NERVOUS POWER. 253 



in a great measure by the particular mode of action of the • 

 blood vessels of that part. The quantity, and sometimes 

 even the quality of the secretions, are dependent, in like 

 manner, on the conditions of the circulation; and the action 

 of the ducts, which convey the secreted fluids to their re- 

 spective destinations, is also resolvable into the cflfccts of a 

 muscular power. 



The immediate cause which, in these organs, excites the 

 muscular fibre to contraction, may frequently be traced to 

 the forcible stretching of its parts. This is the case in all 

 hollow and tubular muscles, such as the stomach, the heart, 

 and the blood vessels, when they are mechanically distended, 

 beyond a certain degree, by the presence of contained fluids, 

 or other substances. At other times, the chemical quality 

 of their contents appears to be the immediate stimulus in- 

 citing them to contraction. But numerous instances occur, 

 in the higher orders of animals, in which these causes alone 

 are inadequate to explain the phenomena of the vital func- 

 tions. No mechanical hypothesis will suffice to account for 

 the infinite diversity in the modes of action of the organs 

 which perform these functions, or aflford any clew to the 

 means by which they are made to co-operate, with such 

 nicety of adjustment, in the production of the ultimate ef- 

 fect. Still less will any theory, comprising only the agency 

 of the muscular power, and the ordinary chemical affinities, 

 enable us to explain how an irritating cause, applied at one 

 part, shall produce its visible efiects on a distant organ; or 

 in what way remote and apparently unconnected parts shall, 

 as if by an invisible sympathy, be brought, at the same mo- 

 ment, to act in concert, in the production of a common ef- 

 fect. Yet such co-operation must, in innumerable cases, be 

 absolutely indispensable to the perfect accomplishment of 

 the vital functions of animals. 



Nature has not neglected objects so important to the suc- 

 cess of her measures, but has provided, for the accomplish- 

 ment of these purposes, a controlling faculty, residing in the 

 nervous system, and denominated the 7iervous power. Ex- 



