THE NERVOUS SYSTEM 423 



enceplialon), and hindbrain {rhombencephalon) . In the forebrain two 

 main divisions are usually distinguished, the endbrain {telencephalon) 

 and the interbrain {diencepJialon). The basal part of the endbrain 

 forms the corpora striata and rhinencephalon, while the dorsal part ex- 

 pands into the pallium {cerebral hemispheres) . In the interbrain may 

 be distinguished a dorsal part, the epithalamus; a middle (largest) 

 part, the thalamus; and a ventral expansion, the hypothalamus. 

 In the midbrain the basal part becomes the tegmentum, and the dor- 

 sal part expands into the corpora qiiadrigemina or inferior and superior 

 collicuU. The narrower part, connecting midbrain and hindbrain, is 

 the isthmus. The basal part of the hindbrain forms the medulla ob- 

 longata and part of the dorsal wall expands into the cerebellum. In 

 the later development of the brain, the pallium, and with it parts 

 of the cerebellum, becomes enormously enlarged and structures are 

 formed constituting connections between the pallium and the rest of 

 the brain. The most massive of these are the pes pedunculi, added 

 ventrally to the thalamus and midbrain; the pons Varolii, added 

 ventrally to part of the midbrain, isthmus and part of the hind- 

 brain; and the pyramids, added ventrally to the medulla. The 

 basal part of the mid- and hindbrain, thus covered ventrally by the 

 pes and pons, is the tegmentum. Other portions of the dorsal walls 

 of the forebrain and hindbrain form thin-walled expansions which, 

 together with vascular mesodermic coverings, are the chorioid plexuses 

 of the lateral, third, and fourth ventricles. The cavities of the cere- 

 bral hemispheres are the lateral ventricles; the cavity of the interbrain 

 is the third ventricle; that of the midbrain is the iter or aquaductus 

 Sylvii; that of the hindbrain is the fourth ventricle. 



The wall of the neural tube is at first composed of a single layer of 

 epithelial cells. By proliferation of these cells the epithelium soon 

 becomes many-layered, and forms a syncytium — the myelospongium 

 of His — although some of the original epithelial cells appear to extend 

 through the entire thickness of the wall. 



Some of the syncytial cells which extend through the entire thick- 

 ness of the wall of the neural tube {spongioblasts of His) increase in 

 length as the wall increases in thickness. The inner ends of these 

 cells form the lining of the tube, while other parts of the cells between 

 the lumen and the surface tend to collapse, forming cord-like struc- 

 tures. The outer ends of the cells, on the other hand, become per- 

 forated and unite to form a thick network^ — the marginal veil of His. 

 Of these cells, some retain this position, with nuclei near the lumen, 



