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



(2) two divisions of the abductor minimi digiti ; (3) the abductor ossis metatarsi minimi 

 digiti ; (4) the dorsal interossei. 



The abductor hallucis (fig. 5, d l ) is a strong muscle, which arises from the cartilage of 

 the sole, and a sesamoid bone which glides upon the plantar surface of the scaphoid. Its 

 chief insertion is by fleshy fibres into the tibial sesamoid bone of the hallux, but it is also 

 prolonged onwards by a slender tendon to the tibial side of the distal phalanx. By a 

 third slip it gains an insertion into the extensor tendon. 



The three abductors of the little toe (fig. 5, d 6 d 6 ' d e ") arise side by side from the 

 outer and under surface of the tuber of the os calcis. They are inserted into the base of 

 the metatarsal bone, into the outer sesamoid bone, and into the extensor tendon. The 

 abducting apparatus on the outer aspect of the foot of the Cuscus, therefore, is more 

 differentiated than in any of the preceding animals. In the Thylacine, a separate slip 

 for the sesamoid is only an occasional occurrence ; it is always very feeble, and springs 

 from the base of the metatarsal. In Dasyurus it seems to be constant, and it arises 

 further back in the foot. In Cuscus it reaches its full development, and arises from the 

 os calcis with its neighbours. 



The dorsal interossei are placed more in the sole than the corresponding muscles of 

 the hand. They are not bipenniform, and they show a slight tendency to fuse with the 

 flexores breves. 



The first or abductor indicis (fig. 6, d 2 ) is the most powerful of the series. It arises 

 by two heads — («) from the internal cuneiform (b) from the base of the first metatarsal, and 

 it is inserted into the tibial side of the base of the proximal phalanx of the index, and also 

 into the extensor tendon. The index and medius, as we have seen, have no independent 

 power of movement as they are enclosed within the same integumental covering ; they 

 constitute, in fact, from a functional point of view, one digit. In consecpuence of this, 

 the second dorsal interosseous muscle is so completely amalgamated with the inner head 

 of the flexor brevis medii that its presence cannot be detected as an independent muscle. 1 

 The first dorsal interosseous muscle serves as the abductor of both digits. 



The third and fourth dorsal interossei (fig. 6, d i , d v — d 5 , d 5 ') are arranged in a very 

 peculiar and interesting manner. Each consists of two slips. In the case of the third, 

 one of these (cZ 4 ) arises from the base of the third metatarsal, and is inserted into the 

 outer aspect of the base of the proximal phalanx of the medius, whilst the other (d*') 

 takes origin on the dorsum of the foot by two delicate heads which spring from the 

 adjacent bases of the third and fourth metatarsal bones. Near the roots of the toes this 

 muscular slip ends in a tendon which bifurcates, and in this manner is inserted into the 

 adjacent sides of the bases of the proximal phalanges of the medius and annularis. The 

 arrangement of the fourth dorsal interosseous muscle is very similar. One slip (d 5 ) takes 



1 In fig. 6, Plate VI., d 3 represents the deeper part of the inner head of the flexor brevis medii — the portion supposed 

 to represent the second dorsal interosseus — artificially separated and left in situ. 



