286 PHYSIOLOGY OF MUSCLES AND NERVES. 



quale irritants^ according to which there is such a 

 sufficient irritant for each sense-nerve, that is, an 

 irritant in its nature adapted to the nature of the 

 sense-nerve, and that this was alone able to excite it. 

 We know now that this is not true. Yet the expres- 

 sion may be used to indicate the irritants which are 

 especially able to act on the terminal organs of the 

 nerves. 



In the same way we may look upon the idea of 

 so-called sjpecific energies of the sense-nerves, if by 

 this it is intended to express any character of the 

 nerves, as disproved-. But we must ascribe specific 

 energies to the individual nerve-cells in which the sen- 

 sations are originated. It is these alone which are 

 able to produce in us dififerent kinds of sensation. If 

 all the nerve-cells of the sensations were alike, sensa- 

 tions could indeed be elicited in us by the influence 

 of the outer world on our sense organs ; but these 

 would only be of one and the same kind, or at most it 

 could only be in the strength of this one undefined 

 sensation that differences would be perceptible. There 

 may be animals which are only capable of such a single 

 undefined sensation, their nerve-cells being all alike 

 and not yet differentiated. Such animals would be 

 able to form a conception of the outer world as 

 distinguished from their own bodies, that is, they 

 would be able to evolve self-consciousness ; but they 

 would not be able to attain a knowledge of the pro- 

 cesses in the outer world. The development of such 

 knowledge in us is greatly assisted by a comparison 

 of the different impressions brought about by the 

 different organs of the senses. A body presents itself 

 to our eye as occupying a certain space, being of a 



