190 PEACTICAL PHYSIOLOGY 



process must be retarded, so that we may have time to observe the 

 different stages. Moreover, it is inconvenient to study the process in 

 blood as it contains solid elements in suspension, so that we 

 usually separate the plasma for this purpose, and, in order to do this, 

 we must retard clotting. 



Since the blood does not clot so long as it is contained in the 

 healthy blood-vessels, we may prevent the process by excising a blood- 

 vessel (such as the jugular vein of the horse), after ligaturing it in two 

 places, so as to retain the blood in it. The excised piece of vein is 

 popularly called a " living test-tube" and, since the walls do not die for 

 a considerable period, the blood remains fluid. If the test-tube be hung 

 up, the solid elements of the blood (the corpuscles) sink to the bottom, 

 and the fluid portion (the plasma) can be separated from them by 

 means of a pipette. This is a very interesting method, but one which 

 is, of course, impracticable for obtaining large quantities of plasma. For 

 this purpose certain neutral salts (magnesium sulphate, sodium sulphate, 

 sodium citrate, etc.) are mixed with the blood to prevent it clotting. 

 The corpuscles and plasma are then separated by allowing the blood to 

 stand, when the corpuscles sink to the bottom of the vessel, or this 

 separation may be hastened by placing the salted blood in a centrifuge. 

 The supernatant plasma called salted plasma is then removed by a 

 pipette or syphoned off'. There are many other methods for preventing 

 clotting, but the description of these we will leave till after the process 

 itself has been studied. 



EXPERIMENT I. Fresh blood has been mixed with an equal volume 

 of saturated sodium sulphate solution, or with one-quarter its volume 

 of saturated magnesium sulphate solution. 1 The corpuscles have settled 

 to the bottom of the vessel, and the straw-coloured or reddish plasma 

 is pipetted off. Divide this into three portions, and label them a, b, c. 

 To each add ten times its volume of water, so as to remove the action 

 of the salt by dilution. Place a in water-bath at 37 C. To b add a 

 piece of blood-clot or a few drops of serum. To c add a few drops 

 of a 1 per cent, solution of potassium oxalate. Observe that first 

 b, then a, pass into a jelly-like state clotting but that c remains fluid. 



From this we learn that plasma itself can clot, but that the process 

 is accelerated by adding blood which has already clotted, and is 

 hindered by adding a soluble oxalate. The clotted blood must, there- 

 fore, contain an excess of some element necessary for clotting, and since 

 the previous heating of it to about 60 C. robs it of the power of 

 assisting clotting we assume that it contains some ferment fibrin 



1 In order to do this the artery or vein is allowed to bleed into a vessel con- 

 taining one of these solutions. 



