ix.] THE NERVOUS SYSTEM. 377 



and the pituitary body (pt~) below. The wall at the anterior end of the first 

 vesicle (or fore-brain) is the lamina terminalis (/). 



3. This figure shows the cerebrum (cr) budding from the first vesicle, its anterior 

 part (o) being prolonged as the olfactory lobe (the so-called olfactory nerve), 

 the cavity of the cerebrum (or incipient lateral ventricle) communicating with 

 that of the olfactory lobe in front and with that of the first cerebral vesicle (third 

 ventricle) behind. The latter communication takes place through the foramen 

 of Monro. The walls of the three primitive vesicles are becoming of unequal 

 thickness, and the cavity (b) of the middle vesicle (iter a tertio ad quartum 

 ventriculiim) is becoming reduced in relative size. 



4. The cerebrum is here enlarged, and the inequality in thickness of the wall of 

 the primitive vesicles is increased. The thickened upper part of the wall of 

 the cerebrum is the fornix (./). 



5. This figure shows the cerebrum still more enlarged, and with a tri-radiate 

 cavity (/, i, 2, 3). The fornix has now come to look slightly downwards ; 

 dotted lines indicate the downward extension of its anterior part, into the 

 corpora mammillaria. 



6. Here the cerebrum is still more enlarged and backwardly extended. The 

 fornix is shown bordering the descending cornu and extending into the 

 temporal lobe (tl) of the cerebrum, which lobe is destined to descend (when 

 the brain is fully developed) so much more that it comes to advance forwards, 

 as in Fig. 325. The fornix borders the margin of the very thin outer wall of 

 the descending cornu, which when torn forms the fissure of Bichat. The 

 bending back of the cerebrum has now almost enclosed (between the fornix 

 and the velum) the space (x] which in Fig. 4 is widely open, making what is 

 morphologically called the outside of the brain come practically to be in its 

 very centre. 



a, fore-brain ; b, mid-brain ; c, hind-brain ; cb, cerebellum ; cr, cerebrum ; d, 

 cavity of the medulla ; f, fornix ; /, lateral ventricle ; m, medulla oblongata ; 

 ma, corpora mammillaria ; o, olfactory lobe ; /, pons Varolii ; //, pineal gland : 

 //, pituitary body ; <?, corpora quadrigemina ; r, crura cerebri ; f, lamina 

 terminalis; //, temporal lobe of the cerebrum ; x, space enclosed by the 

 extension backwards of the cerebrum ; i, anterior cornu of lateral ventricle ; 

 2, its middle or descending cornu , 3, its posterior cornu. 



The velum 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 continues 

 in that part of the ependyma which passes through the 

 foramen of Munro into the lateral ventricles, but of course 

 the pia mater and arachnoid do not pass through that fora- 

 men, 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 ot 

 the third) merely portions of the ependyma, which happen 

 to be very vascular, and not intrusions from without. 



Each cerebral hemisphere is a bag with walls of very un- 

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

 the descending cornu of the lateral ventricle is reduced to 

 the ependyma (with the pia mater and arachnoid), and 

 readily tears (forming the fissure of Bichat), and this rupture 

 having been mistaken for a natural opening, each lateral 

 ventricle has been supposed to communicate with the exterior 

 close to the crus cerebri. Each corpus striatum is an out- 



