350 Journal of Agriculture. [8 June, 1908. 



probably in constant operation. It is only when bacteria get temporarily 

 the upper hand that inflammation, with all its signs and symptoms, takes 

 place. These white cells can not only digest bacteria but they_ can also 

 eat away and remove small blood-clots and small portions of tissue that 

 have died through injury or stoppage of the circulation. White cells 

 undoubtedlv plav some part in digestion as their number is always increased 

 after a meal. That they can carry fat globules from the surface cells 

 of the villus into the central lacteal is undoubted ; probably they help 

 also in the absorption and rebuilding of proteins and carbohydrates. 



Both varieties of corpuscles are slightly denser than the plasma in 

 v/hich they float. If blood be kept in a vessel and prevented from 

 clotting the corpuscles will sink by gravity ; this action can be hastened 

 by the centrifuge, an instrument on the same principle as the cream 

 separator, only that in this instance the corpuscles, unlike the cream, are 

 slung outwards. 



THE CLOTTING OF BLOOD. — The spontaneous coagulation or 

 clotting of blood serves as a natural means of arresting haemorrhage. Were 

 it not for this property, a small and unperceived cut would continue to bleed 

 until death resulted. The essential change in blood clotting is a trans- 

 lormation of a soluble globulin called fibrinogen, which exists in a small 

 amount in the plasma, into an insoluble, stringy substance, fibrin. When 

 blood clots as a whole the fibrin meshwork entangles the corpuscles so that 

 the clot consists of fibrin and corpuscles together. Such a clot at first 

 extends over the whole mass of blood affected, but very soon a clear 

 fluid will be seen exuding from the clot ; this fluid is serum and is 

 really plasma bereft of its fibrinogen. If, while blood is clotting, it is 

 stirred briskly with a bunch of feathers, the fibrin is collected into a mass 

 and does not entangle the corpuscles. Such blood is called defibri.nated 

 blood and when it is centrifuged or allowed to settle it separates into 

 corpuscles and scrum. If blood be separated into plasma and corpuscles 

 before clotting has taken place, then the clot will form in the plasma 

 and of course will contain no corpuscles. 



The change of fibrinogen intO' fibrin is conditional on the presence of an 

 enzyme called fibrin ferment. But this enzyme does not occur as such in 

 the blood. It is there in an inactive form called thrombogen. Now before 

 thrombogen can change into fibrin ferment, two conditions must be fulfilled 

 - — firstly there must be lime salts present and secondly there must be a 

 substance called kinase which is produced whenever a tissue is torn. The 

 interaction of these factors can be given in a simple diagram. 



Kinase ... ■ ) 



Throml)ocren ... V fibrin ferment ... "\ ^., . , ^ 



T ■ , ° ci • I^ nlji'in or clot 



Lime salts ... ) hbiinogen ... . J 



The coagulation of blood may be prevented or delayed by cold, which 

 depresses all chemical change ; and further by the removal of any of the 

 precursors of fibrin. A favourite method is to add to blood, as soon as 

 it is shed, some sodium oxalate or sodium citrate which removes the lime. 

 Such lilood can be centrifuged and plasma obtained from it. On adding 

 to this blood, or its plasma, suflicient lime salts, e.g., calcium chloride, 

 a clot forms very quickly. Leeches prevent clotting around the point 

 of their puncture bv destroving the thromlx)gen, some snake venoms prevent 

 clotting by destroying the kinase. If blood be received into a vessel 

 smeared with oil or parafiin, clotting is greatly delayed; the reason for 

 this is not clearlv understood. 



