216 STUDIES IN PHYSIOLOGY 



branch off from the thoracic aorta and supply minute capil- 

 laries to the walls of the air tubes, air sacs, and to the various 

 blood vessels; for the larger blood vessels, as well as the 

 heart, must receive nutriment from the outside, since they 

 cannot absorb it from within. Like the liver, then, the 

 lungs are supplied with two kinds of blood. 



The Pleura. The outer surface of each lung is covered 

 with a thin layer of serous membrane, and the walls of the 

 chest cavity are lined with the same kind of tissue. These 

 two layers constitute the pleu'ra. Both surfaces secrete a 

 serous liquid resembling that found between the two layers 

 of the pericardium. Hence the lungs can glide over the 

 chest wall without friction. 



The Structure of the Chest Cavity In the upper portion of 



the trunk is the cone-shaped chest cavity, which is more or 

 less inclosed by the sternum, the ribs, the collar bones, and the 

 spinal column. This bony framework is covered by muscle 

 and skin. The floor of the chest cavity is formed by the 

 tough sheet of muscle and connective tissue known as the 

 di'aphragm (Greek diaphragma = partition wall). In this 

 way there is formed an air-tight compartment which is com- 

 pletely filled by the heart, the blood vessels, the esophagus, 

 and the lungs. 



Enlargement of the Chest Cavity. The chest cavity is not, 

 like most boxes, inclosed by rigid, immovable walls. Let one 

 empty one's lungs as completely as possible, and then place 

 one hand on each side of the body, with the finger tips touch- 

 ing on the ventral surface in front. On taking in a long 

 breath, one feels the chest cavity enlarging at the sides and 

 ventrally, so much so that, if the palms of the hands are 

 pressed to the sides of the body, the finger tips may be 

 separated by a considerable space. At the same time the 

 ventral wall of the abdominal cavity is seen to be pushed 

 forward. These movements prove that the chest cavity can 

 be enlarged in three directions, namely, from side to side, 

 from dorsal to ventral surface, and from anterior end to 



