CHAPTER XX 



THE CIRCULATION OF THE BLOOD 



WE have now to approach the physiological side of the subject, 

 and study the means by which the blood is kept in movement, so 

 that it may convey nutriment to all parts, and remove from those 

 parts the waste products of their activity. 



Previous to the time of Harvey, the vaguest notions prevailed 

 regarding the use and movements of the blood. The arteries were 

 supposed by some to contain air, by others to contain a more subtle 

 essence called animal spirits; the animal spirits were supposed to 

 start from the ventricles of the brain, and they were controlled by 

 the soul which was situated in the pineal gland. How the animal 

 spirits got into the arteries was an anatomical detail which was 

 bridged across by the imagination. 



There was an idea that the blood moved, but this was considered 

 to be a haphazard, to-and-fro movement, and confined to the veins. 

 The proofs that the movement is in a -circle were discovered by 

 William Harvey, and to this eminent discoverer also belongs the 

 credit of pointing out the methods by which every physiological 

 problem must be studied. In the first place there must be correct 

 anatomical knowledge, and in the second there must be experiment, 

 by which deductions from structure can be tested; moreover, this 

 second method is by far the more important of the two. Harvey's 

 proofs of the circulation came under both these heads. The structural 

 or anatomical facts upon which he relied were the following : 



1. The existence of two distinct sets of tubes in connection with 

 the heart, namely, the arteries and the veins. 



2. The existence in the heart and also in the veins, of valves 

 which would only allow the passage of the blood in one direction. 



His experimental data were the following: 



3. That the blood spurts with great force and in a jerky manner 

 from an artery opened during life, each jerk corresponding with a 

 beat of the heart. 



4. That if the large veins near the heart are tied, the heart 



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