118 



THE STRUCTURE OF MAN 



Fio. 75. DEEP DORSAL MUSCLES OP THE 

 FOREARM. One-fifth natural size. (After 

 Ranber. ) 



hu, huraerus ; ul., olecranon process of ulna ; 

 rd., radius; pr., processus styloideus 

 ulnse ; inc., os metacarpeum secundum. 

 ., M. anconaeus ; f.p., M. flexor pro- 

 fundus digitorum ; f.c., flexor carpi 

 ulnaris, separated from the fascia of the 

 forearm ; e.b., extensor carpi radialis 

 brevior ; e.L, the tendon of the extensor 

 carpi radialis longior ; e.p'., M. ext. 

 metacarpi pollicis ossis ; e.p". M. ext. 

 primi internodii pollicis ; e.p"'. , M. ext. 

 secundi interuodii pollicis ; e.L, M. ext. 

 indicis ; e.m., insertion of the extensor 

 tendon into the middle finger, and its 

 connection with the second and third 

 dorsal interossei. 



of the lower Vertebrates these 



We saw above that a num- 

 ber of muscles and tendons 

 meet in the thumb; and the 

 same applies, though to a lesser 

 degree, to the great toe. To it 

 offshoots of the extensor hallucis 

 longior and the tibialis anticus 

 or their tendons pass; these, 

 however, do not indicate the 

 commencement of a new de- 

 velopment, but rather a rever- 

 sion to a former condition, in 

 which the great toe was capable 

 of freer movement. 



It would be difficult to 

 decide to what extent the 

 variations which occur on the 

 ulnar border of the forearm 

 and hand, in the region of the 

 extensor and flexor carpi ulnaris 

 and the extensor digiti quinti 

 proprius, may be the beginnings 

 of a progressive development. 

 On the other hand, there can 

 be no doubt that the changes 

 at the fibular border of the 

 foot, which have already been 

 mentioned (ante, p. 112), are 

 degenerations. 



The already described dif- 

 ferentiation of a flexor longus 

 pollicis and a flexor longus 

 hallucis out of the original 

 simple flexor masses, finds a 

 parallel in the Ontogeny and 

 Phylogeuy of the superficial 

 and deep common flexors of the 

 fingers. The two latter are 

 connected by an interchange of 

 fibres which may amount to 

 complete fusion ; and in many 

 muscles may not only be con- 



