Flexor carpi ulnaris Soleus-gastrochnemius 



Extensor carpi radialis longus ) 



-. . . .'.-.. . . \ Tibialis anterior 



Extensor carpi radiolis brevis j 



Extensor carpi ulnaris Peronaus \ 



longus 

 brevis 



