CAUSE OF COAGULATION. 253 



the stasis becomes injurious to the intima, coagulates. Probably 

 the chemical chauges going on in the one are useful for the nutri- 

 tion of the other, and that they mutually supply one another 

 with some material essential for their life. This is apparent in 

 those cases where coagulation takes place during life in the ves- 

 sels. It never occurs so long as -the intima of the vessel is per- 

 fect, and the blood flow constant, but it follows lesion of this deli- 

 cate membrane whether caused by injury or by mal-nutrition. 



The gradual occurrence of this impairment of function of the 

 intima can be watched under the microscope in the small vessels 

 of a transparent part during the initial stages of inflammation. 

 Owing to the arrest of the flow of blood, the walls of the small 

 vessels suffer from defective nutrition, and may be seen to allow 

 some elements to escape, while the disks adhere together and the 

 blood coagulates. 



In the larger vessels the same thing occurs when inflammation 

 of their lining membrane destroys its capability of keeping up 

 the necessary nutritive equilibrium. Thus clots form on the inner 

 lining to the walls of an inflamed vein, often growing so as to 

 fill the entire vessel, and give rise to a condition called thrombosis. 



On the left valves of the heart and in the arteries, where the 

 delicate intima is subjected to great mechanical strain, it is com- 

 mon enough to find slight injuries of it covered over with thin 

 clots. To the surgeon this mutual nutrition of intima and blood 

 is of the utmost importance in studying the occlusion of vessels, 

 for it is upon this fact he has mainly to depend for the stoppage 

 of hemorrhage from a wounded artery. A tightly-tied ligature 

 either injures the inner coats mechanically or starves the iutima 

 by checking the flow of blood through the vessel up to the next 

 branch, and that portion of the vessel is filled with stationary 

 blood, which soon clots and forms an adherent plug. But if the 

 ligature be applied too loosely, a slight blood current passes 

 through the point where the vessel is tied, and this suffices for the 

 nutrition of the intima, by the renewal of the blood's contact, so 

 that no clot is formed, the vessel is not closed, and most probably, 

 when the ligature has cut through the outer coat, secondary hem- 

 orrhage occurs. 



