NERVOUS SYSTEM 65 



tions of the optic thalami, and forms the posterior 

 wall of the third ventricle, just in front of the cor- 

 pora quadrigemina. (Plate VII., c.) 



1 6. The Aqueduct of Sylvius is the narrow canal 

 passing posteriorly from the third ventricle, be- 

 neath the corpora quadrigemina and above the 

 crura cerebri. (Plate VII., c.) 



1 7. The Fourth Ventricle is the cavity of the brain 

 posterior to the aqueduct of Sylvius, bounded 

 above by the valve of Vieussens and the cerebel- 

 lum, and below and laterally by the medulla ob- 

 longata. It is continuous posteriorly with the 

 central canal Q{ the spinal cord. (Plate VII., c.) 



1 8. The Arbor Vitae is the name given to the 

 branching structure of white matter in the cere- 

 bellum. It is best seen in the section just made. 

 (Plate VII., c.) 



DISTRIBUTION OF THE CRANIAL NERVES. 



1. The Olfactory Nerves (I.) are delicate fibres 

 which arise from the olfactory lobes of the brain, 

 pass through the ethmoid bone, and are distrib- 

 uted to the Schneiderian membrane of the nose. 



2. The Optic Nerve (II.) passes through the op- 

 tic foramen, and enters the posterior surface of the 

 eyeball. 



3. The Oculo-motor Nerve (I II.) passes through the 

 sphenoidal fissure of the skull, and is distributed to 

 certain muscles of the eyeball (superior, inferior, 



