226 THE CIRCULATION OF THE BLOOD. [CH. xix. 



CHAPTER XIX. 



THE CIRCULATION OF THE BLOOD. 



IN the preceding chapter, we have seen what the course of the 

 circulation is, and we have devoted considerable space to a study 

 of the structure of the heart and vessels. We have now to 

 approach the more strictly 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 con- 

 sidered to be a haphazard, to and fro movement, and confined to 

 the veins. The proofs that the movement is a movement 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 one of these, the veins, of valves which 

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



His experimental facts 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 



