440 THE BLOOD [CH. XXVI. 



vaccinia. This disease is either closely related to smallpox, or 

 maybe it is smallpox modified and rendered less malignant by passing 

 through the body of a calf. At any rate, an attack of vaccinia renders 

 a person immune to smallpox, or variola, for a certain number of 

 years. Vaccination is an instance of what is called protective inocula- 

 tion, which is now practised with more or less success in reference to 

 other diseases like plague and typhoid fever. The study of immunity 

 has also rendered possible what may be called curative inoculation, or 

 the injection of antitoxic material as a cure for diphtheria, tetanus, 

 snake poisoning, etc. 



The power the blood possesses of slaying bacteria was first dis- 

 covered when the effort was made to grow various kinds of bacteria 

 in it ; it was looked upon as probable that blood would prove a suit- 

 able soil or medium for this purpose. It was found in some instances 

 to have exactly the opposite effect. The chemical characters of the 

 substances which kill the bacteria are not fully known ; indeed, the 

 same is true for most of the substances we have to speak of in this 

 connection. Absence of knowledge on this particular point has not, 

 however, prevented important discoveries from being made. 



So far as is known at present, the substances in question are 

 proteid in nature. The bactericidal powers of blood are destroyed by 

 heating it for an hour to 55 C. Whether the substances are enzymes 

 is a disputed point. So also is the question whether they are derived 

 from the leucocytes ; the balance of evidence appears to me to be in 

 favour of this view in many cases at any rate, and phagocytosis 

 becomes more intelligible if this view is accepted. The substances, 

 whatever be their source or their chemical nature, are sometimes 

 called alexins, but the more usual name now applied to them is that, 

 of lacterio-lysins. 



Closely allied to the bactericidal power of blood, or blood- serum, 

 is its globulicidal power. By this one means that the blood-serum of 

 one animal has the power of dissolving the red blood-corpuscles of 

 another species. If the serum of one animal is injected into the 

 blood-stream of an animal of another species, the result is a destruction 

 of its red corpuscles, which may be so excessive as to lead to the 

 passing of the liberated haemoglobin into the urine (haemoglobinuria). 

 The substance or substances in the serum that possess this property 

 are called hcemolysins, and though there is some doubt whether 

 bacterio-lysins and haemolysins are absolutely identical, there is no 

 doubt that they are closely related substances. 



Another interesting chemical point in this connection is the fact 

 that the bactericidal power of the blood is closely related to its 

 alkalinity. Increase of alkalinity means increase of bactericidal 

 power. Venous blood contains more diffusible alkali than arterial 

 blood, and is more bactericidal ; dropsical effusions are more alkaline 



