X. CENTRAL NERVOUS SYSTEM. 



INTRODUCTION. 



The Unity of the Central Nervous System. The human nervous system 

 is formed by a mass of separate but contiguous nerve-cells. As each nerve- 

 cell is always in close relations with some other nerve-cell, this system differs 

 from those formed by the bones, muscles, or glands, since these tissues are dis- 

 tributed through the body in masses more or less isolated. Isolated groups of 

 nerve-cells do not occur. Indeed a group of nerve-cells disconnected from the 

 other nerve-tissues of the body, as the muscles or glands are disconnected, 

 would be without physiological significance. It is desirable, therefore, to 

 emphasize the fact that by dissection the nervous system is found to be con- 

 tinuous throughout its entire extent. 



Subdivisions Artificial. When, therefore, the nervous system is described 

 as formed of a central and a peripheral portion, and the peripheral portion is 

 further analyzed into its spinal and sympathetic components, the parts distin- 

 guished are found to have no sharply marked boundaries separating them, but 

 really to merge one into the other. 



The convenience of these subdivisions is undoubted, but the physiological 

 processes which it is our purpose to study, overstep in so large a measure such 

 conventional limits, that the picture of events in the central nervous system 

 would be very incomplete, should they be traced only within such prescribed 

 anatomical boundaries. 



By virtue of its continuity, the nervous system puts into connection all the 

 other systems of the body. Conforming as it does in shape to the framework 

 of the body, its branches extend to all parts. These branches form pathways 

 over which nerve-impulses travel toward the central system the brain and 

 spinal cord, enclosed in the cranial cavity and vertebral canal and in conse- 

 quence of the impulses that come in, there pass out from the central system 

 other impulses to the muscles, glands, and blood-vessels. 



All incoming impulses must reach the central system. Most important in 

 this arrangement is the absence of any device for short-circuiting the incoming 

 impulses. It is a fact of the greatest significance, that until they have entered 

 the central system the incoming impulses do not give rise to those outgoing, 

 and thus all incoming impulses are first brought to the spinal cord and brain, 

 and the outgoing impulses are there aroused and co-ordinated by them. 



By means of the central system there are established reactions in those tis- 

 sues not directly affected by the variation of the external conditions, and thus 



605 



