16 A MANUAL OF VETERINARY PHYSIOLOGY 



we are unable to account, but it is suggested that by their death 

 they influence the composition of the blood plasma, as in this 

 fluid their component parts must become dissolved after their 

 death. 



During the life of the white corpuscle great activity prevails ; 

 it is constantly giving up and taking in material which must 

 affect the composition of the plasma. It is known that the 

 white cell possesses the power of digesting certain substances, 

 both solid and liquid. The researches of Metschnikoff have 

 paved the way towards a better understanding of the probable 

 manner in which protection against certain diseases is obtained. 

 He has shown that the white cells take up the bacteria into their 

 interior and digest them, a process termed phagocytosis; it is 

 really a fight between bacteria and leucocytes. The difference 

 in the resisting power to disease possessed by ' fit ' over ' unfit ' 

 animals, and the greater protection afforded by maturity as 

 compared with youth, are facts which may be directly connected 

 with the question of phagocytosis. 



The polynuclear cell above described appears to be the leuco- 

 cyte best adapted to ingesting bacteria, but all are capable of 

 turning out good work, and the thoroughness with which this is 

 done depends upon the composition of the plasma. It appears 

 essential that this should contain a substance which acts upon 

 the bacteria, and renders them an easy prey to the leucocyte. 

 This substance, known as an opsonin, may be conveyed to the 

 plasma by the leucocytes, and it may also be artificially increased 

 by the injection into the body of suitable bacteria or of products 

 obtained from them. 



It is also probable, as distinct from the doctrine of phago- 

 cytosis, that the white cells of the blood may be closely con- 

 cerned in the production of certain protective substances which 

 destroy bacteria, bacteriolysins, the existence of which help to 

 explain the theory of immunity. 



Coagulation. — We are now brought to a consideration of the 

 subject of blood-clotting, a process by which the naturally fluid 

 blood becomes converted into a solid. 



If blood be drawn from the body and left at rest, it will be 

 found within a few minutes to have undergone the process of 

 clotting. The fluid first becomes a jelly and then a firm clot or 

 crassamentum, taking a complete cast of the vessel in which 

 it is placed, and so firm in consistence that it may be inverted 

 without any blood being lost. In a short time the clot begins 

 to contract, and by so doing squeezes out a fluid known as serum 

 (Fig. 7). This gradually accumulates, and as it becomes abun- 

 dant the clot sinks. The blood of the horse is remarkable for 

 the slow rate at which coagulation occurs, and the red cells, 



