116 PRINCIPLES OP VETERINARY SCIENCE 



in diameter. Their walls are very thin being composed of a 

 single layer of epithelial cells, and supported and given elasticity 

 by fibers of yellow elastic tissue. Capilliary blood-vessels 

 course in the walls of the air cells (Fig. 34). The total area of 

 the lung alveoli has been estimated at 100 times the area of the 

 skin surface. The inner surface of the lungs of an average sized 

 man, if spread out flat, would cover a space of 108 square feet. 



The pleura is the serous membrane which lines the thoracic 

 cavity. It is arranged to form two sacs, and is reflected at the 

 roots of the lungs to invest these and other organs in the cavity. 

 The median space between the two sacs is called the mediasti- 

 num. The disposition of the pleura is similar to that of the peri- 

 toneum. It is kept moist by a serous fluid, the liquor pleura; 

 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. 36). It is a large muscle with a 

 peripheral 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 kidneys. There are three foramina in the 

 diaphragm — the dorsal one is for the jpassage 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. This keeps 

 the lungs stretched from without and prevents them from col- 

 lapsing, an essential to efficient preformance of work. In the 

 average 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 ad- 

 mitted to the lungs at birth it immediately causes these disten- 

 sible and elastic organs to inflate, because the normal atmospheric 



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