the myology of amiurus catus. 339 



Erectors and Depressors. 



These are on the same plan as the muscles of the posterior rays of 

 the dorsal fin. The erectors arise from the interspinals supporting 

 the preceding ray and the hsemal process (or fascia connecting the 

 haemal arches) of the corresponding vertebra. The depressors arise 

 from the interspinals supporting the rays to which they belong. 

 These muscles are concealed by the lateral trunk muscles, which 

 require to be pulled aside to expose them. 



Innervation. — Supplied by branches from a longitudinal collecting 

 stem which form a plexus into which the ventral branches of spinal 

 nerves xix.-xxx. enter. 



Lateral muscles. 



These are not represented in the dorsal fin. They consist of a 

 number of small muscles, one on each side for each ray, arising from 

 the fascia covering the outer surface of the lateral musculature, and 

 which, passing downwards and towards the median line, are inserted 

 into the lateral surfaces of the bases of the rays ventral to the inser- 

 tion of the erectors and depressors. 



Innervation. — Supplied by a superficial plexus similar to that 

 which innervates the preceding muscles, and coming from the same 

 spinal nerves. 



Action. — By the successive contractions of the muscles of one side 

 from before backwards, a corresponding relaxation of the opposing 

 muscle occurring at the same time, the sinuous motion characteristic 

 of the anal fin is produced. 



XI.— MUSCLES OF THE CAUDAL FIN. 

 The muscles of the caudal fin are formed principally of the pos- 

 terior portions of the lateral muscles of the trunk. From the inter- 

 muscular septa of the last few myomeres a fascia (Fig. 8, f) is given 

 off, which is fastened posteriorly to the bases of the fin-rays. On 

 contraction of the myomeres, this fascia acts on the rays and draws 

 them either to one side or the other, as the case may be. The upper- 

 most and lowermost portions of the myocomma forming the posterior 

 boundary of the last myomere are prolonged into separate tendons 

 (Fig. 8, My 1 and My 2 ) inserted into the abaxial 1 surface of the outer- 



1 The terms abaxial and axial, refer to the surfaces of the rays looking respectively away 

 from or towards the axis of the body. 



