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



Intermediate layer (f 2 to/ 5 ). — The muscles composing this layer are four in number, 

 and are placed one upon the plantar surface of each metatarsal bone, but further, each 

 muscle consists of two slips, which may arise separately, or by a common origin from the 

 base of the metatarsal bone with which they are associated. No definite rule seems to 

 apply to their mode of origin. In the feet of the same animal the muscle of any given 

 digit may be se,en arising in the one pes by two heads, whilst in the other it mav 

 take origin by a single head. At the root of the toe the two slips embrace the base of the 

 proximal phalanx, and are inserted one into each sesamoid bone, and sometimes also into 

 the extensor tendon as well. 



These muscles, then, are true flexores breves when the two slips act in unison. When 

 they act independently of each other, however, they exercise an abducting or adducting 

 influence, according to the slip employed. 



The dorsal layer (d 2 to d e ) includes the abductor minimi digiti, the abductor ossis 

 metatarsi minimi digiti, and the four dorsal interossei. 



The abductor minimi digiti (d Q/ ) is a small fleshy slip which arises from the outer 

 aspect of the tuberosity of the os calcis, and soon ends in a long narrow tendon. This is 

 inserted into the extensor tendon of the minimus on the fibular side of the first phalanx. 



In the foot of the large male Thylacine an additional abductor of the little toe was 

 present. This consisted in an exceedingly minute fasciculus (d 6 "), which took origin 

 from the base of the metatarsal, and extended downwards upon the surface of the outer 

 head of the flexor brevis to its insertion into the outer sesamoid bone. 



The abductor ossis metatarsi minimi digiti (d 6 ) is a strongly marked muscle which 

 springs from the outer aspect of the tuberosity of the os calcis in common with the 

 abductor minimi digiti. It presents the usual insertion into the base of the fifth 

 metatarsal bone. 



The splitting up of the abductor of the little toe into three parts points to the usual 

 triple insertion of this muscle (viz., into the metatarsal bone, into the sesamoid bone, 

 and into the dorsal expansion of the extensor tendon), and indicates a tendency for the 

 fibres belonging to each of these insertions to separate and constitute a distinct muscle. 

 The abductor metatarsi is very constant throughout the mammalian series, but it is rare 

 to find the other two parts separate. In the discus, however, it is even more evident 

 than in the Thylacine. 



The dorsal interossei. — The first dorsal interosseus or abductor indicis, owing to the 

 absence of the hallux, is marginal in its position, and therefore more highly developed 

 than the others. It consists of two parts (fig. 2, dr, and d 2 '). Of these one arises from 

 the base of the metatarsal, in common with the inner head of the flexor brevis indicis, 

 ami is inserted into the inner sesamoid bone ; the other wanders backwards for its 

 origin (thus exhibiting the usual tendency of a marginal muscle), and springing from 

 the scaphoid is inserted into the extensor tendon on the inner aspect of the index. 



