COMPARATIVE ANATOMY 75 



The part of the brain remaining posterior to the cerebellum 

 is the medulla oblongata or myelencephalon. The medulla is 

 continuous posteriorly with the spinal cord. If the thin covering 

 of the medulla, the choroid plexus, is removed, a large triangular 

 depression is visible. This is the fourth ventricle. The lateral 

 extensions of the medulla at its anterior end are the restiform 

 bodies. 



Cranial Nerves of the Shark 

 The ten pairs of cranial nerves of the dogfish need be dis- 

 sected on one side only. They are numbered, beginning 

 anteriorly. 



I. Olfactory. — The olfactory nerve consists of a bundle 

 of short fibers extending from the olfactory bulb to the nasal 

 capsule.' It is a sensory nerve. 



II. Optic. — The optic nerve arises from nerve endings in 

 the retina of the eye and extends to the ventral side of the 

 diencephalon, where it crosses the nerve from the other side. 

 This relation will be better understood after the ventral surface 

 of the brain is studied. It is a sensory nerve. 



III. Oculomotor. — This nerve arises from the mesen- 

 cephalon on the ventral side. It may be seen as a slender thread 

 entering the orbit back of the optic nerve. It is a motor nerve 

 and supplies four of the eye muscles, namely, the internal, 

 inferior, and superior rectus, and the inferior oblique. 



IV. Trochlear. — This begins on the dorsal side of the 

 brain behind the optic lobes, and beneath the anterior part of 

 the cerebellum. It is very slender. It passes through the wall 

 of the orbit dorsal to the optic nerve. It is motor in function 

 and supplies the superior oblique muscle of the eye. 



VI. Abducens. — The sixth nerve is best studied before the 

 fifth. This is a slender nerve arising from near the mid-ventral 

 line of the medulla. It is a motor nerve and supplies the 

 external rectus muscle of the eye. 



V. Trigeminal. — The fifth, seventh, and eighth nerves 

 arise very close together from the side of the medulla. The 

 separate roots of these three nerves are difficult to distinguish. 

 The trigeminal enters the orbit and immediately divides into 

 several branches. It is both motor and sensory. 



