14 



LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



developed at an early stage, in almost all vertebrates, two smaller 

 vesicles, the hemispheres or the secondary fore-brain. These, 

 which at first are small, insignificant structures, soon grow to 

 relatively enormous proportions in mammals, and, bending back- 

 ward, cover most of the other brain-vesicles. At last they lie, 

 cap-like, over the inter-brain (thalamus), the mid-brain (corpora j 

 quadrigemina), and the hind-brain (cerebellum and pons). Of 

 course, in spite of this bending backward of the anterior vesicle, 

 all the cavities (which are the ventricles of the adult brain) still 

 communicate with one another. After the development of the 

 secondary fore-brain, the primary fore-brain is called the inter- 

 brain. The roof of 

 this inter-brain remains 

 throughout life as a 

 simple layer of epithe- 

 lium. At the point of 

 confluence with the fore- 

 brain numerous blood- 

 vessels are developed 

 downward from the cra- 

 nial cavity, which push 



FIG. 5. 



Longitudinal section through the whole brain of a 

 newborn cat. The inter- and mid- brains covered by the 

 fore-braiu. Magnified 1 :2. 



Hinterhirn, Hind-brain. Mittelhirn, Mid-brain. 



Kachhirn, After-brain. Tun!, rhirn, Fore-brain. 



Zicischenhirn, Inter-brain. 



tli is epithelial layer on 

 before them. This mass 

 of blood-vessels covered 

 with epithelium, which thus projects into the cavity of the brain, 

 is called the choroid plexus. Inasmuch as the hemispheres have 

 been developed from the inter-brain, their inner border must 

 be continuous with this plexus. 



In the frontal section of an early human embryo, shown in 

 Fig. 6, this is plainly to be seen. In this cut, too, you can see 

 the cavity of the undivided primary fore-brain, as the ventriculus 

 medius, while the cavities of the hemispheres are marked as 

 vcntriculi laterales. The choroid plexus sends processes into 

 the lateral ventricles, which are called the lateral choroid 

 plexuses. The point where the wall of the hemisphere merges 



