98 



THE MECHANISM OF LIFE 



lower parts of the sides are thickened to form the optic thalamij 

 one of which is represented in the lateral wall in the figure. The 

 enormous lobes, called the cerebral hemispheres, are the roofs of 

 the primary lateral brain vesicles thickened to form the cerebral 

 cortex with its systems of projection nerve fibres. In one corner; 

 of the third ventricle a little opening, called the foramen of MonroA 

 is shown, and dotted lines indicate how this leads into a cavity 

 in each cerebral hemisphere, called the " lateral ventricle." The] 

 cross-hatched part, called the corpus callosum, is the junction of 



Junction of R. and L hemispheres 



Corpus , 

 Sthatum " 



n ng 



^Kl. inTo " 



1st. ventricle 



L atera I ventricle 



_ _ ^Pineal body 

 - Cor bora. 



--Cerebellum 



-Pituitary ', 

 body I 



7eduncle 

 of the cerebrum 



cord. 



FIG. 27. THE HUMAN BRAIN: AN IMAGINARY SECTION ALONG THB I 

 MIDDLE PLANE. 



The cross-hatched areas represent the cut surfaces; the numbers I, lit 

 and IV, show the lateral, third and fourth ventricles respectively. 



the two (right and left) hemispheres. Just round the foramen] 

 of Monro the corpus striatum of the right-hand hemisphere is 

 shown. This ganglion appears to bulge into the third vontricloj 

 but it really belongs to the lower part of its cerebral hemisphereJ 

 Because of the enormous growth backwards of the latter orgaM 

 the relations of the various parts of the brain become difficult 

 to visualise. Thus the corpus callosum appears to form thl 

 roof of the third ventricle, but this is not really the case. The- 

 reof consists of a delicate vascular membrane, which is now 

 easily representable in a diagram, and the corpus collosmn, with; 

 some other parts which are omitted, are the sections of the 



