CHAP. VII.] THE NERVOUS SYSTEM. 85 



Brain and cranial nerves. The brain, the most complex and 

 largest mass of nervous tissue in the body, is contained in the 

 complete bony cavity formed by the bones of the cranium. It 

 is covered by three membranes (also named meninges), the 

 dura mater, pia mater, and arachnoid. 



The dura mater, a dense membrane of fibrous connective tissue, 

 lines the bones of the skull, forming their internal periosteum, 

 and covers the brain. It sends numerous prolongations in- 

 wards for the support and protection of the different parts of 

 the brain ; it also forms sheaths for the nerves passing out of 

 the skull. It may be called the protective membrane. 



The pia mater is a delicate membrane of connective tissue, 

 containing an exceedingly abundant network of blood and 

 lymph vessels. It dips down into all the crevices and depres- 

 sions of the brain, carrying the blood-vessels which go to every 

 part. It may be called the vascular or nutritive membrane. 



The arachnoid is a delicate membrane which is placed outside 

 the pia mater. It passes over the various eminences and de- 

 pressions on the surface of the brain, and does not dip down 

 into them like the pia mater. Beneath it, between it and the 

 pia mater, is space (sub-arachnoid space) in which is a certain 

 amount of fluid. The sub-arachnoid space at the base of the 

 brain is of considerable size, and contains a large amount of 

 this clear limpid fluid, called the cerebro-spinal fluid. This 

 fluid probably acts as a sort of protective water-cushion to the 

 delicate nervous structure, and prevents the effects of concus- 

 sions communicated from without. 



The brain is a semi-soft mass of white and gray matter. 

 The white matter consists of very small, medullated nerve- 

 fibres, running in various directions, and supported by a deli- 

 cate connective tissue framework. The gray matter consists 

 of cells and fine gray fibres, also supported by connective 

 tissue. 



The brain is divided into four principal parts : the cerebrum, 

 the cerebellum, the pons Varolii, and the medulla oblongata. 



The medulla oblongata is continuous with the spinal cord, 

 which, on passing into the cranial cavity through the foramen 

 magnum, widens into an oblong-shaped mass. It is directed 

 backwards and downwards, its anterior surface resting on a 

 groove in the occipital bone, and its posterior surface forming 



