88 PHYSIOLOGY. 



thus free, except at its root, where the air and blood tubes 

 enter. A very small quantity of liquid moistens the con- 

 tiguous surfaces of the pleurae on the outside of the lung 

 and the inside of the chest wall, so they move easily one 

 upon the other during respiration. As the lungs are 

 always distended enough to fill the chest cavity, these two 

 surfaces are always in contact. In pleurisy (inflammation 

 of the pleurae) pain is felt in breathing from friction or 

 adhesion of these surfaces. 



Important Facts concerning Respiration. In study- 

 ing respiration, let us constantly keep in mind these 

 facts : 



1. The lungs are highly elastic, and 



2. Highly porous, each air vesicle being in direct com- 

 munication with the outer air by means of 



3. Air tubes that always stand open 



4. And are always moist internally. 



5. The pulmonary capillaries closely invest each air 

 vesicle. 



6. The lungs are always expanded enough to fill all 

 the space in the chest not occupied by other organs, and 



7. Freely movable, except at the place of entrance of 

 the bronchi and blood tubes. 



8. The smooth, moist pleurae. 



The Diaphragm. The diaphragm is a thin muscle 

 making a complete partition between the abdominal cavity 

 and the chest cavity. It is convex anteriorly, concave pos- 

 teriorly ; its ventral border is attached to the inside of the 

 chest wall about opposite the lower end of the breast bone, 

 thence obliquely along the border of the ribs (as felt in 

 front), and the dorsal attachment is posterior to the ventral 



