116 THE VOYAGE OF H.M.S. CHALLENGER. 



the Cercopithecus, the Macacus, the Pithecia, and the Hapale, all possess a two-headed 

 flexor brevis hallucis. Iu many cases, as for example in the Gorilla and Hylobates, the 

 outer head is very weakly developed, and as this author expressively terms it " pressed 

 into the deep " by the adductor obliquus. In the Orang it is fused with the oblique 

 adductor, and the double constitution of the muscle thus formed is manifested by its 

 receiving its nerve-supply partly from the internal plantar nerve, and partly from the 

 deep division of the external plantar nerve (Ruge x ). 



But Ruge is of opinion that the outer head of the flexor brevis hallucis must be 

 regarded as a derivative of the inner head of the same muscle. He says : 2 — " In Ateles 

 the flexor brevis is a single muscle which is sharply separated from the adductor obliquus 

 by the tendon of the long flexor. It is inserted into the inner sesamoid bone. In 

 Cercopithecus the outer flexor is only represented by distal muscular fibres that run 

 from the inner sesamoid bone, under the tendon of the long flexor to the outer sesamoid 

 bone. In Cebus these muscular bundles under the long flexor tendon are separate, and 

 now indeed represent a perfectly distinct muscle. It lies between the inner flexor and 

 the oblique head (of the adductor), covered by the tendon of the long flexor." 



From these facts, however, we might argue in the opposite direction, and suppose a 

 process by which the outer head, already developed, is gradually reduced in size, and 

 finally absorbed by the inner head. We consider this the more likely interpretation, 

 seeing that a two-headed flexor brevis hallucis is by no means an uncommon occurrence 

 in the lower Mammals. 



In the Cynocephcdus sphinx a very beautiful example is afforded of the manner in 

 which the outer head of the flexor brevis hallucis is reduced in size, and pressed deeply 

 into the interval between the adductor obliquus and inner head of the flexor brevis so as 

 to assume, in fact, a position corresponding to that of a plantar interosseous muscle. 

 In the Lemur there is no trace of the outer head. The oblique adductor 3 is largely 

 developed, but as it draws its nerve supply from the deep division of the external plantar 

 alone, it is not likely that it contains in its midst the lost head of the flexor brevis hallucis. 



Flexor brevis minimi digiti. — It is commonly asserted that this muscle in the Ape 

 consists of a single external head. I believe, on the other hand, that it is almost 

 invariably two-headed. 4 The inner head, however, (as is so frequently the case with the 

 outer head of the flexor brevis hallucis) is very often pressed deeply into the sole so that 

 its identity becomes obscured. It is then described under the cognomen of the " third 

 plantar interosseous," muscle. 



1 Loc. cit., p. 651. - Loc. cit., p. 654. 



3 Murie and Mivart describe and figure this muscle under the name of flexor brevis hallucis (loc. cit, p. 86, 

 fig. 30, pi. vi.J.b.h). 



4 In the Chimpanzee which I dissected, the inner head of the flexor brevis minimi digiti (third plantar 

 interosseus) was alone present. The outer head was absent, or rather completely converted into an opponens. In 

 the Chimpanzee which Rolleston examined, this also was the case, but Champneys found an outer head in his specimen. 



