46 INTRODUCTION. 



of the body; behincl the intestines, txvo nervous branches es- 

 tablish a communication with the various ganglia, and em- 

 brace in their lateral extension, the viscera; other filaments 

 are distributed to the different parts of the body. In the mol- 

 lusca provided with a head, there is a nervous enlargement or 

 a principal medullary mass called brain, situated across and 

 over the aesophagus which it envelops with a nervous ring, 

 which terminates underneath in another but larger ganglion: 

 these enlargements send filaments to the head and to the va- 

 rious parts of the viscera. In some of them, there are besides 

 other small ganglia. The cephalopoda alone have their brain 

 enveloped in a kind of cartilaginous cranium. 



The general character of the nervous system of the inver- 

 tebrate animals, particularly consists in the dissemination of 

 the nervous centres, and in the circumstance that the parts 

 either external or internal, or those which belong to the vege- 

 tative functions, or those which belong to the animal func- 

 tions, receive their nervous filaments from the same centres. 

 We shall see, on the contrary, that in the vertebrata the ner- 

 vous system is differently disposed, and in a manner which 

 entirely distinguish them from other animals. 



30. Nervous action or innervation, presents in animals 

 varieties corresponding to those which are observed in the 

 disposition of the nervous organs. In 1 those animals which 

 have no nervous system, and in those in which this system has 

 no centre, (the radiated animals,) impressions are immediate- 

 ly followed by movements; a part or an animal is called irri- 

 table whose movements are produced by impressions. In the 

 radiated animals the mouth or the orifice through which they 

 take their nourishment is the most irritable part; it is also at 

 this place the nervous system begins to appear in animals of 

 this class provided with it. All other animals have also irri- 

 table parts. In the mollusca and in the insecta in which the 

 divers ganglia of the nervous system are connected with each 

 other by nervous cords, in such a manner as to form a centre, 

 and in which there are organs of a special sensation, the im- 

 pressions received by the senses produce sensations, and the 

 movements are caused by volition. The internal movements, 



