260 THE SENSORIAL FUNCTIONS. 



into final causes be at once abandoned as utterly vain and 

 hopeless. But it surely requires no laboured refutation to 

 overturn a system that violates every analogy by which our 

 reasonings on these subjects must necessarily be guided; and 

 no artificial logic or scholastic syllogisms will long prevail 

 over the natural sentiment, which must ever guide our con- 

 duct, that animals possess powers of feeling, and of sponta- 

 neous action, and faculties appertaining to those of intellect. 



The functions of sensation, perception, and voluntary mo- 

 tion require the presence of an animal substance, which we 

 find to be organized in a peculiar manner, and endowed with 

 very remarkable properties. It is called the medullary sub- 

 stance; and it composes the greater part of the texture of 

 the brain, spinal marrow, and nerves; organs, of which the 

 assemblage is known by the general name of the nervous 

 system. Certain affections of particular portions of this me- 

 dullary substance, generally occupying some central situa- 

 tion, are, in a way that is totally inexplicable, connected with 

 affections of the sentient and intelligent principle; a princi- 

 ple which we cannot any otherwise conceive than as being 

 distinct from matter; although we know that it is capa- 

 ble of being affected by matter operating through the me- 

 dium of this nervous substance, and that it is capable of 

 reacting upon matter through the same medium. Of the 

 truth of these propositions there exist abundant proofs; 

 but as the evidence which establishes them will more con- 

 veniently come under our notice at a subsequent period of 

 our inquiry, I shall postpone their consideration; and pro- 

 ceeding upon the assumption that this connexion exists, shall 

 next inquire into the nature of the intervening steps in the 

 process, of which sensation and perception are the results. 



Designating, then, by the name of brain this primary and 

 essential organ of sensation, or the organ whose physical af- 

 fections are immediately attended by that change in the 

 percipient being which we term sensation; let us first in- 

 quire what scheme has been devised for enabling the brain 

 to receive impressions from such external objects, as it is 



