ANATOMY OF A MONODACTYLOUS FOETUS 407 



to be inserted into the terminal phalanx. In its course it passes 

 under the digital portion of the median nerve which divides on 

 the digit, allowing the tendon to pass out under it in a manner 

 similar to that usually shown by the tendons of the flexor digi- 

 torum sublimis muscle. 



The flexor poUicis longus muscle is apparently entirely absent 

 or much more probably its muscle mass is indistinguishably 

 fused with that of the flexor digitorum profundus, since the 

 primitive condition of the deep flexors is a single muscle mass 

 giving tendons to the thumb and other digits. Man is one of 

 the very few mammals possessing a flexor pollicis longus muscle 

 and McMurrich ('03) has shown that in the other mammals its 

 absence is not due to a lack of the muscle but to the fact that it 

 has not differentiated out from the common deep flexor mass 

 to the digits. It is thus present as the most radial portion of 

 the flexor digitorum profundus in these forms. 



5. Flexor digitorum sublimis (Deep origin) (fig. 8, F.D.S.) 



Origin. From middle third of volar aspect of radius just 

 medial (apparently ventral) to flexor digitorum profundus. 



Joining the proximal part of this muscle is the superficial origin 

 described above. 



Insertion. The common mass so formed passes into a slender 

 tendon inserted at the lower end of the radius and beginning 

 of the carpus. 



The reason of the failure of the tendon of this muscle to 

 reach the digit I think must be sought in the failure of the palmar 

 aponeurosis to which it is attached, to differentiate into a tendon. 

 McMurrich ('03) has shown that primitively the sublimis muscle 

 ends at the wrist inserting into the palmar aponeurosis. Muscles 

 developed in this aponeurosis later fuse end to end with the 

 flexor sublimis thus producing its tendons in the mammalia. 

 The palmar structures included in the sublimis have evidently 

 failed to form here, leaving the sublimis to end at the wrist. 



