RATE OF MOTION OF ARTERIAL CURRENT. 661 



flow, or jerking escape of the blood, is observed in the most remote 

 arteries, not only in those next to the capillary vessels, but even in 

 the capillaries themselves. The elasticity of the arteries engenders 

 no new force in the circulation, but utilizes that of the heart. Without 

 it, the force of this organ would probably rupture the microscopic 

 arteries, or the capillaries of many delicate structures, and so give rise 

 to internal hemorrhages or apoplexies; such accidents, indeed, occur 

 when the coats of the arteries are converted, by disease or degenera- 

 tion, into more or less rigid tubes. Besides acting in the distension of 

 the coats of the arteries, a certain part of the heart's force is lost, 

 being propagated, by disturbance of those vessels, to the neighboring 

 hard or soft tissues. 



The frequent branchings and bendings, and especially the anasto- 

 moses of the arteries, or their communications with one another, as 

 they approach the organs to which they are distributed, as well as in 

 the interior of those organs, serve to diminish, as well as to equalize, 

 the force of the heart's action. The multiplication of the smallest 

 arteries, and, therefore, of their points of entrance into certain delicate 

 organs, as seen in the ciliary arteries of the eyeball, and in the pia- 

 mater of the brain, must also lessen the pressure upon each of them. 

 Moreover, the frequent anastomoses of the arteries, as in the vicinity 

 of the joints in the limbs, and especially at the base of the brain, serve 

 to secure a due and constant supply of blood to a given part through 

 certain vessels, when others are temporarily obstructed by external or 

 internal pressure, or permanently interrupted by aneurisms, tumors, 

 or accidental division of the ligature of an arterial trunk. Anasto- 

 mosing branches given off above and below the seat of ligature, gradu- 

 ally, or even rapidly, enlarge, forming large collateral vessels, through 

 which the so-called collateral circulation is .carried on. Such enlarge- 

 ment of an artery is due, not to a mere relaxation of its coats, and 

 consequent dilatation, but to an increased nutrition of its walls, by 

 which it undergoes a positive enlargement; in like manner, arteries 

 which are no longer traversed by blood, though, in the first instance, 

 they merely contract, afterwards become reduced in size, by a positive 

 atrophy or absorption of their coats. 



The mpply of blood to a given organ, depends primarily upon the 

 size of the main artery distributed to it ; but secondarily also, upon 

 the rate of motion of the blood through those vessels, which varies, as 

 we shall see, according to many circumstances. But, as previously 

 mentioned, the calibre of the arteries, especially of the smaller ones, 

 is not constant ; for it undergoes changes in accordance with the state 

 of relaxation or contraction of their muscular coat, being sometimes of 

 normal size, and sometimes larger or smaller than usual. The in- 

 creased redness of the cheeks in blushing, or that of irritated and 

 inflamed parts, depends partly upon a temporary change in the calibre 

 of the smallest arteries, which are then manifestly dilated. Hence 

 the supply of blood to a part or organ, may also be regulated by the 

 contractility of its arteries, which is itself controlled by the nervous 

 system. 



The rate of motion of the blood in the arteries has been calculated 



