CHAPTER VIII 

 A STUDY OF THE CIRCULATION OF BLOOD 



Definition of the Circulation We have seen in the pre- 

 ceding pages that the blood takes up oxygen in the lungs, 

 that it absorbs food materials while coursing through the 

 villi of the intestines, and that it loses waste materials in 

 the excretory organs. It is evident, therefore, that this 

 important liquid must be kept moving from one organ to 

 another. By the term circulation of the blood is meant the 

 ceaseless movement of the blood through a system of tubes 

 called blood vessels. 



Organs of Circulation. The force that drives the blood 

 around through the body is furnished by the contraction of 

 the muscular walls of the heart. Any blood vessel that 

 carries blood away from the heart is called an ar'te-ry (Greek 

 aer = air -+- terein = to hold, a name which was given by 

 the early anatomists to those tubes because they were found 

 empty after death, and were therefore supposed to carry 

 air). TJie veins are the blood vessels that bring the blood back 

 to the heart. Connecting the arteries and the veins in every 

 part of the body are countless microscopic blood vessels called 

 cap'il-lar-ies (Latin eapi#tt*=hair, so called from their minute 

 size). 



1. THE HEART 1 



Position, Shape, Size. If any one closes upon the palm 

 the fingers of his right hand and places his fist in the middle 

 of his chest in such a way that his thumb points obliquely 



1 See "Laboratory Exercises," No. 27. 

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