CIRCULATION OF THE BLOOD. 171 



Ventricle of the Heart through the Pulmonary Artery into the Lungs, 

 and thence through the Pulmonary Veins into the Left Auricle, thence 

 into the Left Ventricle." 



. Proofs of the Circulation of the Blood. The following are the 

 main arguments by which Harvey established the fact of the circulation: 



1. The heart in half an hour propels more blood than the whole mass 

 of blood in the body. 



2. The great force and jetting manner with which the blood spurts 

 from an opened artery, such as the carotid, with every beat of the heart. 



3. If true, the normal course of the circulation explains why after 

 death the arteries are commonly found empty and the veins full. 



4. If the large veins near the heart were tied in a fish or snake, the 

 heart became pale, flaccid, and bloodless; on removing the ligature, the 

 blood again flowed into the heart. If the artery were tied, the heart be- 

 came distended; the distension lasting until the ligature was removed. 



5. The evidence to be derived from a ligature round a limb. If it be 

 drawn very tight, no blood can enter the limb, and it becomes pale and 

 cold. If the ligature be somewhat relaxed, blood can enter but cannot 

 leave the limb; hence it becomes swollen and congested. If the ligature 

 be removed, the limb soon regains its natural appearance. 



6. The existence of valves in the veins which only permit the blood 

 to flow toward the heart. 



7. The general constitutional disturbance resulting from the introduc- 

 tion of a poison at a single point, e. g., snake poison. 



To these may now be added many further proofs which have accumu- 

 lated since the time of Harvey, e. g. : 



8. Wounds of arteries and veins. In the former case haemorrhage may 

 be almost stopped by pressure between the heart and the wound, in the 

 latter by pressure beyond the seat of injury. 



9. The direct observation of the passage of blood corpuscles from 

 small arteries through capillaries into veins in all transparent vascular 

 parts, as the mesentery, tongue or web of the frog, the tail or gills of a, 

 tadpole, etc. 



10. The results of injecting certain substances into the blood. 

 Further, it is obvious that the mere fact of the existence of a hollow 



muscular organ (the heart) with valves so arranged as to permit the blood 

 to pass only in one direction, of itself suggests the course of the circula- 

 tion. The only part of the circulation which Harvey could not follow 

 is that through the capillaries, for the simple reason that he had no lenses- 

 sufficiently powerful to enable him to see it. Mafpighi (1661) and Leeu- 

 wenhoek (1668) demonstrated it in the tail of the tadpole and lung of the 

 frog. 



