392 



ELECTRO-PHYSIOLOGY 



CHAP. 



systems of transversely - divided fin muscles (external and 

 internal, Fig. 255 [mp]), which are especially numerous and 

 well developed in Gymnotus, and increase posteriorly in number 

 and extension. The small organ increases similarly in height, 

 while the number of prisms in the large organ, on the other 

 hand, decreases towards the caudal end, " since they become 

 rolled up underneath, and leave the lateral surface of the 

 body." In the region where the small organs are fully developed, 

 the upper bundles of the deep fin muscles are wanting, so that 

 the genetic relation between the two is unmistakable. So, 



a 



Fir;. 250. Homology between the electrical organs (0) of Torpedo, and the muscle of the 



common Ray (KM). (Pritsch.) 



too, in the Torpedinidce, where the powerful jaw muscles of 

 the common ray (Fig. 256, I [-Of]), which are balled into a 

 clump, fail almost entirely, and are reduced to an insignificant 

 vestige. This chiefly involves the external or ventrally- 

 developed muscles of the gill chamber, representing the system 

 of the adductores arcnum and the so-called constrictor corn- 

 munis supcrftcialis (Fritsch), which here correspond functionally 

 with the masseter, pterygoid, and temporal muscles. The con- 

 strictor seems more especially to have yielded the elements for 

 the posterior part of the organ, while the broad, anterior 

 portion seems to derive more particularly from the modification 



