THE CIRCULATION OF THE BLOOD. 273 



46.; In fainting, the heart ceases to act) It may be owing 

 to various causes, acting on the nervous system, or on the 

 blood-vessels. Mental emotion, loss of blood, or any thing 

 that renders the blood-vessels about the heart less full or 

 tense than usual, will cause a person to faint. This state is 

 soonest relieved bjflying dow^ probably because the action 

 of the heart is sufficient to force the blood along horizontal 

 tubes, but not to raise it in a perpendicular position. Where 

 blood has been lost in sufficient quantity to endanger life, it 

 has been supplied from the veins of another person, by a pro- 

 cess called transfusion. 



Questions. When was the circulation of the blood discovered? 

 and by whom ? Why not discovered before ? Why is the circulation 

 called a vital function ? How is life maintained ? Have all animals 

 a circulation &* Describe the structure of the heart. What is the peri- 

 cardium? In what animals is the heart double ? Describe the cavi- 

 ties of the heart the valves. W T hat is their use ? What are arteries ? 

 How many coats have they f; Describe the venous system. Have the 

 veins valves ?>' What is the capillary system ? Describe the circulation 

 in reptiles worms fishes. What facts did Harvey bring forward to 

 prove the circulation of the blood ? What is the motion of the blood 

 chiefly owing to ? Are the veniricles thicker than the auricles ? Why ? 

 What is said of the velocity of the blood? Of the force of the 

 heart ? Describe the composition of the blood ? What proportion of 

 the body does it constitute ? How long is the blood in performing a 

 revolution ? What is the last part of the body that continues to act ? 

 Why are the arteries found empty after death ? What circulates the 

 blood in the veins 1 What aids in this process ? Does the heart de- 

 pend on the brain for its action ? What facts connected with this sub. 

 ject ? What occurs in fainting ? What position is most favorable in 

 recovering from fainting ? 



