98 



THE MECHANISM OF LIFE 



lower parts of the sides are thickened to form the optic thalami, 

 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 Monro, 

 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 



^Laferal ventricle 



Corpus , ,^ 

 Striatum "-/^ 



Opening 



of3trJ. inTo 



ht ventricle 



_ _ IPineal body 



-Corbora. , . 



'a ua.dri0emi no. 



fmtd -brain J 



^--Cerebellum 

 ^^-J/leduHcL 



seduncle 

 of the cerebrum 



Fig. 27. 



-The Human Brain: an Imaginary Section along the 

 Middle Plane. 



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

 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 ventricle, 

 but it really belongs to the lower part of its cerebral hemisphere. 

 Because of the enormous growth backwards of the latter organs, 

 the relations of the various parts of the brain become difficult 

 to visualise. Thus the corpus callosum appears to form the 

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

 roof consists of a delicate vascular membrane, which is not 

 easily representable in a diagram, and the corpus collosum, with 

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



