840 PHYSIOLOGY 



in ice, so as to cool it rapidly to between C. and 1 0. At this temperature it remains 

 fluid for an indefinite time. The corpuscles sink, and the supernatant plasma can 

 be decanted and filtered. 



(5) Methods involving Mixture with Neutral Salts, (a) Magnesium sulphate. Blood 

 from any animal is received into one- quarter its bulk of a 25 per cent, solution of 

 magnesium sulphate. 



(6) Sodium sulphate. Blood is mixed on leaving the vessels with an equal volume 

 of half-saturated sodium sulphate solution. The plasma obtained in either of these 

 ways is known as salt-plasma. Clotting is indefinitely delayed in either case, but it 

 can be induced by suitable treatment of the separated plasma. 



(6) Methods depending on Decalcification of the Blood. Oxalate plasma is obtained 

 by receiving blood into a solution of sodium oxalate so that the total blood contains 

 1 per 1000 of the oxalate. Instead of oxalate we may use sodium fluoride, the propor- 

 tion in this case being 3 parts of NaF per 1000 blood. 



(7) Methods depending on the use of certain Substances of Animal Origin, (a) Peptone 

 plasma is obtained by injecting rapidly into the veins of a dog or cat in a fasting con- 

 dition a solution of commercial peptone in the proportion of 0*3 grm. peptone per kilo 

 of the animal. The effect of this injection is to cause a rapid fall of blood-pressure 

 and hurried respiration, and the animal then passes into a state of coma which may 

 last an hour or two. On drawing off the blood immediately after the fall of pressure 

 has taken place it is found to be uncoagulable, and cell-free plasma can be obtained 

 from it by the use of the centrifuge. A number of animal extracts act in a somewhat 

 similar fashion, such as extract of crayfish, of mussels, &c. 



(6) Leech extract or hirudin plasma. Peptone is only efficacious in retarding clotting 

 when it is injected into the animal's veins directly, and has no influence in this direction 

 if it be mixed with the blood as it flows out of the vessels. It has long been familiar to 

 physicians that the bites made by leeches continue to bleed for a considerable time, 

 and it was shown by Haycraft that this is due to the presence of an anticoagulating 

 substance in the buccal glands of the leech. This substance, which has the properties 

 of an albumose, can be extracted by boiling from the anterior half of the leech. It 

 will destroy the coagulability of the blood either when injected into the blood-stream 

 or when blood is received into a solution of hirudin. 



By any of these methods it is possible to obtain blood-plasma free from 

 formed elements. The conditions which will bring about coagulation in such 

 plasmata are strikingly diverse. Thus in cooled plasma a simple rise of 

 temperature is often sufficient to bring about coagulation. If, however, 

 the cooled plasma be filtered several times through, two thicknesses of 

 filter-paper, being kept at a temperature of about 1 C. during the whole time, 

 it loses this spontaneous coagulability on warming. It can still be made to 

 clot by the addition of certain substances such as blood-serum or the washings 

 of a blood-clot, and in some cases by the addition of tissue extracts. 



Oxalate plasma clots on simple addition of lime salts. Sodium sulphate 

 plasma clots on dilution. Magnesium sulphate plasma will not clot on 

 dilution, but needs in addition the presence of some blood-serum or some 

 substance derived from blood-serum. In both these cases tissue extracts 

 have no influence. By a careful study of one or two forms of plasma we can 

 arrive at a conception of the processes of coagulation which enables us to 

 understand the behaviour of all these various types. We may take as 

 our type oxalate plasma. Oxalate plasma, as procured by centrifuging a 

 specimen of horse's blood containing 0-1 per cent, sodium oxalate, is a clear 

 yellow fluid, perfectly free from formed elements, which evinces no tendency 



