NERVOUS POWER. .'519 



comprised under the former class of functions in the 

 higher animals, will convince us that they must be 

 the result of the combined operation of several dif- 

 ferent agents ; but the principal source of mechani- 

 cal force required by the vital organs, is still, as in 

 all other cases, the muscular power. The coats of 

 the stomach and of the intestinal tube contain a 

 large proportion of muscular fibres, the contractions 

 of which effect the intermixture and propulsion of 

 the contents of these cavities, in the manner best 

 calculated to favour the chemical operations to 

 which they are to be subjected, and to extract from 

 them all the nourishment they may contain. In 

 like manner, all the tubular vessels which transmit 

 fluids, are endowed with muscular powers adapted 

 to the performance of that office. The heart is a 

 strong hollow muscle, with power adequate to propel 

 the blood with immense force, through the arterial 

 and venous systems. The blood-vessels, also, es- 

 pecially the minute, or capillary arteries, besides 

 being elastic, are likewise endowed with muscular 

 power, which contributes its share in forwarding 

 the motion of the blood, and completing its circu- 

 lation. The quantity of blood circulating in each 

 part, the velocity of its motion, and the heat which 

 it evolves, are regulated 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 respective destinations, is also re- 

 solvible into the effects of a muscular power. 

 The immediate cause which, in these organs. 



