THE CIRCULATION OF TIE BLOOD. 165 



large plexiform arrangement of the veins, and such arteries as may ad- 

 mit, upon occasion, augmented quantities of blood. 



(3, 4, 5.) The circulation in the Lungs, Liver, and Kidneys will be 

 described under their respective heads. 



Agents concerned in the circulation. 



Before quitting the subject of the circulation it will be as well to 

 bring together in a tabular form the various agencies concerned in 

 maintaining the circulation. 



1. The Systole and Diastole of the Heart, the former pumping into 

 the aorta and so into the arterial system a certain amount of blood, and 

 the latter to some extent sucking in the blood from the veins. 



2. The elastic and muscular coats of the arteries, which serve to keep 

 up an equable and continuous stream. 



3. The so-called vital capillary force. 



4. The pressure of the muscles on veins with valves, and the slight 

 rhythmic contraction of the veins. 



5. Aspiration of the thorax during inspiration, by means of which 

 the blood is drawn from the large veins into the thorax (to be treated of 

 in next Chapter). 



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 

 became distended; the distention 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 can- 

 not 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 towards the heart. 



7. The general constitutional disturbance resulting from the intro- 

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



