THE CIRCULATION OP THE BLOOD. 263 



auricles, which receive arterial blood from the lungs, and 

 venous blood from the body, and in its cavity both are 

 mixed together. I The main artery going from the heart, di- 

 vides into two branches, one of which goes to the lungs, the 

 other is distributed throughout the body. In some of the 

 other classes of animals, such as icorm^ there is no heart, 

 and the circulation consists in the passage of the blood from 

 the surface, where are seated the organs of respiration, to 

 all parts of the animal and back again, which is performed 

 exclusively by vessels^ I Fishes have a single heart, designed 

 only to circulate venous blood, which it conveys to the gills, 

 or lungs ; and from thence it is carried to every part of the 

 body, by an artery which rises from the gills themselves^ 

 Shell-fish have also a single heart, but this only circulates 

 arterial blood. 



17. The following facts were brought forward by Harvey 

 to prove the circulation of the blood. \If the chest of a 

 cold-blooded animal be opened, the heart will be seen dilating 

 and contractingf The valves are so situated throughout the 

 circulating system, as to promote the circulation. Between 

 the auricles and ventricles they are so placed as to allow the 

 blood to pass freely from the former into the latter, and to 

 prevent its return. The valves placed at the orifice of the 

 arteries, permits the blood to enter them from the ventricles, 

 but prevents it getting back into the heart. The valves in 

 the veins allow the blood to go towards the heart, but pre- 

 vent it from going in the other direction. 



18. In bleeding, a bandage or ligature is placed round the 

 arm, above the point where the arm is to be opened. The 

 blood, in its return toward the heart, is interrupted, but the 

 artery which carries the blood to it is not compressed, be- 

 cause it lies deeper. Sometimes, however, the ligature is so 

 tight that the blood will not flow till it has been loosened. 

 If the vein be opened below the ligature it will bleed freely, 

 but no blood is obtained if it be opened above. 



19. When a limb is amputated by a surgeon, he only ties 





