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



the foot have disappeared by suppression, and that the adductors of the hand are follow- 

 ing their example. 



The dorsal layer (fig. 4, d 2 to d 5 ) is represented by four dorsal interossei. These are 

 arranged in a manner almost identical with that of the same muscles of the foot 

 with the exception that the tendons of the common extensor replace those of the 

 extensor brevis in the formation of the two radial expansions on the dorsal aspect 

 of the second phalangeal joints, and that the second dorsal interosseus muscle does not 

 extend upwards upon the carpus, but is confined entirely to the second inter-digital interval. 



In both hand and foot the proper extensor apparatus of the digits is exceedingly 

 weak. In the former there are only two small tendons sent by the extensor communis, 

 to the medius and index, and in the latter two equally small tendons are given by the 

 extensor brevis to the same digits. The dorsal interossei however make up for this 

 deficiency. The annular digit is entirely dependent upon this group of muscles for the 

 production of this movement. 



Intermediate layer (fig. 4 f 2 to f 5 ). — Under cover of the adductors we find five 

 small muscles which without doubt represent the flexores breves. In this case, however, 

 they possess no flexing action upon the digits, but are rather converted into exten- 

 sors by being carried backwards through the interosseous spaces to join the dorsal 

 interossei. 



The flexor brevis pollicis consists of two very minute thread-like fasciculi which 

 spring from the plantar aspect of the base of the rudimentary first metacarpal bone. 

 They sink into the substance of the fibrous cord which prolongs this bone forwards, and in 

 this way they gain an insertion into the radial aspect of the first phalanx of the index. 



A small slip (f 3 r), evidently the radial head of the flexor brevis indicis, takes origin 

 from the outer side of the base of the index metacarpal, and at once turns backwards 

 through the first interosseous space to join the first dorsal interosseous muscle. The ulnar 

 head of the same muscle (f' 2 u) arises from the inner side of the base of the same-meta- 

 carpal, and inclines backwards in the second interosseous space, to join the second dorsal 

 interosseous muscle. This slip is so closely connected with the interosseous muscle that 

 at first sight it appears to be merely a part of it. 



The short flexor of the medius has disappeared altogether. 



The flexor brevis annularis (/ 4 ) arises by two well-marked heads of which one 

 springs from the outer side of the base of the fourth metacarpal bone, whilst the 

 other, considerably longer, takes origin from the unciform bone. These soon unite, and 

 the muscle is then directed through the third interosseous space to join the third dorsal 

 interosseous muscle. 



The flexor brevis minimi digiti (f 5 ) is a small fleshy belly which arises from the base 

 of the rudimentary fifth metacarpal bone, and then passes backwards between it and the 

 fourth metacarpal to effect a junction with the fourth dorsal interosseous muscle. 



