THE BRAIN. 



377 



hollow ; and their cavities, the lateral ventricles, retain through- 

 out life their communications with the third ventricle through 

 the foramina of Monro, a pair of apertures which are at first wide, 

 but which gradually become reduced, by thickening of their lips, 

 to narrow slits. 



The cerebral hemispheres first become prominent about the 

 twelfth day (Fig. 150, Bs) ; from this date they grow actively, 



Fie. 152. — The brain dissected from above. Enlarged. (From Marshall 



and Hurst.) 



C. lateral lobe of cerebellum. CA, pDlars of cerebellum. CB, cut edge of velum 

 medullae posterius. CO, corpus callosum : the right half is remoTed. CD, cut edge of 

 corpus callosum. CF, floccular lobe of cerebellum. CS, corpus striatum. P, anterior 

 limit of body of fornix. H, hippocampus major. M, medulla oblongata. O, olfactory 

 i..i.„ Qp^ anterior optic lobe. P, pineal body. V, fourth ventricle. 



lobe. 



extending forwards, and still more rapidly backwards, so as to 

 overlie and cover the roof of the thalamencephalon, and at a later 

 stage the mid-brain as well (cf. Figs. 151 and 152). 



The wall of each hemisphere is at first thin on all sides, and 

 the cavity, or lateral ventricle, is consequently large (Fig. 151). 

 The inner wall remains thin, but the outer wall (Fig. 152) 

 thickens considerably in the later stages of development ; while 



