122 THE CIRCULATION. 



pathetic system supplied to the organs nourished by these same 

 arteries. Thus, any condition in these organs which causes 

 them to need a different amount of blood, whether more or 

 less, produces a certain impression on their nerves, and by 

 these the impression is carried to the ganglia, and thence re- 

 flected along the nerves which supply the arteries. The mus- 

 cular element of these vessels responds in obedience to the 

 impression conveyed to it by the nerves ; and, according to its 

 contraction or dilatation, is a larger or smaller quantity of 

 blood allowed to pass. 



Another function of the muscular element of the middle 

 coat of arteries is, doubtless, 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 

 present, the pressure exercised by the walls of the containing 

 vessels on the contained blood would be sometimes very small, 

 and sometimes inordinately great. The presence of a muscu- 

 lar element, however, provides for a certain uniformity in the 

 amount of pressure exercised ; and it is by this adaptive, uni- 

 form, gentle, muscular contraction, that the 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, the artery 

 being cut, it first limits and then, in conjunction with the coag- 

 ulated fibrin, arrests the escape of blood. It is only in conse- 

 quence of such contraction and coagulation that we are free 

 from danger through even very slight wounds ; for it is only 

 when the artery is closed that the processes for the more perma- 

 nent and secure prevention of bleeding are established. 



Mr. Savory has shown that the natural state of all arteries, 

 in regard at least to their length, is one of tension that they 

 are always more or less stretched, and ever ready to recoil by 

 virtue of their elasticity, whenever the opposing force is re- 

 moved. The extent to which the divided extremities of ar- 

 teries retract is a measure of this tension, not of their elasticity. 



From what has been said in the preceding pages, it appears 

 that the office of the arteries in the circulation is, 1st, the 

 conveyance and distribution of blood to the several parts of 

 the body ; 2d, the equalization of the current, and the con- 

 version of the pulsatile jetting movement given to the blood 

 by the ventricles, into a uniform flow ; 3d, the regulation of 



