

THE BLOOD. 169 



The reaction of blood to litmus is alkaline, but when it is deter- 

 mined accurately in terms of H ion concentration, it is found to be 

 almost the same as that of distilled water. Under various conditions 

 the reaction may alter slightly, and these changes produce marked 

 physiological effects in the body, although they are usually too slight to 

 affect an ordinary indicator, suoh as litmus. Further, the presence in 

 blood of proteins and phosphates makes it possible for a considerable 

 amount of acid or alkali to be added to blood without any appreciable 

 change being produced in the H ion concentration. The reason is that 

 the acid or alkali thus added combines with proteins or phosphates to 

 form compounds which do not undergo ionic dissociation, and therefore 

 does not alter the concentration of H ions, by which the reaction of the 

 blood is ultimately determined. For example, Xa 2 HP0 4 can be partly 

 converted into NaH 2 P0 4 on the addition of acid, with little or no 

 alteration in the number of free H ions present in the solution. 



THE COAGULATION OF THE BLOOD. 



When blood is shed, it becomes more viscid, and within three to 

 ten minutes it begins to set into a jelly-like clot. The clot gradually 

 contracts, expressing a yellow fluid, the serum, as it does so ; and 

 within ten to forty-eight hours the process results in a shrunken, firm 

 clot floating in the expressed serum. If coagulation has taken place 

 slowly, so that the corpuscles have had time to settle, the upper part of 

 the clot will be paler than the deeper part, because the lighter 

 leucocytes do not sink so quickly as the heavier red cells. 



If a drop of blood be placed on a slide and covered with a cover- 

 slip, the process of clotting may be observed microscopically. It is 

 found that the red corpuscles become aggregated into rouleaux, and 

 that between the aggregations delicate threads of fibrin make their 

 appearance. Clotting thus consists in the formation of a meshwork 

 of threads of fibrin, entangling the corpuscles ; and the subsequent 

 shrinking of the clot is due to the contraction of the newly formed 

 fibres. 



Fibrin may be obtained in quantity by whipping a large volume of 

 freshly shed blood with a bundle of twigs, when the fibrin adheres to 

 the twigs as it forms. Blood treated in this way will not clot 

 subsequently, arid is spoken of as defibrinated blood. The fibrin, when 

 washed free of blood pigment, is a white, stringy substance, easily 

 stretched and possessing considerable elasticity. It is insoluble in 

 water and in dilute salt solutions, but dissolves slowly in 5 per cent, 

 sodium chloride. It swells up and slowly dissolves in 0'2 or 0*4 per 

 cent, hydrochloric acid, with the formation of acid metaprotein. 



