58 The Phylogcny of tlie Crural Flexors 



with these muscles; the evidence for this statement will be presented 

 later. In the opossum it is a well-developed muscle forming what has 

 been termed by Coues, 72, the flexor brevis pollicis obliquus. Leche, 98, 

 however, records it as wanting in the marsupials and Cunningham, 81, 

 remarks that the muscle is wanting in Thylacinus and Dasyurus. 

 Dobson, 83, on the other hand, finds in Dasyurus a band passing from the 

 OS calcis to the under surface of the flexor fibularis tendon and identi- 

 fies it, probably correctly, with the flexor accessorius and Young, 82, 

 in his account of the musculature of Phascolarctos, while stating that 

 " there is no flexor accessorius in the foot," goes on to say that a muscu- 

 lar bundle which arises from the os calcis and passes to a fibro-cartilagi- 

 nous backward prolongation of the plantar fascia is regarded by 

 Macalister as similar to it in its nature. McCormick, 87, suggests the 

 identity of one of the heads of his flexor brevis digitorum with the flexor 

 accessorius in Dasyurus viverriniis, but the brevity of his description of 

 this head and the absence of explanations of his figures prevent an 

 opinion as to the correctness of the suggestion. There cannot be the 

 slightest question as to the existence of the muscle in Didelplnjs virgin- 

 lana, and on account of its importance in the fundamental plan of the 

 crural muscles, to be discussed later, it seems quite probable that it may 

 be found in a rudimentary condition in the majority of the marsupials. 



In the opossum it arises from the outer surface of the os calcis as a 

 distinct bundle of fibers which are directed inwards and distall3^ They 

 early pass over into a tendon which crosses the plantar surface of the 

 tendon of the flexor fibularis and unites with its outer border, that 

 portion with which it unites immediately separating to form the tendon 

 for the hallux. This description differs somewhat from that of Coues, 

 72, wdao regards the tendon of the hallux as representing the direct 

 continuation of the accessorius. Sections show very clearly, however, 

 that this is not the true state of affairs and that the arrangement is as 

 described above. In the cat the accessorius is a strong muscle of con- 

 siderable size, arising from the outer surface of the os calcis. Its thin 

 tendon passes obliquely across the tendon of the flexor fibularis and unites 

 with the greater part of its plantar surface, including the united flexor 

 tibialis tendon. In the mouse it is also well-developed, arising from the 

 outer surface of the os calcis and passing obliquely to the tendon of the 

 flexor fibularis, especially to that portion of it which becomes the long 

 flexor tendon of the hallux. 



There still remains for consideration the muscle which has been 

 termed the pronator tihice in the monotremes and marsupials and in 

 the higher mammals the popUteus, assuming for the present that the 

 two muscles are identical. 



