136 DISEASES OF THE PARENCHYMA OF THE LUNG. 



illogically separate the mechanical form, takes place in all cases in which 

 the pulmonary veins are abnormally filled and their walls unduly 

 stretched. Here the blood flows from the capillaries with difficulty, 

 while the arteries continue to convey blood to them, even though 

 scantily filled themsdves, since even then their walls evince a greater 

 degree of tension than the capillary walls. (Blood continues to flow 

 from the arteries into the capillaries after the heart has ceased to con- 

 tract.) Hence we see that stagnation results in a far greater dilatation 

 of capillaries than fluxion does, as when there is much obstruction of 

 the venous current in the capillaries, which have become, as it were, 

 blind appendices to the arteries, the blood continues to enter them until 

 the tension of the capillary walls is equal to that of. the artery, or until 

 the delicate membrane can no longer support such a pressure, and be- 

 comes ruptured. Stagnation, or engorgement of the pulmonary capil- 

 laries, occurs most typically 



1. From contraction of the left auriculo-ventricular opening and in* 

 sufficience of the mitral valve. Both forms of disease of the heart are 

 accompanied by the most intense hyperaemia of the lung. We know 

 that the brown color of the indurated hypertrophied lung depends upon 

 rupture of the dilated capillaries, the chief cause of which we have 

 found to be valvular disease of the mitral Whether the evacuation of 

 the auricle be retarded, or whether the blood be regurgitated during 

 the systole into the auricle, either process must impede the emptying of 

 the pulmonary vein, and give rise to overcharge of the capillaries. 



2. Enfeebled action of the heart results in imperfect evacuation of 

 its cavities, and hence in impeded efflux of blood from the veins. Here 

 the supply from the arteries is not diminished in proportion as the out- 

 flow from the capillaries is obstructed, and thus asthenic fevers, in which 

 the contractions of the heart are frequent, but incomplete, such as 

 typhus, puerperal fever, or pyaemia, are constantly accompanied by en- 

 gorgement of the pulmonary capillaries. When the heart's action is 

 weakened, gravity furnishes a new impediment to the evacuation of the 

 capillaries in dependent portions of the body. While such an obstacle 

 is easily overcome when the heart contracts with energy, yet when its 

 action is depressed, we soon see evidences of the effect of gravitation, 

 and hyperaemia begins to form at the more dependent places. A healthy 

 person may he in bed for months without the development of this form 

 of hyperaemia (hypostasis) in the capillaries of the back, or the forma- 

 tion of bed-sores, or the different phases of pulmonary hypostasis which 

 are the almost constant accompaniment of a typhus of long duration. 



We have learned that swelling and succulence of the mucous mem- 

 brane, and increase and alteration of the follicular secretion, are the 

 constant result of hyperaemia of a mucous membrane ; similar processes 



