260 EXPERIMENTAL GENERAL SCIENCE 



the blood of the lower animals, however, the corpuscles are 

 nucleated. There are also to be found in the blood certain 

 larger globular cells called white blood corpuscles. These are 

 greatly outnumbered by the red corpuscles, often as much as 

 300 to 1. The blood is contained in a closed system of tubes 

 called blood vessels and is kept in constant motion through the 

 body by a sort of double pump, the heart. The tubes which 

 carry the blood away from the heart are known as arteries, 

 and those which return it to the heart are veins. The blood 

 leaves the heart for its tour of the body through a single large 

 artery, the aorta. This soon branches into many divisions 

 which go to all parts of the body. As they subdivide they be- 

 come smaller and smaller until finally the corpuscles, minute as 

 they are, have to squeeze to get through them. These very 

 small tubes are called capillaries. From the capillaries, the 

 blood flows into somewhat larger tubes, called veins, and these 

 flow into still larger ones until two main veins return the blood 

 to the heart. One may get an idea of how very numerous the 

 capillaries are by reflecting that the slightest break in the 

 flesh will injure the capillaries and allow some of the blood to 

 run out. The arteries are usually deep in the flesh and rarely 

 come to the surface except where they cross a joint, but many 

 of the veins are nearer the surface and may be seen through the 

 skin. 



221. Absorption. Water and mineral salts may be absorbed 

 from any part of the alimentary canal, but the bulk of the 

 food is absorbed by the blood vessels of the small intestine. 

 The walls of the intestine are abundantly supplied with blood 

 vessels and the food passes into them by osmosis (102). 

 In order to pass into the blood in this way, starch has to be 

 changed to sugar and other sugars have to be changed to 

 grape sugar. Peptones, as such, are not found in the blood 

 and therefore appear to undergo a second change in their 

 passage through the mucous membrane and the walls of the 



