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



the palmar aspect of the muscles, two additional bellies are found occupying each of 

 the respective interosseous spaces. These are more or less separable from the bellies seen 

 dorsally, so that four muscular bundles — two dorsal and two palmar — are distinguishable 

 as constituting the third and fourth (second 1 ?) dorsal interosseous muscles. All end 

 inferiorly on a common arched tendon, which by its extremities is attached, as in the 

 Opossum, but on a more anterior plane, reaching indeed as far as the sesamoids." 



It would appear, therefore, that in certain Marsupial animals, a dorsal interosseous 

 muscle, in addition to the usual mode of insertion, may obtain insertion in three different 

 ways all leading the one out of the other. In the manus of the discus, the tendon of 

 insertion of the three outer muscles is fixed by a small transverse band to the head of the 

 metacarpal bone of the digit, which lies adjacent to that into which the muscle is inserted. 

 This acts so as to increase the abducting power of these mucles. In the manus of the 

 Tliylacinc and Cuscus, and in the pes of the Cuscus, the tendon of certain of the 

 interossei bifurcates, and is inserted into the bases of the contiguous digits. An approxi- 

 mating action is thus gained. Lastly, in the foot of the Vulpine Phalanger, and in the 

 manus of certain other Marsupials, a fibrous arch is thrown across between the adjacent 

 bases of two digits, and the muscles are inserted into this. 



In the Vulpine Phalanger, a very distinct opponens minimi digiti similar in all 

 respects to the same muscle in Cuscus is present. 



Nerve Supply of the Intrinsic Muscles in the preceding Animals. 



In Thylacinus, which has no hallux, all the intrinsic muscles are supplied by the 

 external plantar nerve. In the other animals, with the exception of Cuscus, the abductor 

 hallucis and the flexor brevis hallucis are supplied by the internal plantar, whilst the 

 remaining muscles draw their nerve supply from the external plantar nerve. In Cuscus, 

 however, the abductor hallucis appears to be supplied by a special branch to the 

 hallux (fig. 5). 



It is to be noted that in all these specimens, with one exception, the deep division of 

 the external plantar in passing from the outer to the inner margin of the foot lies under 

 cover of the plantar layer of muscles, and superficial to the intermediate and dorsal 

 muscles. The one exception to this arrangement is to be seen, as already pointed out in 

 Dasyurus, in which the nerve in turning inwards runs also under cover of the minute 

 abductor of the minimus, which is inserted into the sesamoid bone. Ruge noticed this 

 also in Dasyurus hallucatus. 1 



My friend, Dr. Young of Manchester, who has had an opportunity of examining the 

 feet of the Koala and the Virginian Opossum, has very kindly furnished me with the notes 



1 Loc. cit., p. 54. 



