THE RABBIT. 101 



is the future brain, becomes swollen up into three hollow 

 structures called the three cerebral vesicles (fig. 55). The 

 first vesicle becomes the primitive fore-brain, or thalam- 

 encephalon ; the second becomes the mid-brain, or mes- 

 encephalon ; the third, the hind-brain, or met-encephalon. 

 From the first vesicle are developed a pair of hollow 

 outgrowths, the cerebral hemispheres, or prosencephalon, 

 prolonged forward into the olfactory lobes. At first, in 

 the embryo, the walls of these vesicles and of the spinal 

 cord consist of a mass of simple cells, but as development 

 proceeds these cells multiply to a greater extent in some 

 regions than in others. They also send out axis- cylinder 

 processes which, pushing their way in various directions, 



Fig. 55. BRAIN OF RABBIT. 

 Side view. (After Wiedersheim.) 



give rise to the bands of white matter that we have 

 seen in the spinal cord, and which are also found in the 

 brain, though not so simply arranged. Thus, by growth 

 of nerve-cells (forming grey matter) in some regions, and 

 the convergence of numerous fibres (forming white matter) 

 in others, the complex brain of the adult is at length built 

 up. Diagrammatic sections of it are shown in figs. 56 

 and 57 ; and actual views of it in figs. 54 and 55. 



The processes of growth in the brain very nearly 

 obliterate the original cavities of the vesicles, but they 

 can still be recognized and now receive fresh names. 

 The cavities of the cerebral hemispheres are called 

 (he lateral (or first and second) ventricles; that of the 

 first vesicle is the third ventricle ; that of the third 



