GENERAL SUMMARY. 641 



CHAPTER XIV. 



OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



1. General Summary. 



674. THE Nervous System of Man, as of Vertebrated animals generally, con- 

 sists of an aggregation of separate ganglionic centres, which are more or less 

 intimately connected with each other by commissural fibres, and which are called 

 into consentaneous activity in a large proportion of the operations of which this 

 apparatus is the instrument. Hence, notwithstanding the abundant evidence 

 that these several centres differ in their respective endowments, there is con- 

 siderable dimculty in the determination of their special functions ; since the 

 destruction or removal of any one portion of the Nervous system not only puts 

 a stop to the phenomena to which it is itself directly subservient, but so de- 

 ranges the general train of nervous activity, that it often becomes impossible to 

 ascertain, by any such method, what is its real share in the entire performance. 

 In this difficulty, however, we may advantageously have recourse to the study 

 of the structure and actions of those forms of the Nervous system presented to 

 us among the lower animals, in which its ganglionic centres are fewer and less 

 intimately connected, and in which, therefore, it is more easy to gain an ac- 

 quaintance with their several endowments. And from an extensive survey of 

 these, we seem able to deduce the following conclusions, which afford the most 

 valuable guidance in the study of the Nervous System of Man. 1 



I. The Nervous System, in its lowest and simplest form, may consist of but 

 a single ganglionic centre, 2 with afferent and motor nerves ( 350), whose func- 

 tion is essentially internuncial ; impressions made upon the afferent fibres excit- 

 ing respondent movements in the muscles supplied by the motor, without any 

 necessary intervention of consciousness. Such movements are properly distin- 

 guished as excito-motor. 



n. A repetition of such ganglionic centres may exist to any extent, without 

 heterogeneousness of function, or any essential departure from the simple mode 

 of action just indicated ; each of these centres may be specially connected by 

 afferent and motor fibres with one segment or division of the body, and may 

 minister peculiarly to its actions; but the several centres may be so intimately 

 connected by commissural fibres, that an impression made upon the afferent 

 nerves of any one of them may excite respondent motions in other segments. This 

 we see effected through the annular gangliated cord of the higher Radiata, and 

 through the longitudinal gangliated cord of the Articulata; the disposition of 

 the ganglia and of their connecting cords having reference simply to the general 

 plan of the body. 



in. A higher form of Nervous System is that in which the multiplication of 



1 For a general view of the facts on which these conclusions are based, see "Princ. of 

 Phys., Gen. and Comp.," CHAP, xx., Am. Ed. 



2 It may, perhaps, be doubted whether any Animal really exists, possessing such nervous 

 system, and yet not endowed with consciousness. It is quite certain, however, that animals 

 do exist (the Tunicated Mollusca, for example), in which the actions above referred to are 

 the only ones of which we have any distinct evidence from observation of their habits. 



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