360 THE CAT. [chap. x. 



choroid 2)kxuses, are but its margins which have become very vascular, 

 and the choroid j^lexuses of the lateral ventricles are also (as has 

 been before pointed out) merely portions of the cpondyma which have 

 become very vascular, and are by no means intrusions from without 

 into the true cerebral cavity. The vascularity is, in fact, continued 

 on in that portion of the ependyma which lines the foramen of Monro 

 and the lateral ventricles. The membranes which invest the brain 

 externally never enter the ventricles at all, but are (as has been 

 already said) reflected back on the under surface of the fornix. 



The THIRD VENTRICLE, wliicli has (as has been said) the velum 

 for its roof, grows out below into the infundibulum. The pineal rjland 

 is formed in connexion with the roof of the third ventricle from behind 

 the velum. The pituitary hody comes from the mouth — see p. 343. 



The sides of the thii-d ventricle, unlike the rest of its walls, 

 become greatly thickened to form the optic thalami, while its 

 anterior wall, the lamina terminal is, becomes thickened in j)laces and 

 so forms a band of transverse fibres, the anterior conunissiire (connect- 

 ing the corpora striata) and also a pair of vertical bundles (the 

 anterior pillars of the fornix) extending into the fornix above, and 

 into the corpora mammillaria below. The two optic thalami also 

 become connected by two sets of transversely extending nervous 

 structures. The first and more anterior is formed of grey matter 

 and is called the soft commissure, the more 2)Osterior, formed of 

 nerve fibres, is called the posterior commissure. The corpora 

 QUADRiGEMiNA ariso as an outgrowth from the roof of the mid- 

 brain, and one containing a cavity continuous with that of the 

 second cerebral vesicle. The corpora are at first of very largo 

 relative size, but they seem, as it were, to lag behind in growth, 

 become solid, and ultimately divided, first into two lateral halves, 

 and ultimately into nates and testes. 



Concomitantly with the development of the corpora quadrigemina, 

 the floor of the mid-brain becomes greatly thickened by the develop- 

 ing crura cerebri, and thus the primitively large cavity of the 

 mid-brain becomes reduced to the very narrow iter a tcrtio ad qiiar- 

 turn fcntriculum. 



Each CEREijRAL HEMisiMiERE is also a bag, the walls of which 

 are very unequally thickened. Part of the inner wall, along the 

 descending cornu, is reduced to mere ependyma, with pia mater and 

 arachnoid, so that it tears very easily, the rupture thus produced 

 having (in man) been called the fissure of Bichat and taken for a 

 natural opening. Each corpus striatum is a thickening of the outer 

 and under wall of each hemisphere abutting against the front and 

 outer part of the optic thalamus behind it, and forms the axis round 

 Avhich the whole hemisphere is developed. 



Slight depressions begin sooner or later to appear on the surface 

 of each hemisphere, the beginnings of the future e/i/ri and sulci. 

 These increase very gradually as the cerebrum grows upwards and 

 backwards. 



The MEM ]}R ANGUS INVESTMENTS OF THE BRAIN aiisC in dificrcut 



