402 THE DOGFISH chap. 



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. 112) — 



1. The efibranchial and efferent branchial arteries, and the dorsal 

 aoi'ta. 



2. The carotid and subclavian arteries. Sec. 



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. 458, or the auditory 

 capsule of one side. After noting the dura mater and pia mater, make 

 out — 



I. 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. 445-447). 



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- 

 geal region on the undissected side, expose the orbit, and remove 

 the delicate connective tissue surrounding its contents. Pin down 

 firmly, and dissect out the following from the side (compare Fig. 116 

 and pp. 445-449). 



1. The ophthalmic division of ihe facial nerve, and immediately 

 below it that of the trigemina : trace them backwards to their 

 foramina in the skull-wall and forwards through a canal between the 

 olfactory capsule and the cranium, to their distribution. 



2. The large mass of sensory (ampullary) canals on the dorsal side of 

 the snout. 



