COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 331 



part. The second or nuchal flexure takes place in the medulla, bending the 

 medulla at an angle to the spinal cord. The pontal flexure in the region ventral 

 to the cerebellum bends the brain in the opposite direction from the other two 

 flexures, with the result that this region of the brain is depressed. 



The anterior end of the brain consists of the two very small olfactory lobes. 

 Posterior to them are the large cerebral hemispheres separated by a deep sagittal 

 fissure. These are so enlarged posteriorly as to completely conceal the dien- 

 cephalon from dorsal view, with the exception of the delicate pineal body which 

 is seen in the posterior end of the sagittal fissure. The large optic lobes of the 

 midbrain are ventral to the posterior ends of the cerebral hemispheres. Posterior 

 to the hemispheres is the curved cerebellum, marked by transverse grooves. Pos- 

 terior and ventral to this is the medulla oblongata, its anterior end depressed 

 beneath the cerebellum. The roof of the medulla is composed as usual of a 

 chorioid plexus. 



Draw the brain from the side. 



4. The cranial nerves. These are somewhat difficult to follow in detail. 

 Work on the side left intact. There are twelve pairs of cranial nerves. 



a) The olfactory nerves: These are two stout and elongated nerves passing 

 from the nasal sacs along the dorsal margin of the interorbital septum to the 

 olfactory lobes. 



b) The optic nerves: On the side where the wall of the orbit was removed note 

 the stout white optic tract in front of the optic lobe. Follow this toward the 

 orbit and find the optic nerve connected with its anterior end. 



c) The trochlear nerve: The cranial origin of this nerve is difficult to see at 

 the present stage of the dissection. It arises in the deep groove between the 

 optic lobe and the cerebellum, and passes ventrally between the optic lobe and 

 medulla. It runs forward in the floor of the cranial cavity to the orbit. To 

 find it in the orbit, expose the intact eye as before. Cut through the superior 

 oblique at its insertion on the eyeball and lifting it note the trochlear nerve pass- 

 ing to it and spreading out on its ventral surface. The ophthalmic branch of 

 the trigeminus runs close to the trochlear nerve. 



d) The oculomotor nerve: The cranial origin of this nerve will be seen later. 

 It branches to the inferior oblique, superior, inferior, and internal rectus muscles 

 Remove the eye which is still in place, cutting the eye muscles as near the eye- 

 ball as possible and preserving the ophthalmic nerve intact. Look for the 

 branches of the oculomotor among the eye muscles in question. The branch 

 to the inferior oblique in the floor of the orbit is the most conspicuous of them. 



e) The abducens nerve: This will be found on examining the posterior surface 

 of the external rectus muscle. This nerve also supplies the pyramid and quadrate 

 muscles. 



f) The trigeminus nerve: This has three branches, the ophthalmic, the max- 

 illary, and the mandibular. The ophthalmic has already been noted in the dorsal 



