510 ELEMENTARY PHYSIOLOGY less. 



a small opening in the pia mater covering the hinder end 

 of the fourth ventricle ; this opening is known as the 

 foramen of Magendie. 



11. The Minute Structure of the Brain.— In the 

 spinal bulb the arrangement of the wliite and grey matter 

 is substantially similar to that which obtains in the 

 spinal cord, that is to say, the white matter is external and 

 the grey internal; but the grey matter, containing, as in the 

 spinal cord, nerve cells, is more abundant than in the 

 spinal cord, and the arrangements of white and grey 

 matter become much more intricate and complex. The 

 structure of the white matter of the brain is essentially 

 the same as that of the spinal cord. 



Above the bulb there are internal deposits of grey matter, 

 containing nerve cells, at various places, more especially in 

 the pons Varolii, the crura cerebri, the corpora quadri- 

 gemina, optic thalami and corpora striata. And there is a 

 remarkably shaped deposit of grey matter in the interior of 

 the cerebellum, on each side. But what especially charac- 

 terises the brain is the presence of grey matter of a 

 special nature, containing peculiarly sliaped nerve cells, 

 on the surface of the cerebral hemispheres known as the 

 cortex, and similarly a special grey matter forms the 

 surface of the cerebellum. This superficial grey matter 

 covers the whole surface of both these organs, dipping 

 down into the fissures (sulci) of the former, and 

 following the peculiar plaits or folds into which the latter 

 is thrown. 



The Cerebellum. — The surface of the cerebellum pre- 

 sents a corrugated or laminated appearance. When a 

 section is made through one of its hemispheres it is seen 

 that the depressions which separate the lamina} give ofi' 

 secondary lateral depressions as they pass towards its 

 centre, so that the surface is really divided up into a very 

 large number of leaf-like foldings which are known as the 

 lamellae. The central part of the cerebellum consists of 

 white matter which is essentially the same as the white 



