231 



CHAPTER VII. 



OF SENSATIONS. 





CXIII. WE have already seen, how the human body, essentially change- 

 able and perishable, maintains itself in its natural economy, carries on 

 its growth, and supplies its decay, by assimilating to its own substance, 

 principles that are yielded to it by the food it digests, and by the air it 

 breathes. We shall now proceed to examine, by what organs man is 

 enabled to keep up, with all nature, the relations on which his existence 

 depends; by what means he is made aware of the presence of objects 

 which concern him, what means he possesses to fit his connexion with 

 them to his welfare, to draw them towards him or to repel them, to ap- 

 proach or to avoid and escape them, as he perceives in them danger, or 

 the promise of enjoyment. 



Man possesses, in all its plenitude, this new mode of existence, which 

 is denied to vegetable Nature. Of all the animals it is he that receives 

 impressions the most crowded and various, that is most filled with sen- 

 sations and that employs :hem, with the most powerful combination, as 

 the materials of thought, and the sources of intelligence : he is the best 

 organized for feeling the action of all beings around him, and re-acting 

 on them in his turn. In the study which we are about to undertake, we 

 shall see many instruments placed on the limits of existence, on the sur- 

 face of the living being ready to receive every impression; conductors, 

 stretching from these in s~.ru ments to one common centre, to which all is 

 carried; conductors through which this central organ regulates the ac- 

 tions, which now transport the whole body from one place to another 

 (locomotion} ; now merely change the relative situation of its parrs (fiar- 

 tial motion); and, at othe* times, produce, in the organs, certain disposi- 

 tions, of which speech and language, in their various forms, are the 

 result. 



CXIV. If we are thoroughly to understand the mechanism of this ac- 

 tion of outward objects on our body, we must follow the natural succes- 

 sion of the phenomena of sensation; studying first, the bodies which pro- 

 duce the sensitive impression, examining next, the organs that receive 

 it, and next the conductors which transmit it to a particular centre, whose 

 office is perception. To take the sense of sight, for instance, we can 

 never understand how it is that light procures us the knowledge of cer- 

 tain qualities of bodies, if we have not learnt the laws to which that fluid 

 is subjected, if we know nothing of the conformation of the eyes, of the 

 nerves by which those organs communicate with the brain, and of the 

 brain itself, whither all sensations, or rather the motions in which they 

 consist, are ultimately carried. 



CXV. Of light. At this day, the greater part of Natural Philosophers 

 consider it as a fluid impalpable from its exceeding tenuity. Many be- 

 lieve it to be only a modification of caloric, or of the matter of heat ; and 

 this last opiniou has received much plausibility from the late observa- 



