COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 307 



3. The dorsal aspect of the brain. The brain is now to be exposed by care- 

 fully picking away the cartilage in small pieces from its roof. The cranial nerves, 

 white strands passing through the cartilage, must not be injured. One side of 

 the head has thus far been left intact for the study of the cranial nerves. This 

 side is now to be exposed along with the brain as far as necessary. Remove the 

 upper eyelid as directed under the eye but leave all structures intact. In 

 removing the cartilage between the brain and the eye the following nerves 

 will be noted : the superficial ophthalmic nerve running in the wall of the orbit 

 near the dorsal surface; the small trochlear nerve passing through the back wall 

 of the orbit to the superior oblique eye muscle; in the skate the larger oculomotor 

 nerve accompanying the trochlear. Dissect forward to the olfactory sacs, expos- 

 ing them dorsally, leaving the ophthalmic nerve in place. Remove the skin 

 behind the spiracle and note the hyomandibular nerve passing posterior to the 

 spiracle; this nerve is also to be preserved. To expose the posterior part of 

 the brain the internal ears of both sides may be cut through and the mass of 

 muscles posterior to the ear removed as much as necessary. Nerves will be 

 seen passing through the cartilage in the ventral part of the ear, but are not to 

 be dissected out for the present. In short, the dorsal side of the brain is to be 

 fully exposed, leaving all of the more superficial nerves intact. The dorsal 

 aspect of the brain will then be studied first, and the cranial nerves afterward. 



The brain is situated in a cavity in the chondrocranium, which it only 

 partially fills. It is covered by a delicate membrane, the primitive meninx, in 

 which the blood vessels of the brain are situated. The meninx is connected by 

 strands with the membrane lining the cartilaginous walls of the cranial cavity. 

 The space between brain and chondrocranium is filled in life by a fluid. 



The most anterior part of the brain is the large olfactory bulb, a nervous mass 

 situated in contact with the dorsal walls of the olfactory sacs. From the olfactory 

 sac a number of very short fibers, which together constitute the olfactory nerve, 

 pass into the olfactory bulb. The olfactory bulb is spherical in the dogfishes, 

 elongated in the skate. The olfactory bulb is connected with the next part of 

 the brain by a stalk, the olfactory tract, which is short in the smooth dogfish. 

 The olfactory tracts pass to enlarged, rounded lobes, the olfactory lobes, at the 

 anterior (or lateral in the skate) end of the main mass of the brain. Posterior 

 to (dogfish) or medial to (skate) the olfactory lobes is situated another pair of 

 lobes, the cerebral hemispheres. The cerebral hemispheres and the olfactory 

 lobes are separated only by a faint groove. All of the parts thus far mentioned 

 belong to the telencephalon. The olfactory bulbs, tracts, and lobes together con- 

 stitute the rhinencephalon or smell portion of the brain. In fishes the cerebral 

 hemispheres are also largely concerned with the sense of smell. 



Posterior to the cerebral hemispheres is a depressed region, the diencephalon 

 or thalamencephalon, very narrow in the smooth dogfish. The roof of this is 

 thin and discolored, due to the fact that it consists entirely of a plexus of blood 



