68 



ELEMENTARY PHYSIOLOGY 



diaphragm. It is somewhat conical in form, and is placed with 

 its broad portion or base uppermost, and with its apex or pointed 

 end turned downward and toward the left. Its size is about 

 equal to that of the closed fist, and in the adult it weighs about 

 nine ounces. The heart contains four cavities through which 

 the blood circulates ; and by its powerful contraction it forces 

 this fluid through all the blood-vessels of the body. 



The thorax contains the great blood-vessels which are con- 

 nected with the heart. These all enter or leave at the base, and 

 are called arteries if they convey blood from the heart, and veins 

 if they convey it to the heart. 



Fig. 78. Transverse Section through the Thorax. 



The section is carried above the heart, but below the division of the trachea. 



i, sternum ; 2, body of dorsal vertebra ; 3, spinous process ; 4, spinal cavity ; 5, rib ; 6, inner 

 layer of pleura; 7, outer layer of pleura; 8, pericardium; 9, right bronchus; 10, left 

 bronchus; 11, oesophagus; 12, heart ; 13, aorta, ascending ; 14, aorta, descending ; 15, left 

 lung ; 16, right lung ; 17, pulmonary arteries. 



The oesophagus passes down the neck close behind the 

 trachea, then between the lungs behind the heart, and through 

 the diaphragm to the stomach beneath. 



The organs just mentioned completely fill the thorax during 

 life ; but, when the chest of a dead animal is cut open, the lungs 

 collapse, even if they are uninjured. If, however, a suitable tube 

 be inserted in the trachea, the lungs may again be inflated by 

 blowing into them. 



Both heart and lungs are invested by double membranes- 



