54 The Phylogeny of the Crural Flexors 



plantaris and lateral gastrocnemius, eventually becoming tendinous and 

 uniting Avith the tendon of the latter muscle. In the opossum, however, 

 it does not appear to exist as a distinct muscle and the conditions in 

 this form probably serve to explain its apparent absence in others. 



In the description of the gastrocnemius lateralis of the opossum it 

 was noted that it possessed an origin from the head of the fibula. This 

 head seems to be unrepresented in the gastrocnemius of the cat and 

 mouse, and its relations to the rest of the muscle in the opossum present 

 some interesting peculiarities. When it is first seen in tracing a series 

 of sections downward it consists of a thin band of fibers ( Fig. 8, s) which 

 arise from a tendon extending downward from the head of the fibula, 

 a portion of the flexor longus digitorum lying between the tendon and 

 the bone. This muscle band is separated at this level from the deeper 

 surfaces of the gastrocnemius lateralis and the plantaris by the tendon 

 which extends downward from the lateral fabella and gives origin to the 

 plantaris. As this tendon gradually fades out below a distinction be- 

 tween the muscle band under consideration and the gastrocnemius 

 lateralis becomes less and less, until, finall}^, there is complete union of 

 the two. 



This fibular head of the opossum seems to represent the soleus of the 

 higher mammalia, and the supposition of Cunningham, 8i, and others 

 that the so-called gastrocnemius lateralis of the marsupials includes also 

 the soleus is correct, and the same is probably true of the dog and the 

 other higher mammalia in which the soleus is stated to be lacking. As 

 regards the monotremes it is to be noted that the lateral superficial 

 crural flexor has been termed the soleus, and the gastrocnemius is re- 

 garded as lacking (Westling in Leche, 98), this nomenclature being 

 adopted no doubt in view of the fact that the muscle arises from the 

 peculiar process developed upon the upper end of the fibula in these 

 forms and has no connection with the femur. If the fibular process 

 represents a true outgrowth of that bone such a nomenclature would be 

 justified, but it seems really to be an epiphysial structure and in all prob- 

 ability represents the lateral fabella of other forms. On this view the 

 distinction which is made between the marsupial and monotreme muscle 

 practically vanishes and it seems necessary to regard it as representing 

 in both groups the gastrocnemius and soleus of higher forms. 



The long flexors of the digits in the mammalia have been thoroughly 

 discussed by F. E. Schulze, 66, and by Dobson, 83, and the former has 

 pointed out that the arrangement occurring in man is quite different 

 from that characteristic of the majority of mammals and as a conse- 

 quence the nomenclature employed in human anatomy cannot be con- 



