136 HANDBOOK OF PHYSIOLOGY. 



moved. The extent to which the divided extremities of arteries retract 

 is a measure of this tension, not of their elasticity. (Savory.) 



The Muscular Coat. The most important office of the muscular 

 coat is, (1) that of regulating the quantity of blood to be received by 

 each part or organ, and of adjusting it to the requirements of each, ac- 

 cording to various circumstances, but, chiefly, according to the activity 

 with which the functions of each are at different times performed. The 

 amount of work done by each organ of the body varies at different times, 

 and the variations often quickly succeed each other, so that, as in the 

 brain, for example, during sleep and waking, within the same hour a 

 part may be now very active and then inactive. In all its active exer- 

 cise of function, such a part requires a larger supply of blood than is 

 sufficient for it during the times when it is comparatively inactive. It 

 is evident that the heart cannot regulate the supply to each part at dif- 

 ferent periods ; neither could this be regulated by any general and uni- 

 form contraction of the arteries ; but it may be regulated by the power 

 which the arteries of each part have, in their muscular tissue, of con- 

 tracting so as to diminish, and of passively dilating or yielding so as to 

 permit an increase of the supply of blood, according to the requirements 

 of the part to which they are distributed. And thus, while the ventricles 

 of the heart determine the total quantity of blood, to be sent onwards at 

 each contraction, and the force of its propulsion, and while the large 

 and merely elastic arteries distribute it and equalize its stream, the 

 smaller arteries, in addition, regulate and determine, by means of their 

 muscular tissue, the proportion of the whole quantity of blood which 

 shall be distributed to each part. 



It must be remembered, however, that this regulating function of the 

 arteries is itself governed and directed by the nervous system i^see p. 145). 



Another function of the muscular element of the middle coat of ar- 

 teries is (2), to co-operate with the elastic in adapting the calibre of the 

 vessels to the quantity of blood which they contain. For the amount of 

 fluid in the blood-vessels varies very considerably even from hour to hour, 

 and can never be quite constant; and were the elastic tissue only pres- 

 ent, the pressure exercised by the walls of the containing vessels on the 

 contained blood would be sometimes very small, and sometimes inordi- 

 nately great. The presence of a muscular element, however, provides for 

 a certain uniformity in the amount of pressure exercised; and it is by 

 this adaptive, uniform, gentle, muscular contraction, that the normal 

 tone of the blood-vessels is maintained. Deficiency of this tone is the 

 cause of the soft and yielding pulse, and its unnatural excess, of the hard 

 and tense one. 



The elastic and muscular contraction of an artery may also be regarded 

 as fulfilling a natural purpose when (3), the artery being cut, it first 

 limits and then, in conjunction with the coagulated fibrin, arrests the 



