REPORT ON THE MARSUPIALIA. 91 



The fourth dorsal interosseous muscle (d 5 ) is also small. It takes origin in the 

 interval between the rudimentary fifth and the fourth metatarsal bones, and almost 

 immediately ends in a slender tendon which is inserted into the expansion formed by the 

 outer part of the third dorsal interosseous muscle on the dorsum of the second phalangeal 

 joint of the annularis. 



But it may be asked, What has become of the other members of the intrinsic group of 

 muscles 1 I believe that the adductors, and the marginal abductors are suppressed whilst 

 the flexores breves have wandered to the dorsum of the foot and coalesced with the 

 dorsal interossei. I have come to this conclusion, from an examination of the corre- 

 sponding muscles in the manus of the same animal, and as the arrangement found in this 

 is even more remarkable than that in the foot, I am induced to give a description of it 

 (PI. IX. figs. 3 and 4). 



In general features the manus of the three-toed Sloth is very similar to the pes. 

 The metacarpus and the phalanges resemble in almost every respect the corresponding 

 bones of the foot. The intrinsic muscles, however, are not banished from the palm. 

 There is a distinct palmar layer of adductors ; rudiments also of the flexores breves are 

 to be found. 



Palmar layer (fig. 3, p 2 p 2x p*). — This layer is composed of two muscles, viz., (1) 

 the adductor annularis (p 4 ), and (2) the adductor indicis. These muscles are flat ribbon- 

 shaped bands which cross each other in the palm like the limbs of the letter X. 



The adductor annularis is the more superficial of the two. It arises from the bases 

 of the first and second metacarpal bones, and extends obliquely across the palm to 

 the inner margin of the hand, where it is inserted partly into the palmar face of the 

 stunted first phalanx, and partly into the head of the metacarpal bone of the annular 

 dip-it. 



The adductor indicis is composed of an oblique (p. 2 x ) and transverse part {p. 2). 

 The former takes origin from the base of the fourth metacarpal bone, and extending 

 obliquely towards the root of the index is inserted into the radial and palmar aspect of the 

 first phalanx, and also into the head of the metacarpal bone of this digit. The transverse 

 adductor is a small slip which springs from the palmar surface of the middle metacarpal 

 bone, near its head, and stretches transversely from this towards the index where it joins 

 the oblique adductor close to its insertion. 1 



As the digits have no more power of independent movement these adductors can 

 have little or no action beyond bracing together the margins of the manus. That they 

 are undergoing retrograde development is rendered likely from the fact that they are 

 largely composed of tendinous fibres. It is probable, therefore, that these muscles in 



1 Although I have described these palmar muscles as adductors of the index and annularis from their insertions 

 into these digits, I am rather of opinion that they represent the adductors of the absent hallux and minimus. This 

 view would account for their palmar insertions. 



