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



tarsal of the hallux ; this has not been described by other authors, nor indeed is there any 

 trustworthy evidence of the existence of such a muscle in any of the Mammalia." 



Macalister (Proceedings of the Royal Irish Academy, vol. i., 2nd series, page 50G) 

 describes an opponens hallucis in the Gorilla. Bischoff, however, was unable to find such a 

 muscle in the specimen which he dissected (Beitriige zur Anatomie des Gorilla, p. 31, 

 loc. cit.). 



An opponens minimi digiti is very common, and often very strongly marked. It may 

 be regarded as belonging in some instances to the adducting or plantar layer, and in 

 others to the intermediate or flexor layer. Thus Ruge has conclusively shown that in 

 the human foot (when present) it is a special development of the flexor brevis minimi 

 digiti. He says ■} " in the earliest periods in the development of the human foot every 

 trace of an opponens of the little toe is awanting. First by the aberration of fibres of 

 insertion of the flexor brevis on to the head, and outer surface of the fifth metatarsal 

 the beginning of an opponens becomes gradually noticeable ; in older subjects this 

 becomes further developed, and more turned in direction until, finally, the whole outer 

 surface of the fifth metatarsal serves for the insertion of the muscle. In preparations 

 from later embryonic periods, the flexor and opponens fibres appear separate in their 

 distal portions, so that the opponens reaches a greater and greater degree of independence 

 the further the process of separation goes on. At the same time the opponens, now 

 distinct, comes to lie under cover of the flexor. This condition remains throughout life, 

 but still the muscle in the adult shows a decrease in its circumference, and may even 

 have disappeared altogether. These conditions occur so very frequently that I may say 

 they are the rule." 



In the Lemur and in the phalanging marsupials the opponens minimi digiti has 

 apparently the same origin as in man. 



On the other hand many of the digitigrade Carnivora afford a beautiful example of 

 its association with the plantar layer. We have already seen it in the Dog, Cat, Puma, 

 Leopard, Lion, Otter, and Pole-Cat, arising in common with the adductor minimi digiti. 



III. The last point that we have to consider is one of great interest, viz., that the 

 relation of the intrinsic muscles of the foot to the metatarsus in many animals corresponds 

 to transitory conditions in the foot of the human embryo. 



Ruge in his memoir upon the Development of the Muscles of the Human Foot shows 

 that the interossei muscles in the foot of the early embryo are plantar in position, and 

 that the upward growth of the dorsal interossei and the formation of the interosseous 

 spaces take place as a subsequent and gradual step. In three of the diagrams which 

 illustrate the text he gives representations of sections through the metatarsus at three 



1 The Development of the Muscles of the Human Foot (loc. cit.), p. 131. 



