THE PERCH 213 



The Arterial System {Continued). We shall now study the ef- 

 ferent branchial arteries and the dorsal aorta and its branches. 

 The afferent arteries have already been seen ; the efferent ar- 

 teries lie immediately posterior to them on the gill arches. Make 

 a cross section of a gill arch and note these two arteries; the 

 afferent is the larger of the two. Remove the ventral wall of the 

 pharynx and the heart. Dissect away the mucous membrane 

 which covers the roof of the mouth and pharynx. Trace the 

 efferent arteries dorsally to the dorsal aorta. Follow the aorta 

 and the arteries which branch from it, as already explained (see 

 page 211). 



Exercise 16. Draw a diagram showing the efferent branchial arteries, 

 the dorsal aorta and its branches, so far as observed. Carefully 

 label all. 



Exercise 17. Draw a diagram of the entire vascular system. 



The Body Muscles. Skin the side of the body. Note the great 

 muscle which forms the entire side and extends from the head 

 to the caudal fin. This complex muscle is made up of a succes- 

 sion of muscle segments, which are separated from one another 

 by connective-tissue septa. Each muscle segment is a plate, be- 

 tween two septa, and consists of parallel fibers which run across 

 from one septum to the other. Note the zigzag shape of the edge 

 of a muscle segment, as it appears on the surface of the body ; 

 note also that each muscle segment may be divided into a dorsal 

 and a ventral half. 



Exercise 18. Make an outline of the fish and draw in it a few of 

 the muscle segments and septa. 



Cut the tail off an inch or two behind the anus and study the 

 cut surface. The muscle segments appear here in groups of con- 

 centric circles. This appearance is due partly to the fact that 

 the inner edge of each muscle segment, which is attached to the 

 spinal column and to its dorsal and ventral processes, is much 

 anterior in position to the outer edge which appears on the sur- 

 face, and also partly to the zigzag shape of the muscle segments. 

 Cut horizontal and dorsoventral sections of the muscles and 

 prove this. 



