CAUSE OF COAGULATION. 247 



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 vessels. It 

 never occurs so long as theintimaof the vessel is perfect, and the 

 blood-flow constant, but it follows lesion of this delicate mem- 

 brane 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 

 elots. 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 haemorrhage 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 haem- 

 orrhage occurs. 



It has also been shown that if any foreign substance, such as a 

 thread, be introduced into the blood while circulating, a coagulum 



