TRANSACTIONS OF SECTION I. 777 



patient an attack of cowpox or vaccinia. This disease is related to smallpox, and 

 some still hold that 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 pi-otective inoculation, 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, &c. 



The power the blood possesses of slaying bacteria was first discovered when 

 the effort was made to grow various kinds of bacteria in it ; it was looked upon 

 as probable that blood would prove a suitable 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 56° 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 bacterio-lysiiis. 



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 (hsemoglobinuria). The 

 substance or substances in the serum that possess this property are called 

 hcemolysins, and though there is some doubt whether bacterio-lysins and hsemo- 

 lysins 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 than normal lymph and kill bacteria more easily. In a condition 

 like diabetes, when the blood is less alkaline than it should be, the susceptibility 

 to infectious diseases is increased. Alkalinity is probably beneficial because it 

 favours those oxidative processes in the cells of the body which are so essential 

 for the maintenance of healthy life. 



Normal blood possesses a certain amount of substances which are inimical to 

 the life of our bacterial foes. But suppose a person gets run down ; everyone 

 knows he is then liable to ' catch anything.' This coincides with a diminution in 

 the bactericidal power of his blood. But even a perfectly healthy person has not 

 an unlimited supply of bacterio-lysin, and if the bacteria are sufiiciently numerous 

 he will fall a victim to the disease they produce. Here, however, comes in the 

 remarkable part of the defence. In the struggle he will produce more and more 

 bacterio-lysin, and if he gets well it means that the bacteria are finally vanquished, 

 and his blood remains rich in the particular bacterio-lysin he has produced, and so 

 will render him immune to further attacks from that particular species of bacte- 

 rium. Every bacterium seems to cause the development of a specific bacterio-lysin. 



Immunity can more conveniently be produced gradually in animals, and this 

 applies, not only to the bacteria, but also to the toxins they form. If, for instance, 

 the bacilli which produce diphtheria are grown in a suitable medium, they produce 

 the diphtheria poison, or toxin, much in the same way that yeast-cells will produce 



