34 CIRCULATION OF THE BLOOD. 



2. The continuous passage of the. blood from the respiratory 

 apparatus toward* all the organs of the body, and the return of 

 the blood from these organs to the apparatus of respiration, con- 

 stitute* the phenomenon of ihr circulation. 



3. This liquid, as we have seen, moves continually in a sort of 

 circle ; after having traversed all the parts which it is destined to 

 nourish, it returns to a particular organ to come in contact with 

 the air, then goes back to the parts whence it came, passes through 

 them, returns again to the apparatus of respiration, and so con 

 tinues as long as life endures. 



4. THE APPARATUS OP THE CIRCULATION, that is to say, the as- 

 semblage of organs destined to effect this conveyance or trans- 

 portation of the blood, is composed : 



FIRST. Of canal*., or pipes, in which the blood runs. 

 SECOND. Of t/te heart which serres to set it, in motion. 



5. The heart is the centre of the apparatus of the circulation ; 

 it is a sort of fleshy pouch communicating with the blood vessels, 

 which receiving the blood into its interior, and which, by contrac- 

 ting on itself from time to time, forces this fluid into the canals, 

 and thus keeps up a continual current in them. 



6. Almost all animals have a heart. This organ exists not 

 only in the mammalia, birds, reptiles, and fishes, but also in snails, 

 oysters, and other animals of the class of mollusca ; in crabs and 

 lobsters ; in spiders, &c. 



7. The blood vessels are of two kinds, namely: 



8. *st. The arteries which carry the blood from the heart to 

 all parts of the body. 



9. 2nd. The veins which bring back this liquid from all parts 

 of the body to the heart. 



1 0. The arteries spring from the heart and divide into branches 

 which decrease in t>izr t and increase in number as they advance, 

 and are distributed to the very numerous parts, distant from the 

 heart. 



2. What constitutes the phenomenon of the circulation ? 



3. After having traversed all the parts it is destined to nourish, wha 

 becomes of the blood ? 



4. Wh it is meant by the apparatus of the circulation ? Of what part* 

 is it composed ? 



5 What is a heart ? 



6. Have all animals a heart ? 



7. How many kinds of blood ves els are there ? 

 8 What is the func'ion of the arteries ? 



9. What is the function of the veins T 

 10. Where do the arteries take their rise? How are they distributed T 



