63 The Phvlogeny of the Crui'al Flexors 



inter the internal popliteal gives branches to the poplitcus and to the 

 plantaris, and shortly thereafter divides into two main trunks each of 

 which is composed of subordinate bundles. These two main trunks lie 

 one behind the other (Fig. 9), and the posterior larger one (pt) descends 

 the leg without taking any part in the innervation of its muscles and 

 below the ankle divides into the external and internal plantar nerves. 



The other trunk is clearly composed of two portions. From one of 

 these (rm) branches are distributed to the soleus and to the flexor 

 fibularis, while the other (rp) early divides into four branches, one of 

 which is distributed to the popliteus, another to the flexor tibialis, 

 a third to the tibialis posticus, while the fourth, which is very small, 

 passes downward in the aponeurosis between the tibialis posticus and 

 the flexor fibularis, gradually becoming smaller. I was not able to trace 

 this last nerve to its termination, but in all its relations it corresponds 

 to the branch to the periosteum in the mouse. 



It may be recalled that in the lower vertebrates the nerves of the 

 flexor surface of the crus were divisible into superficial and deep branches, 

 and that of the former there were two main trunks, one of which, the 

 ramus superficialis medialis, w^as entirely devoted, so far as its muscular 

 branches were concerned, to the supply of the plantaris superficialis and 

 the plantares profundi III and II. The other superficial trunk, the 

 ramus superficialis fibularis, on the contrary', passed downward, supply- 

 ing only the fibulo-tarsalis in the amphibia, and became the external 

 plantar nerve. The deep branch, the ramus profundus, was distributed 

 to the plantaris profundi I and the interosseus, and then was continued 

 into the foot to form the internal plantar nerve. 



Comparing with this the arrangement described above for the opos- 

 sum, considerable similarity will be noticed. Thus descending the 

 entire length of the crus there are two nerves, the external and internal 

 plantar, the former of which has practically identical relations with the 

 ramus superficialis fibularis of the lacertilia. In addition there is given 

 off from the internal popliteal at or slightly above its division into the 

 two plantar nerves a smaller stem which supplies the deep muscles of the 

 crus and is continued down to the ankle joint as an exceedingly fine 

 nerve, which is not, however, continued into the foot. In its topo- 

 graphical relations and in its crural muscular distribution this nerve 

 seems to be the homologue of the reptilian ramus profundus, from 

 v/hich, however, it differs in being limited in its distribution to the crus. 



In my study of the nerves of the antibrachium (McMurrich, 03) it was 

 shown that the ramus profundus of the amphibia and reptilia extended 

 into the manus, supplying in general the radial part of its palmar 



