DIPHTHERIA, 291 Upon potato the bacillus develops only when the reac- tion is alkaline. The potato-growth is not characteristic. Welch and Abbott always secured a growth of the organ- ism when planted upon potato, but do not mention the reaction of the medium they employed. Milk is an excellent medium for the cultivation of the Bacillus diphtherias, and is possibly at times a means of infection. Litmus milk is an excellent medium for ob- serving the changes of reaction brought about by the growth of the bacillus. At first the alkalinity, which is always favorable to the development of the bacillus, is destroyed by the production of an acid. When the culture is old the acid is replaced by a strong alkaline reaction. Palmirski and Orlowskil assert that the bacillus pro- duces indol, but only after the third week. Smith, how- ever, came to a contrary result, and found that when diphtheria bacillus grew in the dextrose-free bouillon that he recommends no indol was produced.2 Diphtheria as it occurs in man is generally a disease characterized by the formation of a pseudomembrane upon the fauces. It is a local infection, due to the presence and development of the bacilli in the pseudo- membrane, but is accompanied by a general toxemia resulting from the absorption of a violently poisonous substance produced by the bacilli. The bacilli are found only in the membranous exudation, and most plentifully in its older portions. As a rule, they do not distribute themselves through the circulation of the animal, though at times they may be found in the heart's blood. The most malignant cases of the disease are thought to be due to pure infection by the diphtheria bacillus, though such cases are more rare than those in which the Streptococcus pyogenes and Staphylococcus aureus and albus are found in association with it. In a series of 234 cases carefully and statistically studied 1 Centralblf. Bakt. u. Parasitenk., Mar., 1895. 2 Jour, of Exper. Med., vol. ii., No. 5, Sept., 1897, p. 546.