BLOOD SYSTEM. 



361 



place for other air. In the case of the blood the circu- 

 lation is kept up by the mechanical action of the heart ; 

 in the case of the air, by the mechanical action of 

 breathing. 



Mechanics of Breathing.— The body cavity is 

 divided by a thin transverse partition into an upper 

 chamber, the thorax, and a lower chamber, the abdomen. 

 In the one is found the lungs and heart, in the other the 

 stomach, spleen, liver, intestines, kidneys, etc. The dia- 

 phragm is not a plane, or it could not be used for respira- 

 tion. It is deeply concave below, or domelike. It is also 

 a muscular partition, the fibers radiating from a clear 

 membranous space in the middle — the skylight of this 

 dome — in all directions and taking hold of the walls 

 of the body cavity all around. Contraction of these 

 fibers brings down the dome and flattens it. The arch 

 of the dome is filled below by the stomach and spleen 

 on the left and the liver on the right. These in their 

 turn rest on the mass of convoluted intestines, and the 

 whole is supported by the abdominal walls. Above the 

 diaphragm the concave lower surface of the lungs rests 

 directly on the upper convex surface of the dome. 

 Although in contact with the viscera above and below, 

 the diaphragm is not united with them. 



Now, the thoracic, like the abdominal, cavity is lined 

 with a smooth serous membrane. In the abdomen it is 

 called the peritonaum, in the thorax the pleura. Like 

 the abdominal, so the thoracic cavity is a closed cavity — 

 i. e., the pleura lines the ribs, etc., and is then reflected 

 to form the investing membrane of the lungs and heart. 

 If it could be successfully dissected off it would form a 

 continuous bag without a hole in it. The manner in 

 which the pleura lines the thorax, is then reflected over 

 the lungs as its investing membrane, and then between 

 the two lungs as the mediastinum, is shown in Figs. 243 



