THE BRAIN AND ITS MEMBRANES. 



707 



The mesencephalon, or mid-brain, simply develops into the corresponding por- 

 tion of the adult brain which is known by the same name, mid-brain. The epen- 

 cephalon becomes the future pons Varolii and cerebellum, while the metencephalon 

 develops into the medulla oblongata. These names, " prosencephalon," etc., which 

 have been given to the five secondary cerebral vesicles, are also used, sometimes, 

 to designate the corresponding divisions of the adult brain. The terms " hind- 

 brain " and " after-brain " are often employed, the former as a name for the pons 

 and cerebellum, the latter for the medulla.* 



It will be observed that in making the above comparison there has been no 

 mention of the hemispheres nor of a 

 corresponding portion of the embryonal 

 brain. This point will now be touched 

 upon. Soon after the formation of the 

 primary, or simultaneously with that of 

 the secondary, cerebral vesicles there 

 grows out from each side of the front 

 part of the fore-brain or prosencephalon 



Mid-brain 



Inter-brain 



'ind-bratn 



After-brain 



brain) 



Mid-brain 



a hollow protrusion. These protrusions 

 from the sides of the prosencephalon are Waetory lobe 

 known as the " hemisphere " vesicles, and Optic lobe 



each one is to form the corresponding 

 <i'h'/)-i} of the adult brain (Fig. 412). 



mi j i - 111 



1 his development is brought about by a 

 process of extension and growth in all 

 directions, forward, backward, upward, 

 and downward, until, as the hemispheres, 

 the enormously enlarged hemisphere 

 vesicles come close together above, and 

 overlie from above downward all the 

 remaining divisions of the encephalon. 

 (The term " fore-brain " is sometimes 

 used to designate the prosencephalon and the hemispheres.) 



It will be remembered that the name " third ventricle " means the cavity of 

 the inter-brain. The cavities of the other divisions are known as follows : That 



Hemisphere 

 tarn ' 



FIG. 412. Brain of a seven weeks' old embryo, seen, 

 A, from the side. B, from above. (Mihalkovics.) 



Choroid 

 plexus 



Com mun i cation 



between both 

 lateral ventricles 



Spinal cord 

 FIG. 413. Diagram showing intercommunication 



Jfid-brain 



Afler-brain 



N opt. 



X. ma. 



FIG. 414. A, brain of a rabbit embryo. B. of a 

 foetal calf. In both the lateral wall of the left hemi 

 sphere vesicle is removed. (Mihalkovics.i 



of each of the hemispheres is the lateral ventricle of the corresponding side ; that 

 of the mid-brain is the aqueduct of Sylvius ; while that of the pons and cerebellum 



