144 



Disturbances of Circulation. 



corresponding at first merely to their distensibility ; but after the 

 dilatation has persisted for a time there also occurs a proliferation 

 of the cells of the vessel walls in consequence of the altered con- 

 ditions of tension, not only causing narrow arteries to assume 

 wide calibre, but changing capillary vessels into arteries. It is 

 in this manner that a tissue whose principal artery has been ligated 

 or occluded by thrombosis has its blood supply restored. The 

 development of collateral circulation, however, requires time before 

 complete compensation is attained ; and should the existing anasto- 



Fig. 12. 



Embolism of an artery with congestion of 

 its two vense comites. 



Fig. 13. 



Supply by anastomotic channels. 



motic channels be very small, few or entirely absent, the tissue must 

 in the interim sufifer from the circulatory loss, be moi'e or less com- 

 pletely deprived of its nutriment and perish. This is particularly 

 true when there are absolutely no arterial anastomoses. Arteries 

 which possess no communicating channels with neighboring arterial 

 areas are commonly known as end-arteries, following Cohnheim. 

 Parts of organs supplied with no arterial anastomoses have of 

 course capillary communications with the adjacent tissues, but the 

 small amount of blood which passes through these minute channels 

 is often insufificient to maintain the nutrition of the area deprived 

 of its arterial supply ; only in case of very small areas is the 



