188 



ON THE EXTERNAL SENSES 



origin of such mischievous and fatal secretions, in some instances thrown 

 forth generally, and in others only from particular organs, as the matter 

 of small-pox, measles, putrid fevers of various kinds, cancers, and hydro- 

 phobia, or the poisonous saliva of mad dogs. 



But the field opens before us to an unbounded extent, and we should 

 lose ourselves in the subject if we were to proceed much farther. It is 

 obvious, that in organic, as in inorganic nature, every thing is accurately 

 arranged upon a principle of mutual adaptation, and regulated by an har- 

 monious antagonism, a system of opposite yet accordant powers, that ba- 

 lance each other with most marvellous nicety ; that increase and diminu- 

 tion, life and death proceed with equal pace ; that foods are poisons, and 

 poisons foods ; and finally, that there is good enough in the world, if 

 rightly improved, to make us happy in our respective stations so long as 

 they are allotted to us, and evil enough to wean us from them by the time 

 the grant of life is usually recalled. 



LECTURE XV. 



ON THE EXTERNAL SENSES OF ANIMAIS. 



The subject of study for the present lecture is the organs of external 

 sense in animals : their origin, structure, position, and powers ; and the 

 diversities they exhibit in different kinds and species. 



The external senses vary in their number : in all the more perfect ani- 

 mals they are five ; and consist in the faculties of sight, smell, hearing, 

 taste, and touch. 



It is by these conveyances that the mind or sensory receives a know- 

 ledge of whatever is passing within or without the system ; and the 

 knowledge it thus gets possession of is called perception. 



The different kinds of perception, therefore, are as numerous as the 

 different channels through which they are received, and they produce an 

 effect upon the sensory which usually remains for a long time after the 

 exciting cause has ceased to operate. This effect, for want of a better 

 term, we call impresdons ; and the particular facts, or things impressed, 

 and of which the impressions retain, as it were, the print or picture, ideas. 



The sensory has a power of suffering this effect or these ideas to re- 

 main latent or unobserved, and of calling them into observation at its op- 

 tion : it is the active exercise of this power that constitutes thought. 



The same constitution, moreover, by which the mind is enabled to take 

 a review of any introduced impression, or to exercise its thought upofl 

 any introduced idea, empowers it to combine such impressions or ideas 

 into every possible modification and variety. And hence arises an en^ 

 tirely new source of knowledge, far more exalted in its nature and infi- 

 nitely more extensive in its range : hence memory and the mental pas- 

 sions ; hence reason, judgment, consciousness, and imagination, which 

 have been correctly and elegantly termed the internal senses, in contra- 

 distinction to those by which we obtain a knowledge of things exterior to 

 the sensorial region. 



Thus far we can proceed safely, and feel our way before us ; but clouds 

 and darkness hang over all beyond, and a gulf unfathomable to the plum- 

 met of mortals. Of the sensory, or mind itself, we know nothing ; we 



