286 LABORATORY EXERCISES IN BACTERIOLOGY. 



following the inoculation (this is indicated by their discovery in isolated condition 

 in increased numbers in the body of the animal). This last constitutes the fourth 

 of Koch's postulates for the proof of the causal relation of a given germ to disease: 

 "From the fluids and tissues of the experimentally diseased animal the same organism 

 inoculated must be recovered." 



The autopsy should be conducted as soon after death as possible, lest contaminat- 

 ing organisms from the intestinal canal or putrefying germs find their way into the 

 tissues and fluids of the body; if it is impracticable to perform the operation at once, 

 the dead animal should be refrigerated in order to prevent such occurrence. In 

 conducting the autopsy the utmost care to prevent contamination must be observed. 

 The usual preliminary observations as to the body, general appearance, external 

 lesions and marks, body weight, degree of nutrition, temperature, rigidity, condition 

 of site of inoculation, etc., are first made. The animal is then laid on its back on a 

 convenient board and the legs extended and fastened to the board by means of tacks 

 or staples. The skin is well wet with a solution of bichloride of mercury, and, a median 

 incision having been made from the pubic region to the throat, the skin is dissected 

 well off the abdomen and thorax and tacked out on the board so as to remove the 

 likelihood of accession of bacteria from it to the parts to be exposed. The site of in- 

 jection of the infection and any other subcutaneous lesions are now inspected, the 

 removal of the skin affording opportunity for close observation ; and from the inocula- 

 tion site are made a number of inoculations on various nutrients, smears on glass 

 slides are prepared for microscopic examination, and bits of the tissue are placed in 

 absolute alcohol, where they are fixed for section-making. The body cavities are now 

 opened by the usual longitudinal incision in the median line, with such additional 

 lateral incisions as may be required for inspection of the interior, with a sterile pair 

 of scissors (boiled and kept in boiled water). On opening the cavities, inoculations 

 and smears for microscopic examination are made from any exudate encountered ; 

 and subsequently inoculations are to be made and smears prepared on glass slides 

 from each of the important organs and the blood as detailed in Lesson V. Likewise, 

 small bits of. tissue from all suspicious points and the principal organs are fixed in 

 absolute alcohol for sections. The remains of the animal should then be cremated, 

 the autopsy board well disinfected, and the hands of the operator disinfected and 

 well washed with soap and water. (In case of severe infections it is a wise practice 

 to have the hands incased in rubber gloves during the above manipulations.) 



The subsequent study of the material thus obtained concerns the discovery of 

 the organisms in the film preparations and in the sections of the tissues, their isolation 

 from some or all of the points from which inoculation to nutrient media was made 

 (by culture of the inoculated media), and the recognition of destructive, degenerative, 

 or inflammatory changes in the various structures; all of which are to be compared 

 with the alterations known to have been produced by the original disease in case 

 the infection was derived from a diseased individual. 



Should the experiment animal or animals have recovered from the effect of the 

 inoculation, after some days it is well to repeat the same with a view of recognizing 

 any degree of immunity which may have been acquired from the original attack. We 

 are accustomed to speak of the natural protection possessed by an individual against 

 the effects of a given virus as his natural immunity against that germ, contrasting 

 with it the protection gained by artificial means or through a prior attack of the same 

 disease (acquired immunity). Natural immunity against the invasion and influences 

 of bacteria in general depends on one or a number of the following factors: (a) The 



