THE MUSCULAR SYSTEM 235 



abductors or adductors of the separate digits (Fig. 62, b, 

 fc).. 



Reviewing the conditions in this, probably the most primi- 

 tive chiropterygium now left to us, several interesting points 

 become manifest. The digits are moved in two ways, either 

 flexed and extended or moved sideways, but while the system 

 which provides for this latter form of motion is extremely well 

 perfected, that for flexion and extension is not. For abduc- 

 tion and adduction there are typically five separate muscles for 

 each digit, that is, two ventral, two dorsal and one intermeta- 

 carpal, while for flexion and extension, aside from the system 

 supplied by an aponeurosis, and evidently a newly introduced 

 feature, there are but three. This extreme perfection of the 

 sideways movement of the digits in the most primitive chirid- 

 ium knozvn, together with the weak and makeshift arrange- 

 ments for bending and straightening the digits, strongly sug- 

 gest the derivation of the chiridial type from one in which the 

 digits (fin-rays?) required to be constantly opened and shut 

 by lateral movements, precisely as in the case of the fins of most 

 fishes. 



During later phylogenetic history there is an evident tend- 

 ency to increase the efficiency of the flexor-extensor system and 

 diminish that of the abductors and adductors, except in the case 

 of the two digits that form the ends of the series (I and V), 

 and the most of these changes have already occurred among 

 the higher urodeles. Thus in Cryptobranchus the dorsalis 

 antebrachii, which in Necturus serves as an abductor-adductor 

 system and terminates at the base of the metacarpals, is con- 

 tinued into four long tendons, which insert into the terminal 

 phalanges, and thus becomes the extensor communis digitorum, 

 although in the hind limb at least, from the sides of the long 

 tendons, small lateral slips extend still to the sides of the 

 metacarpals, the remains of the abductor-adductor system. 

 The short extensors become more complicated than in Nec- 

 turus, but insert along the proximal part of the digits and 

 are no longer continued into long tendons to the ter- 



