20 INFLUENCE OF THE LIVING BLOOD-VESSELS. [BOOK i. 



exposure to air, whereby either an increased access of oxygen or an 

 escape of volatile matters is facilitated, for on the one hand the 

 blood is fully exposed to the air in the lungs, and on the other 

 shed blood clots when received, without any exposure to the 

 atmosphere, in a closed tube over mercury. 



All the facts known to us point to the conclusion, that when 

 blood is contained in healthy living blood-vessels, a certain relation 

 or equilibrium exists between the blood and the containing vessels 

 of such a nature that as long as this equilibrium is maintained the 

 blood remains fluid, but that when this equilibrium is disturbed by 

 events in the blood or in the blood-vessels or by the removal of the 

 blood, the blood undergoes changes which result in coagulation. 

 The most salient facts in support of this conclusion are as follows. 



1. After death, when all motion of the blood has ceased, the 

 blood remains for a long time fluid. It is not till some time 



, afterwards, at an epoch when post-mortem changes in the blood 

 and in the blood-vessels have had time to develope themselves, 

 that coagulation begins. Thus some hours after death the blood in 

 the great veins may be found perfectly fluid. Yet such blood has 

 not lost its power of coagulating ; it still clots when removed from 

 the body, and clots too when received over mercury without 

 r^ i exposure to air, shewing that the fluidity of the highly venous 

 blood is not due to any excess of carbonic acid or absence of oxygen. 

 Eventually it does clot even within the vessels, but perhaps 

 never so firmly and completely as when shed. It clots first in the 

 larger vessels, but remains fluid in the smaller veins, for a very long 

 time, for many hours in fact, since in these the same bulk of blood 

 is exposed to the influence of, and reciprocally exerts an influence 

 on, a larger surface of the vascular walls than in the larger veins. 



2. If the vessels of the heart of a turtle (or any other cold- 

 W blooded animal) be ligatured, and the heart be cut out and 



AT \ vJI$i ace( ^ i n favourable circumstances so that it may continue to beat 

 P> V^V as ^ on a P er *d as possible, the blood will remain fluid within 

 * *$^ ^he heart as long as the pulsations go on, i. e. for one or two days 

 (and indeed for some time afterwards), though a portion taken away 

 at any period of the experiment will clot very speedily. 



3. If the jugular vein of a large animal, such as an ox or 

 horse, be ligatured when full of blood, and the ligatured portion 

 excised, the blood in many cases remains perfectly fluid, along the 

 greater part of the length of the piece, for twenty-four or even 

 forty-eight hours. The piece so ligatured may be suspended in 

 a framework and opened at the top so as to imitate a living 

 test-tube, and yet the blood will often remain long fluid, though 

 a portion removed at any time into another vessel will clot in 

 a few minutes. If two such living test-tubes be prepared, the 

 blood may be poured from one to the other without coagulation 

 taking place. 



The above facts illustrate the absence of coagulation in intact 



