236 



CHAPTER V. 



FUNCTIONS OF THE NERVOUS SYSTEM. 



1. General Summary. 



289. OUR fundamental idea of a Nervous System includes a central or^an 

 or ganglion, essentially composed of vesicles or cells, with a plexus of capil- 

 lary vessels distributed amongst these ; and a set of trunks and ramifying 

 branches, composed of tubular fibres, and connecting the ganglion with differ- 

 ent parts of the fabric. These branches are for the most part distributed, on 

 the one hand, to the sensory surfaces and organs ; and, on the other, to the 

 muscles or motor organs. The former serve for the conveyance of impres- 

 sions, made upon the periphery, towards the centre ; and they may thence 

 be denominated afferent fibres.* The latter, on the other hand, serve to con- 

 vey an influence, originating in the central ganglion, to the muscles, which 

 are thereby thrown into contraction; and these are distinguished as efferent 

 or motor fibres. Although the distinctness of these two sets of fibres has 

 only been proved in the Vertebrata, yet there can be no reasonable doubt of 

 its universality. Now this fundamental idea of a Nervous apparatus, which 

 is based upon what are believed to be the relative offices of its several com- 

 ponent parts (as formerly explained 248), is found to be exactly realized in 

 the simple forms of that system, which we find in the lowest animals in which 

 Nervous structure can be discovered at all ; and even where the apparatus 

 has, to all appearance, a character of much greater complexity, it may still be 

 reduced to the same simple idea, by taking it to pieces (so to speak) and exa- 

 mining its component parts. For it will then be found, that the multiplica- 

 tion of ganglia and trunks is principally due to the multiplication of the 

 organs to be supplied ; as in the case of the nervous ring of the Star-fish, 

 where the ganglia, all of them apparently identical in function, and similar in 

 the distribution of their branches, are repeated in conformity with the num- 

 ber of the radiating parts of the body ; or in the case of the ventral nervous 

 cord of an Articulated animal, in which the ganglia are in like manner re- 

 peated longitudinally, in accordance with the number of segments of the body, 

 and of the pairs of members connected with them. In other instances, the 

 multiplication of ganglia is due to the increased complexity of the functions 

 performed by a set of organs; of this we shall see numerous examples in 

 the higher Vertebrata. In all cases, the individual ganglia remain to a great 

 extent independent of each other; so that the removal of any one (if it can 

 be accomplished without injury to the rest) affects only the particular organ 

 with which it may be connected, and the special function of that organ to 

 which alone it ministers. 



290. Uefore proceeding to inquire into the operations of the Nervous Sys- 

 tem as a whole, it is desirable that we should stop to consider the conditions 



* Such arc commonly denominated sensory fibres; but this designation is objectionable, 

 in as much as many of them serve to excite reflex actions, without necessarily producing 

 sensations. 



