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



fourth dorsal interossei to be inserted into the outer sesamoid bone of the annularis in the 

 one case and the inner sesamoid bone of the index in the other case. I believe that 

 these fibres, thus incorporated with those dorsal interosseous muscles, represent the lost 

 outer head of the flexor brevis indicis, and the lost inner head of the flexor brevis 

 annularis. Then, with regard to the medius, we notice a thin stratum of muscular fibres, 

 covering the plantar aspect of the middle metatarsal bone, and uniting the plantar 

 margins of the second and third dorsal interossei. This bifurcates lower down, and the 

 two fleshy slips thus formed run along the tendons of these dorsal interossei to be 

 inserted into the sesamoids of the medius. I am inclined to believe that these slips, 

 although incorporated with the dorsal interossei, represent the flexor brevis medii. 



In the Lemur the flexor brevis annularis is apparently absent ; the flexor brevis 

 medii has two heads ; but its inner head, almost immediately after its origin, joins the second 

 dorsal interosseous muscle, and cannot be regarded as having an independent insertion; 

 the outer head of the flexor brevis indicis is alone present. Murie and Mivart, in their 

 memoir upon the Anatomy of the Lemuroidea, include these muscles under the head- 

 ing of interossei. They say : — "In Lemur catta there are two to each digit, except the 

 hallux, and counting the flexor brevis minimi digiti as one." They are, however, easily 

 distinguished from the dorsal interossei from the latter being feebly bipenniform and 

 also from the fact that the flexores breves can only be seen from the plantar aspect of the 

 foot. 



Dorset! layer. — This layer is generally represented by its complement of muscles. 

 The abductor minimi digiti is usually more strongly developed than the corresponding 

 muscle of the hallux, and as a rule it springs from the tuber of the os calcis. An abductor 

 ossis metatarsi minimi digiti is also frequently present although not always separable 

 from the preceding muscle. In the Cynocephalus sphinx it is very feebly represented ; in 

 the Ateles (?) it is absent, and in the Lemur it is strongly marked. In the Chimpanzee 

 the abductor ossis metatarsi is absent. 



The abductor hallucis, whilst it takes origin, in many cases, from the os calcis, is often 

 very poorly developed. In Cynocephalus sphinx it is a very small slip which springs 

 from the internal cuneiform bone ; in Ateles (?) it arises from the scaphoid, and in the 

 Lemur from the plantar fascia. 



The dorsal interossei may be one-headed muscles, as in Cynocephalus sphinx, and 

 arise from the bases of the metatarsal bones, or they may be bipenniform, as in the 

 Chimpanzee, Ateles (?), Lemur, &c, and spring from the shafts of the metatarsals between 

 which they he. The first member of the series, owing to the first and second metatarsals 

 being so far apart, is usually two-headed. 



In all the Quadrumana, with two exceptions, the dorsal interossei are inserted so as 

 abduct the toes from a line drawn through the medius, as is the case in the human hand : 

 thus the first is inserted upon the inner side of the base of the first phalanx of the index ; 



