170 THE HUMAN BODY. 



the pushed-in epithelium. These plexuses occupy a consid- 

 erable part of the third and lateral ventricles: and a pair of 

 similar vascular tufts drive in before them part of the thin 

 roof of the fourth ventricle and encroach on its cavity. 



Note. A frequent cause of apoplexy is a hemorrhage 

 into one of the lateral ventricles; the outpoured blood accu- 

 mulating and pressing upon the cerebral hemispheres, their 

 functions are suppressed and unconsciousness produced. 

 When a person is found in an apoplectic fit therefore the 

 best thing to do is to leave him perfectly quiet until medical 

 aid is obtained: for any movement may start afresh a bleed- 

 ing into the ventricle which had been stopped by clots 

 formed in the mouths of the torn blood-vessels. 



Sections of the Brain. Having got a general idea of the 

 parts composing the brain, the best way to continue the study 

 of its anatomy is to examine sections taken in various direc- 

 tions. Two such are given in Figs. 75 and 7G. Fig. 75 rep- 

 resents the right half of a vertical section of the brain, taken 

 from before back in the middle line and viewed from the 

 inner side. Above, the knife has passed between the two 

 cerebral hemispheres, in the longitudinal fissure, without cut- 

 ting either, and the convoluted inner surface of the right one 

 is seen. The sickle-shaped mass lower down, Cc to Cc, rep- 

 resents the cut surface of a connecting band of white nervous 

 tissue called the corpus callosum^ which runs across the mid- 

 dle line from one cerebral hemisphere to the other and puts 

 them in communication. Beneath the corpus callosurn 

 the knife has opened, a cavity, the fifth ventricle, 5, 

 bounded on each side by a very thin wall, which forms part 

 of the inner wall of the corresponding lateral ventricle; the 

 median partition formed by these two walls and containing 

 the\slit-like fifth ventricle is the septum lucidum. The fifth 

 is quite different in origin from the remaining cerebral ven- 

 tricles, not being a continuation of the canalis centralis of 

 the spinal cord. 



Forming the floor of the fifth ventricle and separating it 

 from the third ventricle, 3, is the for nix, mainly made up of 

 fibres running from before back. The anterior downward- 

 curved end of the fornix is thickened, and contains the an- 

 terior commissure, a small cord of transverse nerve-fibres. 

 The cavity of the third ventricle is narrow from side to side, 

 and is bounded laterally by the optic tlialami, of which the 



