292 ZOOLOGICAL PHILOSOPHY 



Before setting forth the proofs of these theories, let us see what 

 general idea we can form of the nature and arrangement of the various 

 parts of the nervous system. 



This system, wherever it occurs in animals, presents a main medullary 

 mass, either divided into separate parts or concentrated into a single 

 whole of varjang shape, and also nervous threads which run into this 

 mass. 



All these organs are composed of three kinds of substances of very 

 different character, viz. : 



1. A very soft medullary pulp of pecuhar character. 



2. An aponeurotic investment, which surrounds the meduUary 

 pulp and provides sheaths to its prolongations and threads, including 

 even the finest. The nature and properties of this investment are 

 different from those of the pulp which it encloses. 



3. A very subtle invisible fluid, which moves in the pulp without 

 requiring any visible cavity, and which is kept in at the sides by 

 the sheath, through which it cannot pass. 



Such are the three kinds of substances which compose the nervous 

 system, and which produce the most astonishing of all organic pheno- 

 mena as a result of their arrangement, relations and the movements of 

 the subtle fluid contained within the system. 



It is known that the pulp of these organs is a very soft meduUary 

 substance, white on the inside, greyish on its outer layer, not sensitive, 

 and apparently albumino-gelatinous in character. It forms, by means 

 of its aponeurotic sheaths, threads and cords which proceed to the 

 larger masses of this medullary substance containing the nucleus 

 (simple or divided), or centre of communication of the system. 



Both for the performance of muscular movement and for sensations, 

 it is necessary that this system of organs should have a nucleus or 

 centre of communication for the nerves. As a matter of fact, in the 

 first case the subtle fluid which acts upon the muscles issues from a 

 common nucleus and travels towards the parts which it has to actuate ; 

 and, in the second case, the same fluid, being set in motion by the 

 affective cause, starts from the extremity of the affected nerve and 

 travels towards the centre of communication, there producing the 

 disturbance which gives rise to sensations. 



A nucleus or centre of communication, in which the nerves terminate, 

 is therefore absolutely necessary in order that the system may carry 

 on any of its functions ; and indeed we shall see that without it the 

 individual would not become cognisant of the acts of the organ of 

 intellect. Now this centre of communication is situated in some part 

 of the main medullary mass, which always constitutes the basis of the 

 nervous system. 



