144 



HABIT AND INTELLIGENCE. 



[chap. XII. 



NOTE. 



THE FUNCTIONS OF THB NERVOUS SYSTEM. 



Lewes's 

 theory of 

 sensatiou 



disproved. 



Impossi- 

 ble to say 

 where 

 sensation 

 begins. 



Nerve- 

 fibre may 

 act with- 

 out gan- 

 glionic 

 influence. 



Mr. Lewes, in an admirable treatise on nervous action, wliich 

 forms part of the second volume of his " Physiology," has ad- 

 vanced the opinion that all mutual action of ganglia and nerve- 

 fibres is accompanied by sensation. He thinks the action of the 

 visceral nerves, of which we are unconscious, goes to make up 

 the sense of being alive. This theory is plausible, and it might 

 appear impossible to prove it wrong ; but I think it is disproved 

 by the fact, that cases of blindness have been observed in which, 

 though there is total insensibility to light, the pupil continues 

 to contract in light and expand in darkness.^ There are special 

 nerves for this expansion and contraction ; and the fact just 

 quoted, I think, proves their action to be quite independent of 

 sensation. 



This instance, and many others, though I know of none so 

 striking and conclusive, show how impossible it is to determine 

 where sensation begins ; and if a large part of man's nervous 

 system is insentient, it does not appear anomalous or improbable 

 that the whole nervous systems of the lower worms and moUusca 

 should be so. I believe the most probable guess we can make 

 is, that in the ascending scale of organization sensation begins 

 with the first appearance of organs of special sense ; and the 

 most generally distributed organs of special sense among animals 

 appear to be the eyes. 



It has been often repeated that the ganglia are generators of 

 nervous energy, and the nerve -fibres its conductors. I agree 

 with Mr. Lewes, however, that this account of the matter is, 

 at least, insufficient. A nerve-fibre is capable of acting even 

 when it is not in communication with a gangUon ; as is proved 

 by the well-known experiment of making the cut-off' leg of a frog 

 kick by means of exciting its nerve with electricity, and also in 

 the experiment mentioned in the foregoing chapter, by which 

 Helmholtz has measured the velocity of the nervous current. 



Carpenter's Human Physiology, p. 533. 



