LEPUS CUNICULUS 353 



constitutes a connection between the two hemispheres of the cere- 

 bellum. The posterior or inferior peduncle is constituted by fibres 

 running to the restiform bodies. The cerebellum itself is deeply 

 cut into by the sulci, and the grey matter on its periphery is thrown 

 into a number of subsidiary folds with the result that the white 

 matter presents a very characteristic tree-like appearance in sagittal 

 section, and is for that reason termed the arbor vitae. The side walls 

 of the fourth ventricle are composed to a considerable extent of two 

 large masses, the areee acusticse, to which the auditory nerves are 

 related. Just behind this and dorso-lateral to it are two transverse 

 masses, the corpora restiformia. The floor of the ventricle is mainly 

 composed of a thick mass of grey matter, the medulla oblongata, 

 continuous behind with the spinal cord and containing a number of 

 important ganglionic masses related to the nerves arising from it. 

 The positions of some of these are indicated by slight internal 

 elevations. 



Origins of Spinal Nerves. 



Now that the general anatomy of the brain has been studied 

 it is easy to pass on to consider the points of origin of the cranial 

 nerves. 



The nervus terminalis is a small nerve with which apparently 

 sympathetic nerves are associated. It originates in the ventro- 

 mesial wall of the hemisphere ventral and lateral to the olfactory 

 tract and quite close to the lamina terminalis. 



The first nerve, or olfactorius, is not a single structure, but is 

 represented by numerous small nerve bundles coming in from the 

 olfactory membrane through perforations in the cribriform plate 

 to enter the olfactory bulb. From a small elevation, the accessory 

 olfactory bulb, on the mesial side of the bulb, arise a group of 

 fibres constituting the small naso-vomerme nerve or nervus 

 septalis. 



As we have seen, the second nerve, or opticus, coming from the 

 eye, pierces the ventro-lateral cranial wall at the optic foramen and 

 runs inwards and backwards to join the ventral side of the optic 

 chiasma which it helps to form. 



The third nerve, the oculomotorius, is medium-sized and arises 

 on each side by three or four roots, leaving the crura cerebri near the 

 middle line about half-way between the corpus mammillare and the 

 front border of the pons. 



The fourth nerve, the patheticus or trochlearis, is quite small, and 

 the only one arising from the dorsal surface of the brain. It springs 

 from the dorso-lateral part of the thickened anterior margin of the 

 anterior medullary velum just behind the colliculus inferior. Thence 



2 A 



