408 



PHYSIOLOGY 



The general features of the structure of the brain will be best 

 understood by reference to the mode of development of this part of the 

 central nervous system. At the front end of the body, the primitive 

 neural tube, formed by the invagination and growing over of the 



epiblast, is somewhat enlarged and is marked 

 off by two constrictions into the three primitive 

 cerebral vesicles, which are named respectively 

 the fore-, the mid-, and the hind-brain, or the 

 prosencephalon, the mesencephalon, and the 

 rhombencephalon (Fig. 178). At their first 

 formation the walls of these vesicles are com- 

 posed of simple epithelial cells, and show no 

 trace of nervous structure. A little later the 

 cells forming the walls present a differentiation 

 into neuroblasts and spongioblasts, the former 

 developing into nerve- cells, while the latter 

 brain of a chick at the form the neuroglial supporting tissues of the 

 . brain and probably also furnish the cells of 



1, 2, 3, cerebral vesi- J 



cles ; 0, optic vesicles, the sheath of Schwann to the outgrowing 



cranial nerves. In some places the wall of 



the vesicles remains undifferentiated ; no nervous tissues develop in 

 it, and it forms a layer of epithelium known as ependyma. By the 

 varying growth of nervous tissue in different parts of the wall, the 

 typical structure of the adult brain is brought about (Fig. 179). 

 Thus in the hind-brain, or 

 rhombencephalon, the roof 

 of the neural canal pos- 

 teriorly fails to develop, so 

 that in the adult brain 

 there is merely a layer of O if 

 epithelium covering the ex- 

 panded central canal, here 

 known as the fourth ven- 

 tricle. This back part of 

 the hind- brain is often called 

 the myelencephalon, the an- 

 terior portion being the 

 metencephalon. The floor 



FIG. 179. Longitudinal section through brain of 

 chick of ten days. (After MIHALKOVICZ.) 



olf, olfactory lobes ; /*, cerebral hemisphere ; 

 Iv, lateral ventricle ; pin, pineal gland ; bg, cor- 

 pora bigemina ; cbl, cerebellum ; oc, optic com- 

 missure ; pit, pituitary body ; pv, pons Varolii ; 

 mo, medulla oblongata ; v 3 , t*, third and fourth 

 ventricles. 



of the myelencephalon 



undergoes considerable thickening and forms the future medulla 

 oblongata. In the metencephalon, nervous tissue is developed all 

 round the canal, the floor of the canal forming the pons Varolii, while 

 the cerebellum is developed by an outgrowth of the dorsal wall. In 

 the region of the constriction between the hind-and mid-brain known 



