536 THE THIRD HUXLEY LECTURE 



the vessels. Not that it had wounded them, nor is there any reason to suppose 

 that it had killed them. No doubt if the animal had been released instead 

 of slaughtered, the veins would in due time have recovered. But the mechanical 

 violence which the hard round cord exerted, being pretty severe and long 

 continued, had prostrated for the time the vital energies of the tissues on which 

 it had acted ; and we had, in coagulation of the blood, a repetition of the class 

 of phenomena we had studied in the blood-corpuscles. 



But how was it that the blood remained fluid in other parts of the vessels ? 

 To my surprise I found that the same continued to be the case for days afterwards. 

 And thus accident led me to recognise what I afterwards found to be the general 

 rule, viz. that the blood, though in mammalia it coagulates soon after death 

 in the heart and main trunks, remains fluid for an indefinite period in minor 

 branches. The clotting in the heart had been an object of familiar obser- 

 vation in post mortem examinations in the human subject, and it seems to have 

 been assumed that the same thing occurred throughout the vascular system. 



The sheep's foot, with the blood retained in its veins by a bandage applied 

 before the animal was slaughtered, afforded the opportunity for very simple, 

 but instructive experiments on the nature of the relations between the living 

 vessels and their contained blood. For that the veins retained their life, even 

 after the lapse of more than twenty-four hours after severance of the foot from 

 the body, was shown by their shrinking by muscular contraction on exposure.^ 



Thus I found that a piece of glass introduced into a vein occasioned coagu- 

 lation in its vicinity. The end of a sewing-needle pushed through the wall 

 of a vein otherwise uninjured, became after a while encrusted with a layer of 

 fibrine deposited upon the part within the vessel, while the rest of the blood in 

 it retained its fluidity.^ On the other hand, having injected air into the vessels 

 on another occasion, I found seven hours later that their contents were a frothy 

 mixture of blood and air, the walls of whose bubbles were fluid, but solidified 

 when shed. Sir Astley Cooper had been of opinion that the living vessels kept 

 the blood within them fluid by acting in some way upon it — in other words, 



^ As regards the ammonia theory, an experiment which proved universally convincing was tliis : 

 Having exposed a vein in the sheep's foot, I pressed the blood out of it at one place, and apphed liquor 

 ammoniac to the empty portion, protecting neighbouring parts of the vessel from the vapour with olive 

 oil. After sufficient time had passed for the volatile alkali to fly off, blood was allowed to return to the 

 part on which the caustic liquid had acted. There it soon coagulated : the very substance a mere 

 trace of which should have kept it fluid according to the theory in question having brought about its 

 coagulation by injuring the tissues of the living vessel. 



^ The results of experiments of this kind vary considerably according to the time which has elapsed 

 after the foot was removed from the body : for the blood undergoes pretty rapid impairment of its 

 coagulabihty within the vessels of the severed part, and finally loses it altogether. It then, of course, 

 remains fluid long after the veins have lost all life. 



