208 ANIMAL AUTOMATISM. [LECT. 



the brain, as our philosophers commonly suppose ; or, at least, 

 we must conceive these images to be something quite different 

 from what they suppose them to be. For, as all they suppose is 

 that these images ought to resemble the objects which they re- 

 present, it is impossible for them to show how they can be 

 formed by the objects received by the organs of the external 

 senses and transmitted to the brain. And they have had no 

 reason for supposing the existence of these images except this ; 

 seeing that the mind is readily excited by a picture to conceive 

 the object which is depicted, they have thought that it must be 

 excited in the same way to conceive those objects which affect 

 our senses by little pictures of them formed in the head ; instead 

 of which we ought to recollect that there are many things be- 

 sides images which may excite the mind, as, for example, signs 

 and words, which have not the least resemblance to the objects 

 which they signify." 1 



Modern physiology amends Descartes' conception 

 of the mode of action of sensory nerves in detail, by 

 showing that their structure is the same as that of 

 motor nerves ; and that the changes which take place 

 in them, when the sensory organs with which they 

 are connected are excited, are of just the same nature 

 as those which occur in motor nerves, when the 

 muscles to which they are distributed are made to 

 contract: there is a molecular change which, in the case 

 of the sensory nerve, is propagated towards the brain. 

 But the great fact insisted upon by Descartes, that no 

 likeness of external things is, or can be, transmitted to 

 the mind by the sensory organs ; on the contrary, that, 



1 Locke (Human Understanding, Book II., chap. viii. 37) uses Des- 

 cartes' illustration for the same purpose, and warns us that " most of 

 the ideas of sensation are no more the likeness of something existing 

 without us than the names that stand for them are the likeness of our 

 ideas, which yet, upon hearing, they are apt to excite in us," a declara- 

 tion which paved the way for Berkeley. 



