LETTERS TO BROTHER JOHN. 273 



organs, and that the degree of sensibility depends 

 upon the degree of mobility of the nerves. I have 

 also shewn you how this mobility, and consequently 

 our sensibility, is increased by a languid circulation, 

 and how it is diminished by a vigorous one by 

 which the blood is driven energetically into all the 

 capillary vessels, causing their coats to be well 

 distended, so as to exert a constant and steady 

 lateral pressure upon the nerves which run between 

 them. 



The circulation, therefore, is a resisting power 

 a power directly opposed to sensibility. And this 

 resisting power may be carried so far, as to produce 

 absolute insensibility; as in those cases of apoplexy, 

 in young, athletic, healthy men, which is the result 

 of plethora that is, of having too mv.ch blood, and 

 too powerful a circulation. 



The circulating and sensitive, therefore, are two 

 antagonizing powers. And, as sensibility is the 

 power by which we receive the consciousness of 

 impressions, so the sanguineous circulation is the 

 power by which we resist the consciousness of im- 

 pressions. 



Now, the same impressions will produce both 

 pleasure and pain ; the difference being only in the 

 force of the impressing cause. The same impressing 

 N 3 



