320 THE VITAL FUNCTIONS. 



excites the muscular fibre to contraction, may fre- 

 quently 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 dis- 

 tended, beyond a certain degree, by the presence of 

 contained fluids, or other substances. At other 

 times, the chemical quality of their contents ap- 

 pears to be the immediate stimulus inciting 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 

 functions. No mechanical hypothesis will suffice 

 to account for the infinite diversity in the modes of 

 action of the organs which perform these functions, 

 oc aftbrd any clue to the means by which they are 

 made to co-operate, with such nicety of adjustment, 

 in the production of the ultimate effects. Still less 

 will any theory, comprising only the agency of the 

 muscular power, and the ordinary chemical affini- 

 ties, enable us to explain, how an irritating cause, 

 applied at one part, shall produce its visible effects 

 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 moment 

 to act in concert, in the production of a common 

 effect. Yet such co-operation must, in innumerable 

 cases, be absolutely indispensable to the perfect ac- 

 complishment of the vital functions of animals. 



Nature has not neglected objects so important to 

 the success of her measures ; but has provided, for 

 the accomplishment of these purposes, a controlling 

 faculty, residing in the nervous system, and deno- 

 minated the 7iervous power. Experiments have 



