118 DISEASES OF THE PARENCHYMA OF THE LUNG. 



in their interstices. There is no intervesicular connective tissue in the 

 lung, it exists only between the lobules. More rarely there are larger 

 cysts by which the pleura is extensively separated from the lung, and it 

 scarcely ever happens that the pleura itself at last gives way, allowing 

 air to enter its cavity, or that the air beneath the pleura penetrates 

 along the roots of the lungs into the areolar tissue of the mediastinum, 

 and thence to that beneath the skin, producing emphysema of the sub- 

 cutaneous cellular tissue. 



SYMPTOMS AND COURSE. Circumscribed, vicarious emphysema, in 

 the vicinity of portions of withered and shrunken lung, cannot be recog- 

 nized during life, and has more pathological and anatomical than clinical 

 interest. 



The more slowly a person has died, the more distinct the symptoms 

 of extensive hypostasis have been, the more forcibly the thorax has been 

 expanded in the death-agony, so much the more surely may we expect 

 to find an acute vicarious emphysema of the anterior and lower pul- 

 monary borders at the autopsy. 



The symptoms of extensive vicarious emphysema and those of sub 

 stantive emphysema are very similar to one another, as the most promi- 

 nent characteristics depend upon a more or less advanced wasting of 

 the intervesicular septa, a nutritive derangement common to either 

 form. 



By-and-by we shall briefly adduce the points upon which we base a 

 differential diagnosis between vicarious and substantive emphysema. In 

 many instances the two forms cannot be distinguished from one another 

 during life. 



With the enlargement of the air-cells, and the destruction of many 

 of the septa, a large number of capillaries have also perished, and the 

 breathing-surface is materially diminished. The more points of contact 

 the air finds with the blood, so much the more favorable are the condi- 

 tions for interchange of gases. On the other hand, the smaller this sur- 

 face is, so much the more incompletely, caeteris paribus, is the elimina- 

 tion of carbonic acid and imbibition of oxygen carried on. The loss of 

 the alveolar septa, and the attenuation of the pulmonary tissue is, there- 

 fore, the first important factor in the dyspncea from which emphysem- 

 atous persons suffer. A sufficient renovation of the air contained in 

 the air-vesicles is as essential to the normal performance of the oxygena- 

 tion as is a due extent of respiratory surface. If the thorax and lungs 

 do not expand properly during inspiration, or if they contract imper- 

 fectly during expiration, the air within the air-cells is not sufficiently 

 renewed, and neither can the carbonic acid formed within the body be 

 eliminated from the blood, nor can the latter obtain the oxygen required 

 by the system. 



