160 DISEASES OF THE PARENCHYMA OF THE LUNG. 



by DO means easy of elucidation. [The most recent and precise ob- 

 servations upon the circulatory disturbance caused by embolism are 

 by Cohnheim. First, both above and below the plug there comes a 

 stage of stagnation. The blood is led off into collateral channels, 

 and the state of circulation in the region beyond the plug then de- 

 pends very much upon whether the obstructed vessel, before breaking 

 up into capillaries, anastomoses with any other artery. If it does, 

 then blood again reaches it by the new route, and the embolus is 

 harmless. In the same way embolisms in the capillaries are harmless 

 (when not too numerous), because only one capillary twig is with- 

 drawn from the circulation, which is then carried on by the re- 

 mainder. If, however, there be no arterial anastomosis above the 

 plug if, as Cohnheim has it, the obstructed artery be a terminal 

 artery then the tissues which it should supply suffer either death or 

 hsemorrhagic infiltration. The spleen, kidney, brain, and retina possess 

 such terminal arteries, as do also in some degree the lungs, especially 

 at their periphery, where the anatomical connection of the several 

 lobules is but imperfectly kept up by interlobular lateral vessels. 

 If there be no anastomosis in the region below the plug, the blood- 

 pressure in it is arrested, so that blood from the neighboring veins 

 backs into the vessels of the obstructed district ; i. e., there is a re- 

 flux through the efferent vessels, through the capillaries, even into 

 the arterioles ; and the blood continues to regurgitate until arrested 

 by counter-pressure in the neighboring veins ( Cohnheim? s Anschop- 

 pung}. Signs of haemorrhage now appear, due to giving way of the 

 capillaries, but, according to Cohnheim, they also arise from diapede- 

 sis, or " bloody sweating " or exudation, the surfaces of the overload- 

 ed capillaries first becoming covered with red blood-disks, until at 

 last even to the naked eye small extravasations become visible. The 

 cause of the vascular dilatation and bleeding Cohnheim does not 

 think to be distention alone ; but believes that the interrupted cir- 

 culation and the withholding of new blood from the obstructed dis- 

 trict exercise some baneful effect upon the vascular walls, which 

 induces a disturbance of their integrity, of the nature of which we 

 are as yet ignorant. 



A different condition prevails when the embolus, instead of be- 

 ing a simple blood-clot forming a mere mechanical plug, contains 

 chemical or fermenting matter or small organisms. In such a case 

 a suppurative pneumonia arises at the seat of infarction, and pro- 

 duces what we call a metastatic abscess. The extent of the ab- 

 scess depends somewhat upon the dimensions of the embolus, but 

 more upon the degree of its virulence and upon the resisting power 

 of the adjacent tissues. 



