154 CIRCULATORY SYSTEM 



tected from friction by the moist membranous pericardium which 

 covers it. The ribs and breast bone furnish protection against me- 

 chanical injury. Large blood vessels collect blood from all parts of 

 the body and return it to the heart ; others take blood from the 

 heart to all parts of the body. The heart tissue, itself, has an in- 

 dependent blood supply called the coronary circulation. When the 

 heart is cut longitudinally, it is found to consist of four chambers, 

 a left auricle, a right auricle, a left ventricle, and a right ventricle. 

 A thick muscular partition through the center of the heart sepa- 

 rates the left side from the right side. The auricles receive blood 

 from the veins and give it to the ventricles. The act of receiving 

 blood does not require much work, so the auricle walls have com- 

 paratively little muscle in them. The ventricles force the blood 

 into the arteries to all parts of the body. In order to ac- 

 complish this pumping, the ventricles have very thick muscu- 

 lar walls. The walls of the left ventricle are much thicker 

 than the walls of the right ventricle. They have to send 

 blood to all parts of the body while those of the right ventricle 

 merely send blood to the lungs. Separating the ventricles from 

 the auricles are trapdoor arrangements called valves. These 

 are made of very strong connective tissue attached by cords of 

 the same tissue to the ventricles. On the left side, there is a 

 valve with two flaps, and on the right side, a valve with three 

 flaps. The blood passes from the auricles to the ventricles, by 

 merely pushing against the valves. If the blood backs up during 

 the contracting of the ventricles, the valves fill up, close, and 

 prevent the return of the blood into the auricles. Thus the 

 blood is kept moving in only one direction. 



Arteries. The tubelike blood vessels which carry the blood 

 from the heart to all parts of the body are called arteries. The 

 largest ones branch from the ventricles, and subdivide into smaller 

 vessels until a network of very fine tubes, practically microscopic, 

 is formed. These connect with the capillaries. The arteries can 



