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 tlie septum lucklum, a space called the Ji/fh 

 ventricle being included between its two layers. 



Below the fornix Ave have evidently cut into a cavity extending 

 down into the infundibulum and bounded in front by the lamina 

 terminalis. This cavity is called the tltird ren trick. A small 

 aperture (the foramen of Monro) opens immediately behind the 



Fig. 12?.— The Braix, as sken when a vertical loxgitvdixal sectkjn has eef.k 



HADE THROX^UH ITS SlIDDLE. 



«C. 



av. 

 c. 

 cm 

 to. 



<■'!■ 

 F. 



/. 



h. 



•id. 

 Ol. 



Anterior roininissiirc. 

 Arl)or vita' of certibelhuu. 

 Crucial sulcus. 

 . Corjius alliicans. 

 Corpus callosuni. 

 Corjiora quadrigeiuiua. 

 Frontal lobe of cerebrum. 

 Fornix. 



. Foramen of Monro (between llio fornix aiul 

 the coriuis callosum is tlie lil'tli ventricle, 

 enclosed by the two vertical layers of the 

 se]>tuni Incidum, which pass from the luruix 

 to the corpus callosum). 

 Ilili]Hicani]ial .nyrus. 

 lleduUa oblongata. 

 Olfactory lobe. 



'pv. Pons Varolii. 



p. Pineal t,'land. 



pt. Pituitary body. 



s. Superior external gyras. 



V. Velum interpositum(l)etween itand thefomix 

 is a space enclosed by the folding ovci- of 

 the cerebrum upon the roof of the third 

 ventricle). 



n. Third vi'iitricle. 



4. Antericjr end of fourth ventricle. 



//. Ojitic nerve, which leads back to the fourth 

 ventrich' l)eneatli the cerebellnm. 



The large white spot above F"ig. 3 is the miildle, 

 or S(ift commissiu'e, cut across. The white 

 s]iot beneath and in front of Fig. 'i is tho 

 cut surface of the optic chiasma. 



anterior part of the fn'nix, 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 relum interpositum, which 

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

 margins are very vascular, and bear the name of the choroid p/exu^es. 

 The vascularity is continued on in that part of the epcndyma 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 tho 

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

 third) merely portions of the epcndyma, Avhich happen to be very 

 vascular, and arc not really intru.'^ior.s irom without. This velum 

 interpositum thickens behind and i'orms a small prominence which 

 projects backwards as the pine(d (jhtnd — reminding us of the pituitary 

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



