J. Playfair McMurrich 71 



The opossum has no muscle which corresponds exactly to the popliteus. 

 It has the homologue of the interosseus well developed as the pronator 

 tibife, but that muscle is entirely confined in its origin to the fibula, 

 even its uppermost portion which has been homologized with the popliteus 

 arising from that bone. It is only in the higher forms that a true 

 popliteus is found and certain peculiarities in its structure in the 

 mouse and cat seem to throw some light upon its significance. In both 

 these forms, as has already been noted, two very distinct portions can 

 be discerned in the muscle, a more tibial portion whose fibers have a 

 very oblique direction and a more fibular portion whose fibers are more 

 nearly vertical. A distinct line of demarcation between the two parts 

 occurs in any transverse section of the upper part of the crus. Further- 

 more, the muscle receives two nerves, a fact which in so small a muscle 

 is in itself noteworthy, and is all the more significant in that one of these 

 nerves arises, in the mouse for instance, with that for the soleus from 

 the internal popliteal stem, while the other arises from the branch which 

 I have identified with the crural portion of the ramus profundus of the 

 lower vertebrates. And, finally, the internal popliteal branch is supplied 

 entirely to the more tibial oblique-fibred portion of the muscle, while 

 that from the profundus passes entirely to the more fibular vertical 

 portion. 



The significance of these facts seems to be evident. The popliteus 

 is a compound muscle, consisting of a portion derived from the plantaris 

 superficialis and a portion which represents a part of the pronator 

 tibiae of the marsupials and the interosseus of the lower vertebrates. In 

 other words the constitution of the mammalian popliteus is exactly equiva- 

 lent to that of the pronator radii teres m the arm. 



The idea that the muscle is a composite one furnishes a simple explana- 

 tion of the condition occurring in some carnivores. Gruber, 78, has 

 shown that in the dog, wolf and fox there exists, independently of the 

 popliteus and lying to a certain extent beneath it, a short muscle extend- 

 ing between the upper portions of the fibula and tibia. This is the 

 m. peroneo-tibialis. The same muscle occurs also in Viverra (Dobson, 

 83), and as an anomaly in man (Gruber, 77 and 78). The fact of the 

 occurrence of such a muscle in certain carnivores while lacking in others 

 is certainly reasonably accouted for on the supposition that its absence 

 in the latter is only an apparent one. That is to sa}-, it seems probable 

 til at the peroneo-tibialis of the dog represents the more vertical portion 

 of the popliteus of the cat, the dog's popliteus being, equivalent to the 

 obliquely fibered portion of the cat's muscle. And similarly, the appear- 

 ance of the peroneo-tibialis as an anomaly in man may readily be ex- 

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