CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 267 



more cavities filled with a viscid fluid and gritty matter formed 

 of earthy salts. The third ventricle is bounded iiiferiorly by the 

 corpus albicans and crura cerebri and by the infimdibulum, into 

 which it extends. 



The space between the upper surface of the velum and the under 

 surface of the closely applied lyra is morphologically the outside of 

 the brain, though, in fact, it is in the middle of the complex whole 

 of the adult structure. 



The cavity just described, the third ventricle, continues on back- 

 wards as a very narrow passage (the iter a tertio ad quartum ventri- 

 culiim), bounded below by the crura cerebri and above by a layer 

 of nervous matter continuous with the pineal gland, and exhibiting 

 the cut surface of a small transverse cord (the posterior commissure), 

 and also two prominences in section parts of the corpora quadrigc- 

 mina. A little further back, this passage expands into the fourth 

 -cent rick bounded in the way as already described. 



The cerebellum in section shows (as might be expected from what 

 has been said about its structure) radiating, tree-like ramifications 

 of nervous substance (grey and white) known as the arbor nice. 



Other sections are. necessary to make clear other matters. Thus, 

 the foramen of Monro is the entrance to a cavity which is placed in 

 the cerebral hemisphere of the same side, these two cavities con- 

 stituting the first and second (or two lateral] ventricles. 



The so-called foramen of Monro is, in fact, a Y-shaped passage. 

 It is single below, where it communicates with the third ventricle, 

 but divides above into two branches, one to each lateral ventricle. 



Each lateral ventricle is said to have two cornua. The anterior 

 cornu passes into the frontal lobe and penetrates the olfactory lobe 

 also. The posterior or descending cornu passes into the temporal 

 lobe. Certain sulci on the surface of the cerebrum extend so deeply 

 as to produce eminences on the inner surface of the lateral ventricles. 

 One such structure in the descending cornu has been termed the 

 hippocampus major, and is a very marked elongated and rounded 

 prominence. 



Each central hemisphere is, in fact, a bag, with walls of very 

 unequal thickness. Thus, part of the inner wall running along the 

 descending cornu of the lateral ventricle is reduced to the e-pendyma 

 (with the pia mater and arachnoid), and readily tears (forming what 

 is called the fissure of Bichat), and this rupture having (in man) 

 been mistaken for a natural opening, each lateral ventricle has been 

 supposed to communicate with the exterior close to the crus cerebri. 



The hindmost part of the roof of the fourth ventricle is formed of 

 the ependyma alone, the pia mater and arachnoid being reflected 

 over the postero-inferior surface of the cerebellum. 



Careful inspection shows that the septum lucidum is really (as 

 already mentioned) double, enclosing a very narrow space the fifth 

 ventricle the laminae of the septum lucidum passing downwards 

 from the corpus callosum to the fornix. This fifth ventricle has no 

 connexion with the other ventricles, and it differs from them, not 



