ii6 Disturbances of Circulation. 



collateral anastomoses. This readily occurs if such paths are at all 

 numerous and large; and where they are of narrow calibre they 

 are gradually widened by the pressure of the obstructed blood, so 

 that even very small branches and capillaries come to assume con- 

 siderable dimensions permitting the blood to escape from the area 

 of obstruction practically unhampered (anastomotic compensation). 

 The possibility of such dilatation depends, however, upon the degree 

 and rate of development of the venous obstruction and requires 

 time. In the case of sudden venous obstruction and the existence 

 of few anastomotic channels, before the collateral branches can 

 adapt themselves to the congestion the disturbance of the circula- 

 tion may well induce important functional and vital changes in the 

 affected organs ; and, of course, such occurrences are to be ex- 

 pected if the obstructed veins have no communication with other 

 unobstructed channels (portal vein, the veins of the kidneys and 

 lungs). Even compression under the finger (in phlebotomy or in 

 a rabbit's ear) will show how the impeded blood causes the veins to 

 dilate and swell up ; all the rest of the phenomena of this form of 

 hypersemia may be followed up experimentally in the exposed 

 tongue of a frog or under the microscope, after the ligation of 

 the larger venous trunks so that the blood can pass out only 

 through the venules. The web of the frog's foot may be employed 

 for the same purpose after ligation of the femoral vein. At first 

 the blood in the engorged and expanding veins and capillaries 

 becomes slowed, then irregular, now flowing forward, now back- 

 ward, and sometimes stagnates totally. It can be seen that in 

 some places the current is reversed, that the blood is passing 

 out of the engorged area through the collateral branches, these 

 gradually dilating, and the stationary or slowly moving corpuscles 

 gradually being drawn into the current and after a time the proper 

 rate of circulation again assumed. 



If a number of veins are occluded or cut off sufficient to entirely 

 or largely prevent collateral compensation, stagnation or stasis of 

 the blood takes place. In the affected parts of the vessel the red 

 blood corpuscles are packed so closely together that their outlines 

 can scarcely be distinguished and the blood completely fills the 

 vessel as a uniform red mass. Stasis becomes especially well 

 marked if, while the venous outlets are completely closed, the 

 blood continues to be forced in from the arteries. The capillaries 

 dilate to their fullest : their walls, permeable to the plasma even 

 before their distention, can no longer hold the fluid blood, but allow 



