STRUCTURE OF THE NERVOUS SYSTEM 



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on either side by the right and left sympathetic nerves which 

 extend vertically from ganglion to ganglion. In addition 

 to the ganglia forming these chains, important ones are 

 found in the head (outside of the cranial cavity) and in the 

 plexuses of the thorax and the abdomen. 



The sympathetic ganglia receive nerves from the cen- 

 tral division of the nervous system, but connect with 

 glands, blood vessels, and the intestinal walls through 

 fibers from their own cell-bodies. Some of these latter 

 fibers join the spinal nerves, and some blend with each 

 other to form small sympathetic nerves. 



Protection of Brain and Spinal Cord. On account of 

 their delicate structure, the brain and spinal cord require 

 the most complete protection. In the first place, they are 

 surrounded by the bones of the head and spinal column ; 

 these not only shield them from the direct effects of 

 physical force, but by their peculiar construction prevent, 

 to a large degree, the passage of jars and shocks to the 

 parts within. In the second place, they are surrounded by 

 three separate membranes, as follows : 



1. The dura, or dura mater, a thick, dense, and tough 

 membrane which lines the bony cavities and forms sup- 

 porting partitions. 



2. The pia, or pia mater, a thin, delicate membrane, 

 containing numerous blood vessels, that covers the surface 

 of the brain and cord. 



3. The arachnoid, a membrane of loose texture, that 

 lies between the dura and the pia. 



Finally, within the spaces of the arachnoid is a lymph- 

 like liquid which completely envelops the brain and the 

 cord, and which, by serving as a watery cushion, protects 

 them from jars and shocks. Thus the brain and cord are 

 directly shielded by bones, by membranes, and by the 



