56 The Phylogeny of the Crural Flexors 



what is usually described as the tendon of the flexor fibularis serves to 

 confirm the homology of that tendon with part of the plantar aponeurosis 

 of lower forms. Eventually all the fibers of the flexor fibularis insert 

 into the tendon, the last of them disappearing some distance above the 

 ankle joint. 



The tendon is continued onward into the foot, lying in the median 

 line between the os calcis and the inner malleolus, and at about the 

 level of the distal row of tarsal bones the tendon of the flexor accessorius 

 passes across it to be attached to its tibial border. This portion of the 

 tendon then separates to pass on to an insertion into the base of the termi- 

 nal phalanx of the first digit, and later the remainder of the tendon 

 divides into four nearly equal tendons, which pass to the remaining 

 digits. The relations of the lumbricales to the tendons will be con- 

 liidered on another occasion. 



The flexor digitoruni tibialis is, in contrast to the flexor fibularis, 

 a. rather slender muscle. It has usually been described as arising from 

 the upper part of the tibia, but in my preparations I have not been 

 able to trace it to that bone. I find it (Fig. 7, FT) taking its origin 

 from the strong aponeurosis which covers the posterior surface of the 

 strong pronator tibi^ (PT), and although it thus comes very close to 

 the upper part of the tibia, no definite connection with that bone could 

 be made out. The difference may be due to the fact that the individuals 

 1 studied were advanced fetuses, and that with advancing age the 

 insertion reaches the bone, a process which, if it really occurs, is interest- 

 ing as denoting a migration of the muscle tibia-wards. Its belly forms 

 an irregularly quadrilateral mass lying between the pronator tibiae 

 internally and the flexor fibularis externally, and resting upon tlie 

 tibialis posticus. At about the middle of the crus its tendon begins to 

 appear upon its outer surface and into it the muscle fibers gradually 

 insert, until in the lower part of the crus only the tendon remains, rest- 

 ing directly upon that of the tibialis posticus, by which it is separated 

 from the posterior surface of the tibia. At the ankle joint it rests upon 

 the internal malleolus and as it passes onward into the foot it separates 

 from the tibialis posticus tendon and approaches the tendon of the flexor 

 fibularis. At the level of the junction of the proximal and distal rows of 

 the tarsal bones it gives origin to muscle fibers which represent a por- 

 tion of the flexor brevis digitoruni and pass downward and inward to 

 join the rest of that muscle which arises from the tendon of the flexor 

 fibularis. 



A little farther on the flexor tibialis tendon becomes connected by 

 fibrous tissue of varying density with the inner border of the flexor 



