NERVOUS POWER. 3-^)5 



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, especially 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 com- 

 pleting its circulation. 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 qua- 

 lity 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 resolvable into the effects 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 mechani- 

 cally distended, beyond a certain degree, by the 

 presence of contained fluids, or other substances. 



