First lines of physiology, being an introduction to the science of life; written in popular language . VALVES OF THE VEINS. 97 nourish. In the higherorders of animals, it isa very strong hollownnuscle, designed to re-ceive a small quantityof blood at a time, andthen, by contracting, tourge that quantity on-ward. The receptacle,when constructed inthis manner, is calleda heart; and the beat-ing of the heart, as itis called, is producedby the motion of thismost important organin pumping its con-tents. 186. When the bloodfrom the veins has filledthe heart or the greatvessel that answers the same p
First lines of physiology, being an introduction to the science of life; written in popular language . VALVES OF THE VEINS. 97 nourish. In the higherorders of animals, it isa very strong hollownnuscle, designed to re-ceive a small quantityof blood at a time, andthen, by contracting, tourge that quantity on-ward. The receptacle,when constructed inthis manner, is calleda heart; and the beat-ing of the heart, as itis called, is producedby the motion of thismost important organin pumping its con-tents. 186. When the bloodfrom the veins has filledthe heart or the greatvessel that answers the same purpose, it is ne-cessary that it shouldbe conducted through. A Vein laid open to show the Valves. a. The trunk of the vein ; h, b, the valves;c, a branch of the vein entering it. another set of channels to all parts of the body, and intothe substance of every organ, in order to nourish it. Forthis purpose another set of blood-vessels, called the arte-ries, is provided. One or more great arteries originatefrom the heart, and pursue their course toward the ex-tremities. Each artery soon branches into two or moretrunks, and each trunk is again and again divided, until atlength the number of branches exce-eds all calculation;and there are few parts of the body into which a pin canpenetrate without wounding one. For a general idea ofthe distribution of the arteries in man, you may refer tothe view of the arterial system as represented in fig. The current of blood produced by the action ofthe heart is very rapid; and you are not to suppose thatany part of the body employs all the blood which is sent 98 THE VASCULAR SYSTEM. to it for its growt
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