38 



ZOOLOGY 



SECT. 



The central parts of the nervous system consist (Fig. 26) of 

 certain aggregations of nerve-matter known as ' nerve-ganglia, 

 containing a large number of nerve-cells ; a relatively large mass 



of this matter may be 

 collected together to 

 form a brain. To or 

 from these central parts 

 pass all the systems of 

 nerve-fibres, constituting 

 the peripheral part of the 

 system ; the former have 

 the office both of re- 

 ceiving impressions con- 

 veyed by the nerve-fibres 

 from the surface, from 

 the organs of special 

 sense, and from the in- 

 ternal organs, and of 

 sending off messages 

 through similar channels 

 to the various parts of 

 the body to muscles, to 

 glands, to alimentary 

 canal, and to vascular 

 system. When a move- 

 ment is to be effected 

 a message passes from 

 the nerve-centre along a 

 nerve-fibre to a muscle 

 and causes it to contract ; 

 when an organ requires 

 the amount of blood sup- 

 plied to it to be in- 

 creased or diminished a 

 message is conveyed 

 along a nerve-fibre and 

 causes the dilatation or 

 contraction of the blood- 

 vessels of the part; and 

 a similar initiatory or 

 controlling influence is 

 exerted over the activities 

 of all the organs. 



In certain groups of animals all the impressions from the 

 external world are received through the integument of the general 

 surface, and this is the case in all animals with the general 

 impressions of touch and of heat; and cold. The sensitiveness of 



FIG. 26. Nervous system of the Frog. (From 

 Howes's Atlas.) 



