246 SUSCEPTIBILITY AND IMMUNITY. 
result from the introduction into the bodies of susceptible animals of 
the toxic substances produced by certain pathogenic bacteria. The 
first satisfactory experimental evidence of this important fact was 
obtained by Salmon and Smith in 1886, who succeeded in making 
pigeons immune from the pathogenic effects of cultures of the bacil- 
lus of hog cholera by inoculating them with sterilized cultures of 
this bacillus. In 1888 Roux reported similar results obtained by in- 
jecting into susceptible animals sterilized cultures of the anthrax 
bacillus. Behring and Kitasato, in 1890, reported their success in 
establishing immunity against virulent cultures ‘of the’ bacillus of 
tetanus and the diphtheria bacillus by inoculating susceptible ani- 
mals with filtered, germ-free cultures of these pathogenic bacteria. 
In 1892 Behring, Kitasato, and Wassermann published the re- 
sults of interesting experiments with a bouillon made from the 
thymus gland of the calf. They found that the tetanus bacillus cul- 
tivated in this bouillon did not form spores and had comparatively 
little virulence. Mice or rabbits inoculated with it in small doses— 
0.001 to 0.2 cubic centimetre for a mouse—proved to be subsequently 
immune. And the blood serum of an immune rabbit injected into 
the peritoneal cavity of a mouse—0.1 to 0.5 cubic centimetre—was 
found to give it immunity from the pathogenic action of a virulent 
culture of the tetanus bacillus. Similar results were obtained with 
several other pathogenic bacteria cultivated in the thymus bouillon— 
spirillum of cholera, bacillus of diphtheria, typhoid bacillus. We 
give here the directions for preparing the thymus bouillon as used by 
the authors named: 
Two or three thymus glands are chopped into small pices immediately 
after they are taken from the animal. An equal part of distilled water is 
added to the mass and stirred for some time ; it isthen placed in an ice chest 
for twelve hours. The juices are now expressed through gauze by means of 
a flesh press. A clouded, slimy fluid is obtained, which constitutes a stock 
solution. This is diluted with water, and a certain quantity of carbonate of 
soda is added to the solution before sterilization. By this means coagulation 
and precipitation of the active substance from the thymus gland are avoided. 
The exact amount of water and of sodium carbonate required to prevent pre- 
cipitation must be determined by experiment, asit differs for different glands. 
Usually an equal portion of water and sufficient soda solution to turn litmus 
paper feebly blue will give the desired result. The liquid is now heated in 
a large flask, which is left for fifteen minutes in the steam sterilizer, The 
liquid is allowed to cool and then filtered through fine linen to remove any 
suspended coagula ; the filtrate has a milky opalescence. It is now placed 
in test tubes and again stérilized. The active principle is precipitated by the 
addition of a few drops of acetic acid. 
In Pasteur’s inoculations against hydrophobia, made subsequently 
to infection by the bite of a rabid animal, an attenuated virus is in- 
