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



Macalister 1 describes an opponens hallucis, springing from the internal cuneiform and 

 inserted into the first metatarsal bone, but in six feet examined by Dr. Young no trace 

 of this muscle could be detected. The small tendon of the preceding muscle which takes 

 origin from the inner side of the base of the metatarsal bone of the hallux may exercise 

 a slight opposing action. 



An opponens minimi digiti (figs. 2 and 3, o.m), however, is always present. It is a 

 thin, but well-developed muscle which springs from the outer margin of the plantar 

 cartilage, and is inserted into the fibular margin of the shaft of the fifth metatarsal bone. 

 The superficial fibres are prolonged to the base of the first phalanx of the minimus, 

 and constitute the only representative of the outer head of the flexor brevis of that 

 digit. 



Didelphys virginiana (Virginian Opossum), (PL VII. figs. 4 and 5). 



The foot of this animal is pentadactylous. The four outer toes are all well-developed, 

 and quite separate the one from the other. The hallux is placed at right angles to the 

 long axis of the foot so as to oppose the other digits, and it is destitute of a nail. 2 



Upon the plantar aspect of the internal cuneiform bone, a well-marked sesamoid bone 

 is situated. From this a tongue-like cartilaginous prolougation proceeds which affords 

 attachment to some of the muscles of the hallux. 



In this foot the trilaminar disposition of the intrinsic muscles is very marked. Dr. 

 Young has also shown that in the hand of the same animal the corresponding muscles 

 are arranged upon a similar plan. 3 



Plantar layer (p 1 to p 5 ). — This group comprises adductors of all the digits with 

 the exception of the medius, which in this case constitutes the centre for the movements 

 of adduction and abduction. 



The adductor hallucis (jj 1 ) and the adductor minimi digiti (p 5 ) are large, flat, 

 triangular muscles. They take origin, by their bases, from a mesial fibrous raphe which 

 extends from the base of the middle metatarsal bone to the base of the proximal phalanx 

 of the medius. The adductor minimi digiti also derives fibres of origin from the 

 ligamentous structures over the proximal ends of the fourth and fifth metatarsal bones. 

 They are inserted by their apices — the one into the outer aspect of the base of the first 

 phalanx of the hallux and the other into the inner side of the base of the proximal 

 phalanx of the minimus. 



Together these muscles constitute a continuous muscular sheet of a trapezoidal form, 

 intersected by the fibrous raphe, which extends from the upper to the lower angle. It 

 almost entirely conceals the other muscles of this group. 



1 Muscular Anatomy of the Koala, Annals and Magazine of Natural History, vol. x. (fourth series), 1872, 

 p. 133. 



2 In Waterhouse's work upon Mammalia, a figure of the foot of Didelphys cancrivora is given pi. xii. vol. i. 



3 hoc. cit., p. 



