COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 325 



lum. In the floor of the medulla the two somatic motor columns are conspicuous, 

 one on either side of the midventral groove. 



Draw the dorsal aspect of the brain. 



4. The cranial nerves. The dissection of the cranial nerves is a matter of 

 some difficulty and the following description is consequently not complete. 



a) The olfactory nerves: These are the two stout nerves extending from the 

 dorsal portions of the olfactory sacs to the anterior end of the olfactory lobes of 

 the brain. 



b) The optic nerves: On cutting through the olfactory nerves and raising the 

 anterior end of the brain, the optic nerves are seen as two stout trunks situated 

 below the cerebral hemispheres and passing out of the orbit. 



c) The trochlear nerve: This small nerve arises on each side of the brain in 

 the dorsolateral angle between the optic lobe and the cerebellum. It passes 

 ventrally and forward, and will be seen by pressing the cerebral hemisphere away 

 from the skull. It lies behind the larger oculomotor nerve. To find the course 

 of the trochlear nerve in the orbit expose the undissected eye as before, clearing 

 away the glands. Cut through the superior oblique muscle at its point of 

 insertion on the eyeball and find below it the trochlear nerve terminating in this 

 muscle. Medial to the trochlear is the ophthalmic branch of the trigeminus. 



d) The oculomotor nerve: This nerve originates from the floor of the midbrain 

 immediately in front of the trochlear and is seen by pressing the cerebral hemi- 

 sphere away from the skull. Loosen the ventral side of the eyeball and raise 

 it. Among the loose tissues between the eyeball and the floor of the orbit, 

 generally adhering to the eyeball, is a nerve, the maxillary branch of the tri- 

 geminus. Free this from the eyeball. Cut away the pyramidal muscle, and 

 raising the eyeball and pressing it as far medially as possible, separate the inferior 

 and external rectus muscles and find between and above them the stout white 

 trunk of the optic nerve. The oculomotor is in contact with the ventral surface 

 of the optic nerve and branches to the same four eye muscles as in the dogfish. 

 These are not readily followed. 



e) The trigeminus nerve: This is a stout trunk whose origin from the anterior 

 end of the medulla will be seen by pressing the cerebellar region of the brain away 

 from the skull. The trunk passes laterally and enters its ganglion, the semilunar 

 ganglion, which lies in a depression in the medial wall of the skull. The tri- 

 geminus has three branches, the ophthalmic, the maxillary, and the mandibular, 

 distributed to the orbit and nose, the upper, and the lower jaw, respectively. 

 Remove the eyeball on the side where it is still present leaving the ophthalmic 

 and maxillary nerves intact. Cut through the roots of the nerves anterior to 

 the trigeminus so as to raise the brain and bend it away from the side being 

 dissected. Follow the ophthalmic nerve forward and note its distribution to the 

 nasal sacs. Follow it posteriorly toward the root of the trigeminus. It enters 

 the skull, runs in company with the trochlear nerve between the dura mater am? 



