THE STRUCTURE OF THE BRAIN STEM 



361 



FIG. 177. Diagram of the 

 cerebral vesicles of the 

 brain of a chick at the 

 second day. (CADIAT.) 



1, 2, 3, cerebral vesi- 

 cles ; 0, optic vesicles. 



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 epithe- 

 lial cells, and show no trace of nervous structure. A little later the cells forming the 

 walls present a differentiation into neuroblasts and spongio- 

 blasts, 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 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. 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 mye- 

 lencephalon undergoes considerable thickening and forms 

 the future medulla oblongata. In the metencephalon, ner- 

 vous 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 as the isthmus, the roof or dorsal wall forms the superior cerebellar peduncles 

 at the side, and between them a thin layer of nervous matter known as the valve of 

 Vieussens, or superior medullary 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 con- 

 spicuous in many of the lower types 

 of brain. The whole of its wall is 

 transformed into nervous tissue, the 

 roof or dorsal wall forming the cor- 

 pora quadrigemina, while the two 

 crura cerebri are developed in its 

 ventral wall. The cavity of the 

 second cerebral vesicle is retained as 

 a narrow canal known as the aque- 

 duct of Sylvius, and connects the 

 fourth ventricle with the third ven- 

 tricle. 



Very soon after its first appearance 

 the first cerebral vesicle is modified 

 by the formation of lateral expansions, known as the optic vesicles, which later on are con- 

 stricted off from the central part of the cavity so as to be connected with this by two short 

 tubular passages, the optic stalks. From the optic vessels are ultimately developed the 

 retinae of the eyes. By the development of nerve cells in the optic cup the ganglion- 

 cell layer of the retinae is produced, and from these cells fibres grow back along the 

 optic stalk and make connection with the grey matter developed in the lateral wall of 

 the fore-brain and with the adjacent parts of the mid-brain, viz. the superior corpora 



cbi 



FIG. 178. Longitudinal section through brain of 



chick of ten days. (After MIHALKOVICZ. ) 

 ol-f. olfactory lobes ; h, cerebral hemisphere ; 

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

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

 missure ; pit, pituitary body ; pv, pons Y r arolii ; 

 mo, medulla oblongata : v*. ?:*. third and fourth 

 ventricles. 



