564 NERVOUS SYSTEM. 



tissue or organ its special "irritability" or the power of performing its particular func- 



tion. 



2. The nervous system connects into a coordinated organism all parts and organs of 

 the body. It is the medium through which all impressions are received. It animates 

 or regulates all movements, voluntary and involuntary. It regulates the functions of 

 secretion, nutrition, calorification, and all the processes of organic life. 



In addition to its functions as a medium of conduction and communication, the ner- 

 vous system, in certain of its parts, is capable of receiving impressions and of generating 

 a stimulating influence, or force, peculiar to itself. As there can be no physiological 

 connection or coordination of different parts of the organism, having an active function, 

 without nerves, there can be no unconscious reception of impressions giving rise to invol- 

 untary movements, no appreciation of impressions, general, as in ordinary sensation, or 

 special, as in sight, smell, taste, or hearing, no instinct, volition, thought, or even knowl- 

 edge of existence, without nerve-centres. 



Possessing, as it does, these varied properties and functions, it is evidently of the 

 greatest physiological importance that the anatomical characters of the nervous sys- 

 tem should be most carefully studied, with a view of connecting, if possible, certain of 

 the nervous properties with peculiarities in structure. It is also important to subdivide 

 the system, as regards general properties and functions, as well as with reference to the 

 special office of particular parts. With this end in view, we shall point out first, the 

 great anatomico-physiological divisions common to nervous matter wherever it exists, 

 and afterward, the subdivisions of the system as regards special functions. 



.Divisions of the Nervous System. 



Nervous matter, whatever may be its special function, presents two great divisions, 

 each with distinct anatomical as well as physiological differences. One of these divisions 

 presents the form of fibres or tubes. This kind of nervous matter is incapable of gener- 

 ating a force or stimulus, and it serves only as a conductor. The other division is in the 

 form of cells, and this kind of nervous matter alone is capable of generating the so-called 

 nervous force. 



The nervous matter is divided into two great systems, as follows : 



1. The cerebro-spinal system, composed of the brain and spinal cord with the nerves 

 directly connected with these centres, This system is specially connected with the func- 

 tions of relation, or of animal life. The centres preside over general sensation, the spe- 

 cial senses, voluntary and some involuntary movements, intellection, and, in short, all 

 of the functions that characterize the animal. The nerves serve as the conductors of 

 impressions known as general or special sensations, and of the stimulus that gives rise to 

 voluntary and certain involuntary movements, the latter being the automatic movements 

 connected with animal life. 



2. The sympathetic, or organic system. This system is specially connected with the 

 functions relating to nutrition, operations which have their analogue in the vegetable 

 kingdom and are sometimes called the functions of vegetative life. Although this sys- 

 tem presides over functions entirely distinct from those characteristic of and peculiar to 

 animals, the centres of this system all have an anatomical and physiological connection 

 with the cerebro-spinal nerves. 



The cerebro-spinal system is subdivided into centres presiding over movements and 

 ordinary sensation, and centres capable of receiving impressions connected with the special 

 senses, such as sight, audition, olfaction, and gustation. The nerves which receive these 

 special impressions and convey them to the appropriate centres are more or less insen- 

 sible to ordinary impressions. The organs to which these special nerves are distributed 

 are generally of a complex and peculiar structure, and they present numerous accessory 

 parts which are important and essential in the transmission of the special impressions to 

 the terminal branches of the nerves. 



