142 THE NERVOUS SYSTEM. 



from the anterior end and outer side of the corresponding olfactory 

 lobe, and from the cerebral hemisphere. The optic nerve (To and 

 11} arises, as the optic tract, from the side of the brain below the 

 optic lobe, whence it passes to the chiasma on the under surface of 

 the brain. The oculo-motor (III) takes its origin from the ventral 

 surface close to the median line and between the crura cerebri. 

 The pathetic or trochlear nerve (IF) is attached to the dorsal 

 surface between the optic lobes and the cerebellum. The trige- 

 minal nerve (V) arises from the side and anterior part of the 

 medulla oblongata. The abducens (VI) arises behind the pituitary 

 body close to the median line from the ventral surface of the 

 medulla halfway between the sulcus and the origin of the vagus. 

 The facial and auditory (VII and Till) nerves arise from the 

 medulla oblongata behind the trigeminal nerve, the facial being 

 in front of the auditory nerve. The glossopharyngeal (IX) nerve 

 arises, in common with the pneumogastric nerve (X), behind the 

 auditory nerve. 



2. The several parts of the brain. 



a. The medulla oblongata (Figs. 98, 102, and 103 Mo) a. External 

 form. The medulla is limited behind by the origin of the first pair 

 of spinal nerves, at which point a very faint constriction is sometimes 

 found : it extends forwards as far as the cerebellum. It gradually 

 widens as it passes forwards until just before it reaches its anterior 

 limit, where it presents a shallow but sharp constriction. The 

 dorsal surface is characterised by the presence of a deep, triangular 

 fossa, the fourth ventricle (Fig. 98 S.r), (ventricuhis quartus, 

 Stieda; sinus rJiomboideus s. sinus triangularis, Reissner; fossa 

 'rJiouiboiilalis) ; the sides of the triangle are, however, not quite 

 straight, but are slightly bent outwards just before they converge 

 towards the posteriorly-directed apex ; the base of the triangle 

 is formed by the cerebelhim. By careful examination, the ventricle 

 is seen to be continued for a short distance under the cerebellum, 

 where it opens into the Sylvian aqueduct. In the floor of the fourth 

 ventricle is a well-marked median longitudinal fissure (sulcus cen- 

 tralis), (Fig. 98 S). Into the posterior part of the ventricle opens the 

 central canal of the spinal cord. As the fourth ventricle is formed 

 by the white matter passing to either side, and the simultaneous 

 flattening of the grey matter, the floor of the fourth ventricle is 

 composed of grey matter. 



The fourth ventricle is closed in by a highly vascular mem- 

 brane, the choroid plexus of the fourth ventricle (plexus cJto- 



