400 GENERAL STRUCTURE AND FUNCTIONS 



into action the organs of the living body is not direct but indirect in its 

 operation. In the healthy and uninjured condition of the frame, the 

 muscles are never made to contract by an external stimulus applied 

 immediately to their own fibres, but by one which first operates upon 

 some other organ, adjacent or remote. The various secreting glands 

 have their functional activity increased or diminished by causes which 

 are directly applied not to themselves but to other parts of the body ; 

 as where a flow of saliva from the parotid is produced by food intro- 

 duced into the cavity of the mouth, or where the discharge of perspira- 

 tion by the skin is modified by the influence of mental conditions. As 

 a rule, therefore, in the natural state of the system, the various organs 

 situated in different parts of the body are connected with each other by 

 a mutual sympathy which regulates their physiological action. This 

 connection is established through the medium of the nervous system. 



The function of the nervous system is therefore to associate the 

 different parts of the body in such a manner, that stimulus applied to 

 one organ may excite the activity of another. 



The instances of this mode of action are as numerous as the different 

 vital phenomena. The stimulus of light falling upon the retina pro- 

 duces contraction of the pupil. The introduction of food into the 

 stomach causes the gall-bladder to empty itself into the duodenum. 

 The contact of alimentary substances with the mucous membrane of 

 the intestine, excites the peristaltic action of its muscular coat. The 

 presence of a growing foetus in the uterus is accompanied by an increased 

 growth of the mammary glands. Every organ is subservient, in the 

 manifestation of its functional activity, to influences from other parts, of 

 a structure different from its own. 



General Structure of the Nervous System, 



The nervous system consists of two kinds of nervous tissue, differing 

 from each other in appearance, structure, and physiological endowments. 

 One of these is the white substance, composed of nerve fibres alone ; the 

 other is the gray substance, which contains, in addition to the nerve 

 fibres, interstitial matter and nerve cells. The white substance is found 

 in the trunks and branches of the nerves, on the surface of the spinal 

 cord, and in the internal parts of the brain. The gray substance forms 

 the external layer or convolutions of the brain, as well as various de- 

 posits about its base and central parts, the central portion of the spinal 

 cord, and a large number of small detached masses in different parts of 

 the body. These two kinds of nervous tissue are so different in their 

 properties and function as to require for each a separate description. 



Nerve Fibres. 



The nerve fibres are cylindrical filaments, arranged in bundles or 

 tracts, in which they run, for the most part, in a direction parallel to 

 each other. Their diameter varies considerably, even in the same 



