REPORT ON THE MARSUPIALIA. 71 



The dorsal layer. — This layer consists of four dorsal interosseous muscles and an 

 abductor ossis metatarsi minimi disjiti. 



The abductor ossis metatarsi minimi digiti is a short powerful muscle which takes 

 origin from the outer side of the tuberosity of the os calcis, and is inserted into the fibular 

 aspect of the base of the metatarsal bone of the minimus. This is the only abductor of 

 the minimus, but the outermost belly of the flexor brevis digitorum, from its being 

 inserted upon the outer side of the first phalanx, must in addition to its flexing power 

 possess a very considerable abducting influence, and in this way help to spread out the 

 integumental web which connects the digits. 



The dorsal interossei (PI. IX. fig. 5) have a very complex arrangement, and in order to 

 obtain a proper understanding of them they must be dissected from the dorsal aspect of 

 the foot before the flexores breves are interfered with. The fourth dorsal interosseous (cZ 5 ) 

 muscle is single headed and inseparably united with the outer head of the flexor brevis 

 annularis. It arises from the dorsal surface and outer side of the fourth metatarsal bone, 

 and is inserted into the outer side of the extensor tendon of the annularis. 



The third dorsal interosseus (rf 4 ) at first sight appears to consist of a single large 

 head which springs from the dorsal aspect of the fourth metatarsal, but on separating 

 the bones between which it is placed a second and much smaller head is discovered lying 

 along its inner side, and taking origin from the outer aspect of the base of the third 

 metatarsal. These slips, but more especially the larger one, are partially fused with the 

 flexores breves of the annularis and medius respectively. Both heads unite near the 

 roots of the toes, and end in a fibrous arch which extends between the contiguous 

 sesamoid bones of the medius and annularis. 



The second dorsal interosseus (d 3 ) is also two-headed, and partially fused with the 

 corresponding slips of the flexores breves of the medius and index. The larger and 

 outer slip springs from the dorsal surface of the third metatarsal near its base, whilst the 

 smaller slip, not so dorsal in position, takes origin from the contiguous bases of the third 

 and second metatarsals. The majority of the fibres are inserted into a fibrous arch 

 thrown across between the adjacent sesamoid bones of the medius and index ; the other 

 fibres end in a tendon which passes forward upon the dorsal surface of the arch (with 

 which it is closely united) and finally splits into two delicate slips. These diverge from 

 each other, and are inserted into the extensor tendons on the dorsal aspect of the medius 

 and index. The second and third dorsal interossei, therefore, act as approximators of the 

 index, medius, and annularis. Further, it would appear as if this were the first stage of 

 the process, which was seen completed in the Cuscus, where certain of the dorsal interossei 

 are differentiated into approximators and abductors. Here each of the muscles in 

 question consists of two parts, but these have not attained separate insertions. The first 

 dorsal interosseus (d 2 ) is a small slip which arises from the dorsum and inner surface of 

 the second metatarsal. In most cases it is inseparably fused with the inner head of the 



