288 LABORATORY EXERCISES IN BACTERIOLOGY. 



protection against invasion of the germs afforded by the skin, mucous membranes, 

 and other protective surfaces ; (6) the action of the cilia present on the cells of certain 

 membranes tending to expel the organisms seeking entrance; (c) the influences of the 

 various secretions with which the bacteria may come in contact, as the acid juice 

 of the stomach or the alkaline juice of the small intestine ; (d) the destructive (phago- 

 cytic) action of the white blood-cells and the embryonic connective-tissue corpuscles; 

 (e) the influences of certain agents antagonistic to bacterial development or their 

 products, present naturally in the fluids of the body (alexins or natural antitoxins'). 

 It is believed that if these influences be sufficiently strong an invading organism is 

 destroyed without toeing able to manifest its presence by the production of noticeable 

 disease; and the limitation of the ordinary acute infections is looked upon as being 

 due to the development of sufficiently strong action of the phagocytes, alexins, or anti- 

 toxic substances formed in the system of the diseased individual during the course 

 of the infection. (Possibly, also, the exhaustion of some substances necessary for the 

 growth of the infection may aid in this limitation of the disease course.) 



Should the protection thus brought about be complete, so as to entirely prevent 

 the reacquirement of the disease, such acquired immunity is said to be absolute; if, 

 however, it permit the return of the disease, only in a less degree of severity, it is said 

 to be partial. Advantage is taken of the possibility of acquiring such protection to 

 induce it artificially (a) by "inoculation" with the natural virus of the disease in ques- 

 tion when known to be of a mild type; (6) by introducing the virus after it has in 

 some way been purposely weakened and is unable to induce the severe manifestations 

 of the disease, but is yet able to confer the desired immunity (vaccination) ; or (c) by 

 the introduction into the system of antitoxic material derived from individuals pre- 

 viously influenced by the disease itself or by the filtered toxins of the disease (as in 

 the use of diphtheria antitoxin obtained from horses which have been subjected pre- 

 viously to the filtered toxins of cultures of Mycobacterium diphtheria until they no 

 longer react to large doses injected into them). The immunity acquired by such 

 methods is spoken of as artificial immunity; other means are now and again available, 

 as the influence of drugs, or rarely, the antagonism exhibited by some associated 

 organism to the development or effects of the virus in question. The immunity which 

 is acquired by an attack of disease (either the severe natural affection, or the mild 

 natural disease transmitted to the individual in the practice of "inoculation," or 

 the purposely weakened disease induced in vaccination) is likely to be more or less 

 persistent, the antitoxic substances upon which it depends being produced within the 

 system of the individual and in some instances continued for an indefinite period 

 after the attack ; such immunity is said to be an active immunity. When, on the other 

 hand, the protection is afforded by the introduction of immunizing substances, as 

 antitoxin, from another individual, or of drugs, into the system, the effect lasts only 

 during the persistence of the dose administered ; when the material has been excreted 

 or destroyed in the system, the protection no longer exists. Such immunity is spoken 

 of as passive. 



Note. The following exercises, for the sake of time, should have been inaugurated 

 one or two weeks before the close of the class work; and provision for them should be made 

 as soon as the class has come to understand the conditions of bacterial culture. 



Exercise Si. Bach two students working together inoculate intra- 

 venously a rabbit with M. pyogenes aureus. In fatal cases let formal 



