384 



HEALTH AND DISEASE 



OLF 



Al - 



-BI. 



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 sulci 



(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 



Fig. 175. Upper Surface and Horizontal Section of the Brain tWO Ventricles are Separated by 



A, AI, Convolutions. B, BI, Sulci. c, Gray Matter. D, White the Septum lucidum, but COm- 

 Matter. COR. STB., Corpus Striatum. OPT. THAL., Optic municate with each other an _ 

 Thalamus. 



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 optic 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- 



MEDULLA OBLONGATA.- > 



H- 



