CHAP. II.] THE BLOOD. 55 



thrust deeply into the tissue of the tongue or mesentery of the frog 

 so as to be in close proximity to an artery or a vein, the inner wall of 

 the vessel at the point which corresponds to the crystal becomes 

 covered by colourless blood corpuscles, whose number continually 

 increases ; soon there are three or four layers of colourless corpuscles 

 closely pressed against the wall, whilst the heap grows ever larger 

 and larger as the blood which flows past continually brings fresh 

 white corpuscles to add to it. Soon the vessel becomes completely 

 plugged by this agglomeration of colourless corpuscles. The subse- 

 quent progress of such a thrombus may be various. In some cases 

 however the following process may distinctly be observed: The 

 Y\rhole mass of cells undergo a fine granulation and the contours 

 of individual cells become less distinct. Then the contours of the cells 

 become lost altogether, and a feebly refracting finely granulated mass 

 results, which is said to be not unlike a mass of fibrin. 



(4) When blood is occluded by ligatures within a living vein, it 

 will be found to remain uncoagulated for many hours, providing the 

 vitality of the vein persist. This remarkable experiment was first 

 performed by Hewson, and was subjected to a careful study by 

 Professor Lister, and more lately by Frederique. If, however, 

 the vitality of the vein be destroyed by the application of caustic 

 ammonia to its exterior, coagulation will soon result (Lister). The 

 experiment is best performed with the jugular veins of horses. The 

 animal having, as is usually done in slaughtering horses, been struck 

 down insensible by a blow on the head, the jugular vein or veins are 

 exposed, and two ligatures are applied to the vein at a distance of 

 several inches apart, so as to include the blood contained within this 

 portion of the vein in a tube with venous walls. The vein may then 

 be dissected out without allowing its contents to escape. Such a 

 vein may be kept for many hours, and on being opened the blood 

 will be found still fluid within it, coagulating however when allowed 

 to flow into any ordinary vessel. After an interval of many hours, 

 however, the vitality of the vein being destroyed, the blood coagulates. 

 This experiment we owe to Hewson. Reasoning from it, it might be 

 surmised that the cause of the coagulation was the opening of the vein 

 and the exposure of its contents to air; that such an explanation is 

 entirely erroneous was shewn by Professor Lister, who determined that 

 blood would remain fluid for hours in a vein after being exposed 

 with the utmost freedom to the air by being poured in thin streams 

 from one venous capsule to another. 



The observation of Lister might lead one to the conclusion 

 which Professor Briicke arrived at from his experiments. That 

 eminent observer, extending the observations of Hewson, shewed that 

 blood injected into the separated, but yet living, contracting, heart 

 of a turtle, would preserve its fluidity for days, and came to the 

 conclusion that the walls of the vascular system possess a power 

 of restraining coagulation a view which was assuredly shared by 

 Hewson, but which in this case appeared to find its most striking 



