II. 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. Indeed, 

 a group of nerve-cells disconnected from the other nerve-tissues of the body, 

 as the muscles or glands are disconnected from each other, would be without 

 physiological significance. To understand, therefore, the physiology of the 

 nervous system, it is important to keep in mind the fact that by dissection it 

 is found to be continuous throughout its entire extent. 



When the nervous system is described as formed of a central and a 

 peripheral portion, and the peripheral portion is further divided into its 

 spinal and sympathetic components, the parts distinguished are found to 

 have no sharply marked boundaries separating them, but really to merge one 

 into the other. 



For topographical descriptions the convenience of such subdivisions is 

 undoubted ; but the physiological processes which it is our purpose to study 

 overstep in so large a measure these limits that the picture of events in the 

 central nervous system would be very incomplete, should they be separately 

 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 docs in shape to the frame- 

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

 pathwavs over which nerve-impulses travel toward the central system — the 

 brain and spinal cord — and, in consequence of the impulses that come in, 

 there pass out from the central system other impulses to the muscles and 

 glands. 



All incoming impulses must reach the central system. It is a fad of the 

 greatesl significance that until they have entered the central system the 

 incoming impulses do not give rise to those outgoing, for thus all incoming 

 impulses are first brought to the spinal cord and brain, where the outgoing 

 impulses are co-ordinated. 



By means of the central system reactions are established in parts of the 

 body not directly affected by the variation of the external conditions. < >wing 

 also to the wide connections of the nervous system and the conduction of all 

 incoming impulses to its central parts, a measure of harmony is maintained 



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