360 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



kept fluid for an indefinite time. The blood-corpuscles soon sink, so that this 

 method is an excellent one for obtaining pure blood -plasma. The cooling 

 probably prevents clotting by keeping the corpuscles intact. 



2. By the Action of Neutral Salts. Blood received at once from the blood- 

 vessels into a solution of such neutral salts as sodium sulphate or magnesium 

 sulphate, and well mixed, will not clot. In this case also the corpuscles settle 

 slowly, or they may be centrifugalized, and specimens of plasma can be 

 obtained. For this purpose horse's or cat's blood is to be preferred. Such 

 plasma is known as " salted plasma ;" it is frequently used in experiments in 

 coagulation for example, in testing the efficacy of a given ferment solution. 

 The best salt to use is MgSO 4 in solutions of 27 per cent. : 1 part by volume 

 of this solution is usually mixed with 4 parts of blood ; if cat's blood is used a 

 smaller amount may be taken 1 part of the solution to 9 of blood. Salted 

 plasma or salted blood again clots when diluted sufficiently with water or when 

 ferment solutions are added to it. How the salts prevent coagulation is not 

 definitely known possibly by preventing the disintegration of corpuscles and 

 the formation of ferment, possibly by altering the chemical properties of the 

 proteids. 



3. By the Action of Albumose Solutions. Certain of the products of 

 proteid digestion, peptones and albumoses, when injected into the circulation 

 retard clotting for a long time. For injection into dogs one uses 0.3 gram 

 to each kilogram of animal. If the blood is withdrawn shortly after the 

 injection, it will remain fluid for a long time. According to Pekelharing, the 

 albumoses act by combining with the calcium salts, or at least by preventing 

 them from reacting normally. 



4. By the Use of Leech Extracts. Extract of the heads of leeches, when 

 mixed with blood, will prevent coagulation. The extract contains some sub- 

 stance formed in the salivary glands of the leech. It is probable that this 

 substance acts normally to prevent the clotting of blood when sucked in by the 

 animal. 



5. By the Action of Oxalate Solutions. If blood as it flows from the 

 vessels is mixed with solutions of potassium or sodium oxalate in proportion 

 sufficient to make a total strength of 0.1 per cent, or more of these salts, 

 coagulation will be prevented entirely. Addition of an excess of water will 

 not produce clotting in this case, but solutions of some soluble calcium salt 

 will quickly start the process. The explanation of the action of the oxalate 

 solutions is simple : they are supposed to precipitate the calcium as insoluble 

 calcium oxalate. 



Total Quantity of Blood in the Body. The total quantity of blood in 

 the body has been determined approximately for man and a number of the 

 lower animals. The method used in such determinations consists essentially 

 in first bleeding the animal as thoroughly as possible and weighing the quan- 

 tity of blood thus obtained, and afterward washing out the blood-vessels with 

 water and estimating the amount of haemoglobin in the washings. The results 

 are as follows : Man, 7.7 per cent, (-fa) of the body-weight ; that is, a man 



