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 evectors arise from the interspinals supporting 
the preceding ray and the hemal process (or fascia connecting the 
hemal 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 X1x.—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’ and My’) inserted into the abaxial! surface of the outer- 
1The terms abaxial and axial refer to the surfaces of the rays looking respectively away 
from or towards the axis of the body. 
