CHANGES IN THE BLOOD. 99 



congestion be in a nervous centre, whilst in otlier instances 

 function becomes diminished. Tiius a long-continued congestion 

 of a secretory organ arrests, or very considerably diminishes, the 

 function of that organ. 



The cons^estion or fulness of the vessels is somewhat relieved 

 by transudation of serum into the tissues, or into cavities, con- 

 stituting oedema, anasarca, and dropsical effusion. For this 

 reason the limbs swell when their vessels are congested from 

 debihty or want of exercise ; and the peritoneal cavity becomes 

 filled with fluid, constituting ascites, when the portal veins are 

 in any way obstructed. The transuded serum generally differs 

 from that of the blood, in containing less of the soHd blood 

 constituents and more w^ater ; it has also a lower specific gravity. 

 If exposed to the air it will sometimes coagulate, forming rather 

 a gelatinous mass than a firm fibrinous clot. 



Wlien the obstruction is very great the serum partakes more 

 of the nature of the blood plasma; and it is now ascertained 

 that in some instances the blood globules may escape through 

 the walls of the capillaries without rupture. This fact was dis- 

 covered by Cohnlieim, who, by applying a ligature to the femoral 

 vein of the frog, observed, when the congestion of the vessels 

 became very great, that the red corpuscles passed through the 

 capillary walls, and in so doing assumed an hour-glass shape, or 

 became constricted in their middle. This fact explains to some 

 extent how, in some cases of purpura, transudation of blood 

 takes place into the tissues without rupture of the vessels. 



Congestion from obstruction prodvicing gangrene is seen in 

 strangulated hernia, and in invagination of a portion of the 

 intestines. Gangrene from obstruction is always of the moist 

 kind, whereas that from arterial incompetency is of the dry kind. 

 In the one the blood supply is scarcely or not at all interfered 

 with, but the return of that blood is arrested, stasis is produced, 

 the blood thus imprisoned transudes into the tissues, and, rapidly 

 dying and decomposing, adds to the rapidity of the death and 

 decomposition of adjoining structures. In the other, or gangrene 

 from want of blood, the part turns pale and dry, shrinks, becomes 

 mummified, or crumbles into fragments. 



The occurrence of thrombosis, or the formation of clots in the 

 congested vessels, results from the blood coagulating within 

 them during life, and may arise in congestion of any part, and 



