478 THE DOGFISH CHAP. 



the precaval sinus of the same side ; by means of a 

 seeker find the apertures of the following veins or sinuses 

 (compare Fig. 122) a, the cardinal and jugular ; b, the 

 inferior jugular ; and c, the lateral vein. 



III. Insert the scissors into the external gill-clefts of one 

 side, one by one, and extend the clefts by cutting dorsally 

 and ventrally, so as to expose the gill-pouches, communi- 

 cating with the pharynx by the internal gill-clefts. Make 

 out 



1. The branchial filaments, and observe that there are 

 four complete gills on the first four branchial arches, and a 

 half-gill or hemibranch on the posterior face of the hyoid 

 arch. Note also the pseudobranch on the anterior side of 

 the spiracle. 



2. The structure of the gills. Remove two entire gills ; 

 dissect one, and cut the other across transversely (compare 

 Fig. 1 1 8), noting the relations of the septum., cartilaginous 

 branchial arch and rays, branchial filaments, and single 

 afferent and paired efferent branchial artery. Sketch. 



IV. Cut through the floor of the pharynx and mouth 

 close to the middle line, just on one side of the ventral aorta, 

 and extend the cut through the lower jaw. On one side, 

 turn the floor outwards, and fix it back in this position so 

 as to expose the roof of the mouth and pharynx and the 

 internal gill-clefts ; dissect away the mucous membrane 

 lining the roof, and trace out on one side (Fig. 121) 



1. The epibranchial and efferent branchial arteries, and the 

 dorsal aorta. 



2. The carotid, subclavian, coronary arteries, &c. 

 F. Nervous System and Sense-organs. 



I. Remove with the knife the rest of the skull-roof and 

 a few of the anterior neural arches so as to expose the entire 

 brain and the anterior part of the spinal cord. In doing so, 

 be careful not to injure the contents of the orbit, the nerves 

 referred to on p. 474, II., or the auditory capsule of one 

 side. After noting the " pia mater," make out 



1. The subdivisions of the brain (olfactory lobes, prosence- 

 phalon, diencephalon, optic lobes, cerebellum, and medulla 

 oblongata] . Sketch. 



2 . The origins of the cerebral nerves from the brain and the 

 points at which they penetrate the walls of the skull 

 (pp. 461-464). 



3. The spinal cord, and the alternating dorsal and ventral 

 roots of the spinal nerves. Then cut through the spinal 



