CIRCULATION OF THE BLOOD. 141 



ing force is removed. The extent to which the divided extremities of 

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



Functions of 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, according 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 suffi- 

 cient for it during the times when it is comparatively inactive. It is evi 

 dent that the heart cannot regulate the supply to each part at different 

 periods; neither could this be regulated by any general and uniform con- 

 traction of the arteries; but it may be regulated by the power which the 

 arteries of each part have, in^ their muscular tissue, of contracting so as 

 to diminish, and of passively dilating or yielding so as to permit an in- 

 crease 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 onward at each contrac- 

 tion, and the force of its propulsion, and while the large and merely elastic 

 arteries distribute it and equalize its stream, the smaller arteries, in addi- 

 tion, regulate and determine, by means of their muscular tissue, the propor- 

 tion 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 (vaso-motor 

 centres and fibres). 



Another function of the muscular element of the middle coat of arteries 

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

 sels 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 present, 

 the pressure exercis'ed by the walls of the containing vessels on the con- 

 tained blood would be sometimes very small, and sometimes inordinately 

 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 escape of 

 blood. It is only in consequence of such contraction and coagulation that 



