478 THE DOGFISH CHAP. 



following veins or sinuses (compare Fig. 122) a, tint jugular ; b, the 

 inferior jugular ; c, the cardinal ; and d, the lateral vein. 



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

 by one, and extend them by cutting dorsally and ventrally, so as to 

 expose the gill-pouches, communicating with the pharynx by \.\\Q 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. 118), 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 pin 

 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 con- 

 tents of the orbit, the nerves referred to on p. 474 II., or the auditory 

 capsule of one side. After noting the " pta mater" make out 



1. The subdivisions of the brain (olfactory lobes, prosencephalon, 

 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 cord just behind the 

 medulla oblongata, and through the origins of the cerebral nerves. 

 Remove the brain and place it in formaline or spirit. 



II. Carefully dissect away the skin covering the head and pharyn- 



