92 THE HORSE IN HEALTH AND DISEASE 



disposition is similar to that of the peritoneum. The pleura is kept 

 moist by a serous fluid, the liquor pleurae; in health there is only- 

 enough to permit the lungs to glide on the walls of the cavity with 

 the minimum amount of friction, but it accumulates rapidly in 

 some forms of pleurisy. 



The diaphragm forms the partition between the thoracic and 

 abdominal cavities (see Fig. 28) . It is a large muscle with a periph- 

 eral fleshy portion, two muscular pillars, and a tendinous center. 

 The anterior surface is convex, covered by the pleura, and related 

 to the bases of the lungs and pericardium. The posterior surface 

 is concave, covered for the most part by the peritoneum, and in 

 contact with the liver, stomach, flexures of the colon, and the kid- 

 neys. There are three foramina in the diaphragm — the dorsal one 

 is for the passage of the aorta, the left one for the esophagus, while 

 the right foramen gives passage to the posterior vena cava. 



THE PHYSIOLOGY OF RESPIRATION 

 During life the lungs lie in an air-tight enclosure, the pleural 

 cavity, in which there is a constant negative pressure. In the aver- 

 age condition of expansion the normal elasticity of the lungs of a 

 man is capable of supporting a column of mercury 30 mm. in height, 

 so they are always tending to collapse. When the atmospheric air, 

 with a pressure of 760 mm. of mercury, is admitted to the lungs at 

 birth it immediately causes these distensible and elastic organs to 

 inflate, because the normal atmospheric pressure is greater, by 

 about 730 mm., than that which tends to collapse them. This 

 explains why the lungs are kept expanded and in contact with the 

 chest wall under natural conditions. If the thoracic wall is per- 

 forated and the external air is allowed to rush into the pleural 

 cavity, the lungs immediately collapse, for now, in addition to the 

 contractility force of the lungs, there is added the atmospheric 

 pressure, against which is only the normal pressure from the air in 

 the lungs. 



The mechanism of breathing may be best understood by study- 

 ing the movements of lungs recently removed from a living rabbit, 

 dog, or pig, and placed in an apparatus similar to that illustrated 

 in Fig. 27. The glass bell-jar represents the thoracic walls. The 

 jar is closed beneath by a sheet of rubber (D), to take the place of 

 the diaphragm. Conditions that closely approximate the normal 

 pleural cavity may be thus obtained. By partially exhausting the 



