134 THE ORGANS OF CIRCULATION. 



in foods, there feeding the organs; here warmed, and 

 there cooled; here loaded with wastes, and there puri- 

 fied. Thus by the flowing blood, every part is cared for. 



3. The Organs of Circulation are the heart, the arteries, 

 the capillaries, and the veins. The heart is a hollow mus- 

 cle which squeezes the blood on, and keeps it moving. 

 The arteries carry blood from the heart and distribute 

 it over the body. The capillaries are very fine tubes 

 with very thin walls, into which the arteries of every or- 

 gan pour their blood. The veins take up blood from 

 the capillaries and carry it back to the heart. 



4. The Blood, as every one knows, is a red liquid which 

 is very widely distributed over the body, since it flows 

 from any part of the surface when the skin is cut 

 through. There are very few portions of the body 

 into which blood is not carried. The outer layer of 

 the skin (Chap. VI.), the hairs and nails, the hard parts 

 of the teeth, and most cartilages contain no blood; 

 these non-vascular tissues are nourished by liquid which 

 soaks through the walls of blood-vessels in neighboring 

 parts. 



5. Arterial and Venous Blood. Although all blood is 

 red, it is not all the same tint of red. In nearly all ar- 

 teries, the blood, just sent out of the heart, is bright 

 scarlet; such blood is named arterial blood. In nearly all 

 veins, the blood, which has just flowed through the cap- 

 illaries of some organ, is of a dark purple-red color; 

 such blood is named venous blood. 



3. Name the organs of the circulation. What is the heart ? What 

 are the arteries ? The capillaries? The veins ? 



4. Distribution of blood in the body ? Portions which get no 

 blood ? How are the non-vascular tissues nourished ? 



5. What is arterial blood ? Venous ? 



