EMPHYSEMA OF THE LUNG, 111 



Vicarious emphysema arises, first, in all cases in which portions of 

 the pulmonary substance become wasted, and shrink without corre- 

 sponding reduction of the capacity of the thorax, by collapse of its wall. 

 If the capacity of the chest remain constant, the size of the individual 

 vesicles which contribute to fill it must, of course, depend upon then- 

 number. If a portion of them perish, thus reducing the bulk of that 

 part of the lung to which they belong, either a vacuum must form in 

 the chest, or else the remaining vesicles must dilate. Secondly, vicarious 

 emphysema arises where all of the vesicles do not participate alike in 

 fiUing out the additional space formed in the thorax by inspiratory 

 dilatation. Normally, all the air-vesicles are inflated uniformly by 

 atmospheric pressure during inspiratory dilatation of the chest. If, 

 then, a part of the air-cells be filled up by exudation or serum, so that 

 no more air can enter them, these cells will not dilate during inspiration, 

 and hence the remaining and accessible ones must expand for them 

 vicariously and undergo an abnormal degree of distension. Thus, in 

 the bodies of all persons who have died of pneumonia or hypostatic en- 

 gorgement, we find vicarious emphysema in the portions of lung spared 

 by the original disease. So, too, extensive rigid adhesions between 

 the pulmonary and costal pleura give rise to vicarious emphysema. 

 Under normal conditions, the air-cells in the apices of the lungs, and 

 those placed near the spinal column, although the portion of the thoracic 

 wall adjacent to them scarcely takes any part in the inspiratory move- 

 ment of the chest, expand equally with the cells in the more movable 

 regions of the thorax, and which are situated near to the diaphragm, 

 and to the anterior thoracic wall. Of course, for this to take place, the 

 adjacent movable parts of the lung must yield and be pushed down- 

 ward and forward. Where the costal and pulmonary pleura are firmly 

 united, such yielding and displacement become impossible, and the 

 vesicles at the apices and along the back-bone cannot fully perform their 

 part in occupying the space created in the chest by its inspiratory ex- 

 pansion. Hence, other portions of the lung, especially the anterior and 

 lower borders, must act for them, and are thus made to undergo an ex- 

 cessive and abnormal distention. Finally, chronic catarrh of the smaller 

 bronchi is often followed by vicarious emphysema. If, in one part of 

 the lung, the calibre of the finer tubes be so much narrowed by swell- 

 ing, or by accumulation of mucus, that the air enters imperfectly, and 

 with difficulty, into the corresponding vesicles, such part of the lung 

 will not do its share in filling up the inspiratory expansion of the thorax, 

 so that other parts of the lung, which are free from catarrh, must ad 

 for them, and become abnormally distended. 



From the foregoing representation it is evident enough that the 

 c inspiratory " theory is a correct one. If vicarious emphysema arise 



