THE FLOW OF BLOOD IN THE BODY l8l 



forced toward the back part of the body through the 

 lymph spaces until it again reaches the tube. Their cir- 

 culation is thus like the circulation of lymph in man. 



Shellfish usually possess a heart and arteries and veins. 

 In the very lowest animals, like the ameba, there seems to- 

 be a flow of fluid within the body, but no part of the body 

 is set aside for the purpose. 



315. History of the knowledge of the circulation. The 



ancients thought the heart was the seat of life, because the heart was 

 seen to be the first organ formed in an egg which was being hatched. 

 The idea was confirmed to them by the heart's constant action, which 

 they thought was caused by the boiling of the animal spirits. The 

 spirits then flowed away in a sluggish stream through the veins, and 

 were not supposed to return to the heart. 



They concluded that the arteries carried only air, because they always 

 found them empty after death. They knew nothing whatever of the 

 capillaries. They thought that food was carried to the liver and was 

 there partly cooked, and was then sent on to the heart where it was 

 cooked still further in the heart's vital flame, until it was turned to 

 blood. Then it was sent out by way of the veins to irrigate the body. 

 The valves of the veins were supposed to oppose its flow and to render 

 it sluggish. The boiling in the heart was supposed to heave the chest 

 up and down, and cause air to rush in and prevent too great a degree 

 of heat. The brain also was supposed to cool the blood. Because of 

 its more violent action during physical exertion or emotion, they con- 

 cluded that the heart, instead of the brain, was the seat of the mind 

 and feelings. We still use the word heart with this meaning in such 

 expressions as kind-hear ted2c&& free-hearted. 



Incredibly few discoveries were made for thousands of years, for 

 until within two hundred years the law forbade any one to dissect a 

 human body. In 1628 a true explanation of the heart and the course 

 of the blood was first published by Harvey, an English physician. 

 The only point which he omitted was the explanation of how the blood 

 gets from the arteries to the veins. Three years after his death micro- 

 scopes were made powerful enough to reveal the capillaries for the 

 first time, and thus the truth of our present ideas concerning the cir- 

 culation was fully established. 



