Etiology. 85 



Hyperemia seen in the early stages of acute inflannnatory 

 processes also belongs to this group. 



A collateral hyperemia is produced when the blood current 

 in the lungs meets with a rapidly forming extensive impediment 

 such as occurs in pneumothorax on the healthy side and in 

 consideral)le elevation of intra-abdominal pressure on both 

 sides. 



Congestive hyperemia is one of the usual concomitant con- 

 ditions in stenosis of the left venous ostium or in mitral in- 

 sufficiency. In the majority of cases, however, it develops in 

 consequence of cardiac weakness in dilatation of the heart, in 

 non-compensated valvular disease of the heart in consequence 

 of myocardial degeneration in many acute infectious diseases. 

 Compression of the pulmonary veins in pericarditis, decrease 

 of the negative intrathoracic pressure in bloating of the stomach 

 or of the intestines may be the cause of congestion of the lungs. 

 The prolonged recumbent position on one side may also cause 

 hyperemia in the lower portions of the lungs without the con- 

 tribution of the above factors (hyperaemia hypostatica), be- 

 cause in such a position the venous blood has to overcome the 

 effect of gravity. 



Basch and others have shown that hyperemia of the lungs 

 with increase of the pressure in the capillaries makes the 

 latter stifFer and so impedes the expansions of the pulmonary 

 tissue ; it is also claimed that the capillaries become stretched 

 and thus bring about a certain amount of dilatation of the 

 alveoli. The shickening of the blood-current in congestion also 

 acts unfavorably. 



Edema of the lungs is also due to various causes. All 

 those factors which produce hyperemia of the lungs may also 

 produce edema of the lungs if they are acting more intensely 

 or over a long period of time. Those types of edema which 

 are due to alterations of the vessel walls brought about by 

 irritating substances are easily explained, for in these cases 

 the extravasation of blood serum is a sequel of disease of the 

 vessel walls as is generally the case in inflammations. A similar 

 mode of origin of pulmonary edema is that seen in association 

 with acute pneumonia and in the course of some infectious 

 diseases (anthrax, malignant edema and other septicemic affec- 

 tions). Bacteria and their toxins then need not necessarily 

 act directly upon the pulmonary tissue, but their inflammatory 

 action may l)e exerted through the blood circulation. A similar 

 effect may be manifested by certain vegetable or mineral poisons 

 or by decomposition products of the organism ; intense edema 

 of the lungs which is sometimes seen in severe disease of the 

 kidneys is probably of a similar origin. 



In the majority of cases the edema is due to stasis of the 

 blood. It frequently develops on this basis shortly before 

 death from cardiac weakness, combined with insufficient expan- 

 sions of the lungs. Valvular lesions in the left venous opening 



