412 STUDIES IN ADVANCED PHYSIOLOGY. 



finer architecture of the nervous system, it being the inten- 

 tion in these preliminary paragraphs to deal simply with 

 those points of the nervous system included under its gross 

 anatomy. 



THE BRAIN AND SPINAL CORD. 



By far the most important system, both with reference 

 to its special psychical functions and its general control 

 over the other systems, is the cerebro-spinal system. This 

 consists of the brain and spinal cord, and the nerves issu- 

 ing from them. The brain and spinal cord are continuous 

 through the foramen magnum, a large opening in the occip- 

 ital bone. 



The Membranes of the Cerebro- Spinal System. 



In the examination of these systems one is peculiarly 

 impressed with the efficient way in which they are enclosed 

 within bony and membranous coverings. The brain is en- 

 cased in the bony cranium, while the spinal cord is almost 

 equally protected in the neural arches formed by the verte- 

 brae. In addition to this bony envelope both brain and 

 spinal cord are covered with three membranes. Lying 

 next to the nervous tissue is a delicate thin membrane 

 called the pia-mater. This dips down into all the convo- 

 lutions and configurations of brain and spinal cord, and 

 serves especially to carry the blood-vessels nourishing them. 

 Ikying next to the bone is an exceedingly tough dense mem- 

 brane formed almost wholly of white closely woven con- 

 nective fibers called the dura-mater. Between the dura- 

 mater and the pia-mater there is a spongy membrane called, 

 on account of its web-like nature, the arachnoid membrane. 

 The dura-mater figures not only as an enveloping membrane 

 of the brain and spinal cord, but serves as a periosteum for 

 the cranial bones as well. While these membranes sur- 

 round the brain very closely the dura-mater does not invest 

 the spinal cord in the same way, but here frequently leaves 

 quite a space between the pia-mater and itself, in this way 

 covering even the spinal root ganglia lying along the spinal 



