384 ANATOMY FOR NURSES [Chap. XIX 



membranes (also named meninges), — the dura mater, pia mater, 

 and arachnoid. 



The dura mater is a dense membrane of fibrous connective tissue 

 containing a great many blood-vessels. It is arranged in two 

 layers and the layers are attached except in a few places. The 

 external layer is adherent to the bones of the skull, and forms their 

 internal periosteum. The internal layer covers the brain and 

 sends numerous prolongations inward for the support and protec- 

 tion of the different lobes of the brain. These projections also 

 form sinuses that return the blood from the brain, and sheaths 

 for the nerves that pass 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 depressions 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 serous membrane which is placed 

 between the dura mater and the pia mater. With the exception 

 of the longitudinal fissure,^ it passes over the various eminences and 

 depressions on the surface of the brain and does not dip down into 

 them like the pia mater. Between the arachnoid and the pia 

 mater is a space called the sub-arachnoid space in w'hich is a certain 

 amount of fluid. This fluid is secreted by the arachnoid and is 

 called the cerebro-spinal fluid. It serves to lubricate the other 

 membranes and prevent friction. In cases of meningitis, i.e. in- 

 flammation of these membranes, the amount of this fluid is very 

 much increased. 



Structure of the brain. — The whole brain appears to consist of 

 a number of isolated masses of gray matter — some large, some 

 small — connected together by a multitude of medullated fibres 

 (white matter) arranged in perplexing intricacy. But a general 

 arrangement may be recognized. The numerous masses of gray 

 matter in the interior of the brain may be looked upon as forming 

 a more or less continuous column, and as forming the core of the 

 central nervous system, while around it are built up the great 

 mass of the cerebrum and the smaller mass of the cerebellum. This 

 central core is connected by various bundles of fibres with the 



iSee page 389. 



