RAN Z AN I ON SENSATION 



223 



nerves appear they are not isolated and dispersed, but communicating 

 and united together, forming an assemblage, which is called the 

 nervous system. This communication, and this re-union of the nerves 

 should persuade us that those persons are in error who pretend that 

 the nervous system in animals has no centre of this kind. It is not 

 besides necessary that the centre, thus spoken of, should be a mere 

 spot, and we have reason to believe that it has an extension, which it 

 is not in our power to determine. Nor would it be reasonable in us to 

 indicate with precision the situation of such a centre in the absence of 

 those observations and those experiments which alone could establish 

 the fact, and which, perhaps, we shall never be able to accomplish. 

 The nervous system- in different animals presents great differences, of 

 which there are four principal, and the remainder may be considered as 

 varieties. 



In more perfect animals the nervous system is composed of a pri- 

 mary mass, called the brain, of a large trunk called the spinal marrow ; 

 then of filaments which ramify, and sometimes closely unite, to form 

 those minor nervous masses which have obtained the name of nerve- 

 knots or ganglia. 



In other animals the nervous system is composed of scattered 

 nervous masses, connected by filaments, one of which, situated above 

 the gullet (oesophagus), is called the brain, and out of this proceeds 

 a collar which surrounds the gullet. Other filaments which ramify and 

 distribute themselves to the parts, issue as well from the brain as from 

 the other nervous masses. 



There are, besides, animals over the belly of which run two longitu- 

 dinal nervous cords, which frequently swell out, and sometimes form 

 nodes or ganglia, distributed in a regular row. Of such nodes, that 

 which remains upon the gullet is denominated the brain. From the 

 same part a collar which encompasses the gullet, and the nerves des- 

 tined for the different parts, as well from this first node as from the 

 subsequent ones, derive their origin. 



Finally, in animals whose structure is tolerably simple, the nervous 

 system consists of filaments without ganglia and without brain ; one 

 of these filaments surrounds the mouth, and from it again proceed 

 others like rags, and ramify and distribute themselves to the different 

 parts. 



Such is the principal division which we are, at present, able to make 

 of the different forms of the nervous system. It is not improbable 

 that, as discoveries advance, we shall afterwards have occasion to 



