CHAP. I.] OF A NERVOUS SYSTEM. 23 



These bodies are interposed so as to constitute part of the 

 actual path of the stimulus wave, and accordingly, they 

 may be, in effect, junctions for ingoing impressions or 

 dividing stations for out-going impressions. The matter 

 composing them seems to be endowed with extreme mole- 

 cular mobility. It is owing to the multitudinous com- 

 binations of these bodies with one another, and with 

 ingoing and outgoing fibres, in modes which will be 

 sketched in the next chapter, that the complex work of 

 the nervous system is enabled to be carried on. 



Nerve tissue, in the lower forms of animal life, is 

 essentially subservient to the bringing about of move- 

 ments in more or less immediate response to external 

 shocks or other localized impressions, or of movements 

 and glandular activity as a result of impressions upon 

 internal surfaces. These various movements gradually 

 become more definitely related and appropriate as 

 responses, in proportion as the organism becomes better 

 able to discriminate the differences between the several 

 kinds of impressions made upon different parts of its 

 surface. 



Even amongst Medusae definite responses to stimuli 

 are occasionally met with. Thus in the hemispherical 

 Tiaropsis, from the inside of which hangs a long funnel- 

 like body or polypite, this structure, as Romanes says, is 

 found to be capable of " localizing with the utmost pre- 

 cision any point of stimulation situated in the bell. For 

 instance, if the bell be pricked with a needle at any 

 point, the polypite immediately moves over and touches 

 that point. ... If immediately afterwards any other 

 part of the bell be pricked, the polypite moves over to 

 that part, and so on." From this it may be concluded 

 " that all parts of the bell must be pervaded by lines oi 

 discharge, every one of which is capable of conveying a 



