HEALTH AND DISEASE 



principally of blood-vessels; the arachnoid, which is a serous mem- 

 brane, and is next outwardly placed; and the dura mater, which is a 

 tough fibrous membrane, and lines the interior of the skull. 



The outer layer of gray nerve substance being very much larger than 

 the surface of the brain, is folded, and, as it were, crumpled, and made 

 to dip down some distance into the organ. The folds are named con- 

 volutions (fig. 175 a), and the depressions or grooves between them sidci 



(fig. 175 b). 



The convolutions appear at 

 first sight to be quite irregu- 

 larly disposed, nor will their 

 arrangement be found exactly 

 the same in any two brains; 

 yet by tracing their develop- 

 ment, and by observing the 

 effects of injuries, a general 

 similarity has been demon- 

 strated in their position, and 

 the function of each has become 

 pretty well known. 



If the brain be sliced hori- 

 zontally a little below the level 

 of the corpus callosum, a cavity 

 is opened on each side named 

 the lateral ventricle. These 

 two ventricles are separated by 

 the septum lucidum, but com- 

 municate with each other an- 

 teriorly, and with the third 

 ventricle by the foramen of Monro. The floor of each lateral ventricle is 

 formed by the corpus striatum in front, and the otitic thalamus behind, and 

 upon these lie numerous blood-vessels forming the velum interpositum and 

 choroid plexus. The third ventricle is situated between the two corpora 

 striata and optic thalami, and is crossed by a gray and two white com- 

 missures. Behind, it ends in the aqueductus sylvii, which is a tunnel 

 running underneath the corpora quadrigemina and pons, and opening 

 behind into the fourth ventricle, which is again continuous with the 

 central canal of the spinal cord. 



The cerebellum is situated behind the brain and above the medulla 

 oblongata. Its convolutions are more numerous than those of the brain, 

 but present the same crumpled disposition of the gray and white sub- 



Upper Surface and Horizontal Section of the Brain 



A, Ai, Convolutions. B, Bi, Sulci. C, Gray Matter. D, White 

 Matter. COR. stb., Corpus Striatum. opt. thal.. Optic 

 Thalamus. 



