CHAPTER XL 

 THE BRAIN AND NERVES. 



THE central nervous system is differentiated very early in 

 development. Beginning as a neural plate, it soon, by the up- 

 growth of laminae dorsales, which bend over and eventually 

 meet in the mid-line, becomes converted into a neural tube 

 (see p. 102). We have seen that the anterior part of this tube 

 becomes differentiated into three swellings or vesicles, the fore- 

 brain, mid-brain, and hind-brain. These in the course of 

 development become further differentiated, so that we have 

 eventually five regions, viz. : 



(1. Myelencephalon, or region of the medulla 

 Hind-Brain, -< oblongata. ( 



( 2. Metencephalon, or region of the cerebellum. 

 Mid-Brain, 3. Mesencephalon, or region of the optic lobes. 



/ 4. Thalamencephalon, or region of the thalamus^ 

 -p , -n, . J 5. Prosencephalon, or region of the cerebral 

 ' j hemispheres, with their outgrowths, the 



olfactory lobes. 



These regions are somewhat differently developed in Frog, 

 Pigeon, and Rabbit. We will consider them seriatim from 

 behind forwards. 



(A.) Hind-Brain 1. Myelencephalon. The floor and sides of 

 the canal in this region become much thickened, while the roof 

 becomes very thin. The central canal here expands to form the 

 fourth ventricle of the brain (iv.). The pia mater covering this 

 ventricle becomes very vascular, forming the tela vasculosa. 

 Into this region of the brain the dorsal and ventral fissures of 



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