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



latter do not, strictly speaking, lie upon a plane posterior to the palmar and intermediate 

 muscles. They are palmar in their orgin, but they cross the margins of the hand 

 obliquely, and they are clearly dorsal in their insertion. Their palmar orgin is for the 

 purpose of giving them a more powerful action upon their respective fingers. But it is 

 their abducting action more than the position which they occupy in the hand which 

 indicates their association with the dorsal interossei. The intermediate group of muscles 

 is a series of paired muscles, which includes the flexor brevis pollicis and the flexor brevis 

 minimi digiti. They are placed upon the palmar aspect of each metacarpal bone, and 

 they act chiefly as flexors of the fingers at the metacarpo-phalangeal joints. 



The manus of each of the three animals presents distinctive peculiarities in the arrange- 

 ment of these muscles, and it is therefore necessary to study each in turn, As the 

 Phascogale exhibits the simplest arrangement, we will begin with it. 



Phascogale. 



The hand of this animal is very minute. It measures only one inch in length and a 

 quarter of an inch in breadth, and consequently the intrinsic muscles are extremely 

 delicate. 



Dorsal group. — The abductor pollicis is a very distinct muscle, which stretches over 

 the radial margin of the palm. It arises from the annular ligament, and from the 

 scaphoid and trapezium, and is inserted into the radial aspect of the base of the first 

 phalanx of the thumb, on the same plane as the insertions of the dorsal interossei. 



The abductor minimi digiti is about the same size as the preceding muscle. It arises 

 from the unciform bone and the annular ligament, and is inserted into the ulnar side of 

 the first phalanx of the little finger, on the same plane as the dorsal interossei. 



The dorsal interossei are four in number, and occupy the intermetacarpal spaces. 

 They have the same connections as the corresponding muscles in man. The dorsal group 

 of muscles, therefore, abduct the fingers from a line drawn through the middle finger. 



The palmar group — This group is composed entirely of adductors. They are four in 

 number, and are arranged so that they antagonise the abductors. They all arise close to 

 each other, from the palmar aspect of the carpus, and from this point they radiate to 

 their insertions. The first is inserted into the ulnar side of the first phalanx of the 

 thumb, and is the representative of the adductor pollicis in man ; the second goes to the 

 ulnar side of the index ; the third goes to the radial side of the ring finger, and the fourth 

 is inserted into the radial side of the little finger. 



In this way, then, the abductor minimi digiti, the abductor pollicis, and the first and 

 fourth dorsal interossei, are antagonised by four special adductors, whilst the second and 

 third dorsal interossei act alternately as adductors and abductors of the middle finger, 

 as in the human hand. 



