266 



THE CAT. 



[CHAP. ix. 



outer wall or bag of the cerebrum enclosing the lateral ventricles. 

 Filling up the interval between the corpus callosum and fornix is a 

 double membrane called the septum lucidum, a space called the fifth 

 ventricle being included between its two layers. 



Below the fornix we have evidently cut into a cavity extending 

 down into the infundibulum and bounded in front by the lamina 

 terminalis. This cavity is called the third ventricle. A small 

 aperture (the foramen of Monro) opens immediately behind the 



Fig. 120. THE BRAIN, AS SEEN WHEN A VERTICAL LONGITUDINAL SECTION HAS BEEN 



MADE THROUGH ITS MIDDLE. 



nc. Anterior commissure. 



uv. Arbor vitte of cerebellum. 



c. Crucial sulcus. 



cm. Corpus albieans. 



cc. Corpus callosum. 



ctj. Corpora quadrigemina. 



F. Frontal lobe of cerebrum. 



/. Fornix. 



/MI. Foramen of Monro (between the fornix and 

 the corpus callosum is the fifth ventricle, 

 enclosed by the two vertical layers of the 

 septum lucidum, which pass from the lornix 

 to the corpus callosum). 



7i. Hippocampal gyms. 



wi. Medulla oblongata. 



ol. Olfactory lobe. 



pv. Pons Varolii. 



p. Pineal gland. 



2>t. Pituitary body. 



s. Superior external gyms. 



v. Velum interpositum (between it and the fornix 

 is a space enclosed by the folding over of 

 the cerebrum upon the roof of the third 

 ventricle). 



3. Third ventricle. 



4. Anterior end of fourth ventricle. 



II. Optic nerve, which leads back to the fourth 

 ventricle beneath the cerebellum. 



The large white spot above Fig. 3 is the middle, 

 or soft commissure, cut across. The white 

 spot beneath and in front of Fig. 3 is the 

 cut surface of the optic chiasma. 



anterior part of the fornix, and a little behind this aperture is the 

 cut edge of a bundle of transverse fibres which form what is called 

 the soft (or middle) commissure. The third ventricle is bounded 

 above by a delicate membrane, the velum interpositwn, which 

 consists only of the ependyma, the pia mater, and the arachnoid. Its 

 margins are very vascular, and bear the name of the choroid plexuses. 

 The vascularity is continued on in that part of the ependyma which 

 passes through the foramen of Monro into the lateral ventricles, 

 but of course the pia mater and arachnoid do not pass through that 

 foramen, as they never get inside the ventricles at all, but are 

 reflected back on the under surface of the fornix. Thus the 

 " choroid plexuses " of the lateral ventricles are (like those of the 

 third) merely portions of the ependyma, which happen to be very 

 vascular, and are not really intrusions from without. This velum 

 interpositum thickens behind and forms a small prominence which 

 projects backwards as the pineal aland reminding us of the pituitary 

 body below. It is reddish and very vascular, and contains two or 



