362 



PHYSIOLOGY 



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. 

 177). At their first formation the walls of these 

 vesicles are composed 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 form 

 the neuroglial supporting tissues of the brain and 

 probably also furnish the cells of the sheath of 

 Schwann to the outgrowing cranial nerves. In 

 some places the wall of the vesicles remains un- 

 differentiated ; no nervous tissues develop in it, and 

 * it forms a layer of epithelium known as ependyma. 

 brain of a chick at the By the varying growth of nervous tissue in different 

 second day. (CADIAT.) tg of the wa] j the t ical stru cture of the adult 



1, 2, 3, cerebral vesi- t . . , i i . T 



cles ; optic vesicles. brain is brought about (Fig. 178). Thus in the hind- 

 brain, or rhombencephalon, the roof of the neural 



canal posteriorly fails to develop, so that in the adult brain there is merely a 

 layer of epithelium covering the expanded central canal, here known as the 

 fourth ventricle. This back part of the hind-brain is often called the 

 myelencephalon, the anterior portion being* the metencephalon. The floor 

 of the myelencephalon undergoes 

 considerable thickening and 

 forms the future medulla ob- 

 longata. 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 



T . FIG. 178. Longitudinal section through brain of 



wall. In the region oi the con- chick of ten days. (After MIHALKOVICZ.) 



Striction between the hind- and olf olfactory lobes; h, cerebral hemisphere; 

 j i i ,T .! Iv, lateral ventricle; pin, pineal gland ; bg, cor- 



mid-bram known as the isthmus, pora bigemina . c u\ cerebellum ; oc, optic com- 



the roof Or dorsal wall forms the missure ; pit, pituitary body ; pv, pons Varolii ; 



, mo, medulla oblongata ; t' 3 , v*, third and fourth 



superior cerebellar peduncles at ventricles. 

 the side, and between them a thin 



layer of nervous matter known as the valve of Vieussens, or superior medul- 

 lary velum. The cavity of the third vesicle corresponds in the adult brain 

 to what is known as the fourth ventricle. 



The mesencephalon, or second cerebral vesicle, takes a relatively small 

 part in the formation of the adult human brain, though very conspicuous 

 in many of the lower types of brain. The whole of its wall is transformed 



cbl 



