202 RESPIRATION, BLOOD, AND EXCRETION 



Figure 216. — Diagram of Capillaries. 



The artery breaks up into minute 

 branches, the capillaries, which in turn 

 unite to form veins. 



to the vessels which return blood to the heart. There is 

 little structural difference between the veins and arteries 



except that the walls of 

 the arteries are thicker, 

 and there are no small 

 valves as in the veins. 

 As the branches of the 

 arteries become minute, 

 the walls become much 

 thinner, thus allowing 

 the food and oxygen to pass more easily to the individual 

 cells. These minute branches are called capillaries 

 (Latin, capillus, hair). From a cluster of capillaries a 

 small vein begins which soon connects with a slightly 

 larger vein, which leads back 

 to the heart through larger 

 and larger veins. 



The blood follows a regular 

 course through the body, pass- 

 ing from the left ventricle into 

 the aorta, which is the largest 

 artery in the body. As soon 

 as the aorta leaves the heart, 

 smaller arteries branch from it, 

 and the aorta itself also branches 

 until the entire body is sup- 

 plied with blood. The right 

 ventricle gives off a short 

 arterv which divides, and a 

 branch enters each lung. At 

 the point where an artery 

 leaves a ventricle, there are three half -moon-shaped valves 

 which prevent the blood from flowing back into the heart 

 (Figure 215). 



To Brain 



To Skin 

 --To Arm 

 "-To Lung 



To Kidneys^ 



To Stomach 



•To Small 

 Intestine 



:_ ";:To Back 



To Large Intestine 

 -To Leg 



Figure 217. — Main Arteries 

 of Frog. 



