THE NERVOUS SYSTEM. 



THE nervous system, for description, is divided into two 

 parts: central and peripheral. 



The central nervous system, or cerebro-spinal axis, consists 

 of the brain and spinal cord. 



The peripheral nervous system consists of the cranial and 

 spinal nerves and ganglia, and the sympathetic nerves and 

 ganglia. 



The nervous system may also be divided into the cerebro- 

 spinal system, that supplying muscles, skin, and mucous mem- 

 branes; and the sympathetic system, presiding over organs and 

 blood-vessels. 



Structure. Nervous tissue consists of three distinct sub- 

 stances, combined in variable proportions in the different parts 

 of the nervous system (white, or fibrous substance; gray, or 

 vesicular substance, and neuroglia) . 



(a) White substance is found in the cortex of the cord, the 

 interior of cerebrum, in nerves, etc., and is made up of medullated 

 nerve-fibres. Those are smooth, round fibres, measuring one two- 

 thousandth to one twelve-thousandth of an inch in diameter, and 

 have each three parts : 



1. Axis cylinder is a round or band-like striated structure 

 consisting of fibrillee; 



2. The medullary sheath, or white substance of Schwann, 

 which is made up of fatty substance and probably insulates the 

 axis cylinder; and 



3. The neurilemma, or sheath of Schwann, a delicate, struc- 

 tureless membrane, closely surrounding the medullary sheath, and 

 forming the surface of the nerve-fibres. 



Near their termination the nerve-fibres lose their medullary 

 sheath and become non-medullated (Eemak's) fibres. Such are 

 the olfactory and most of the sympathetic nerves. 



(b) Gray substance, found in the middle of the spinal cord, 

 in the cortex of the brain, in ganglia, etc., consists of three ele- 

 ments: (1) nerve-fibres; (2) nerve-cells; and (3) blood-vessels 

 and connective tissue. 



Nerve-fibres. These are medullated and non-medullated 

 nerve-fibres, axis-cylinder processes, and dendritic processes of 

 nerve-cells. 



Nerve-cells are of two kinds large branched cells, bipolar 

 and nmltipolar, etc., and small round cells resembling free nuclei. 



(283) 



