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



Dorsal layer (d} to d h ). — This layer includes — 



1. Abductor hallucis (c^ 1 ). 



2. Abductor minimi digiti (d 5 ). 



3. Abductor ossis metatarsi minimi digiti {a.m). 



4. Dorsal interossei. 



The abductor hallucis (d 1 ) arises from the sesamoid bone at the base of the first 

 metatarsal bone, and from the adjoining ligaments. It is inserted into the base of the 

 proximal phalanx of the hallux, along with the inner head of the flexor brevis. 



The adductor minimi digiti (dH and d 5 f) arises by two distinct heads. Of these one 

 springs from the outer surface of the os calcis and consists of a fusiform muscular bundle, 

 ending in a long tendon, whilst the other, fleshy throughout, takes origin from the lower 

 border of the annular ligament. Both heads are inserted into the fibular side of the base 

 of the first phalanx of the minimus. 



The abductor ossis metatarsi minimi digiti (a.m) has a similar disposition to the 

 same muscle in Koala. 



Here again, therefore, is another instance of the abducting apparatus on the outer 

 margin of the foot showing a triple arrangement. 



The dorsal interossei are four in number. They are single headed muscles, and are 

 inserted so as to act upon the index, medius, and annularis with reference to a line 

 drawn through the middle digit. This toe is therefore provided with two. A slight 

 tendency to fusion between these muscles and the flexores breves is exhibited. 



Neither the hallux nor the minimus is provided with an opponens. 



Macropus robustus, 



The foot of this animal is far from being symmetrical It is tetradactylous, the hallux 

 being completely suppressed. The annularis is enormously developed ; the minimus 

 is well marked but not so long as the fourth toe, whilst tbe index and medius are 

 exceedingly slender, and are enclosed within the same covering of integument. The 

 intrinsic muscles in this foot are deficient both as regards number and development. 



Plantar layer. — This layer has only one representative, viz., the adductor minimi 

 digiti, and even this shows signs of regressive change. The proximal two-thirds of this 

 muscle consist of a thin tendinous band, which arises from the fibrous textures upon the 

 plantar surface of the base of the fourth metatarsal bone. The distal third alone is 

 fleshy, and is inserted upon the base of the first phalanx of the minimus, partly into the 

 sesamoid bone, and partly into the extensor tendon. It is distinctly plantar in position. 



Intermediate layer. — Each digit has in some form or other a flexor brevis, but that 

 for the annularis is the only one which is well developed. The flexor brevis minimi 

 digiti, like the adductor of the same toe, is fibrous in its proximal two-thirds, and composed 

 of a round tendinous cord which takes origin from the plantar surface of the base of the 



