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90 THE MICRO-ORGANISMS OF THE HUMAN MOUTH. —

14. Bacillus g.
15. Bacillus h.
16. Leptothrix.
Vignal's table is characterized by the predominance of bacilli.
With the exception of the two pyogenic bacteria, he met with
but a single coccus form (Coccus a), while other investigators
Black, Biondi, Gysi, and myself—found a preponderance of
coccus forms. An examination of a few preparations derived
from mucous tooth deposits or carious dentine will, as a rule,
show a prevalence of micrococci.
This difference in the results of different investigators can be
explained, I think, by the circumstance that Vignal cultivated all
of his bacteria on gelatine. Since, however, very many mouth-
bacteria do not grow on gelatine, cultures made upon agar-agar
may lead to different results.
In America, Black ''^ has entered upon the study of the micro-
organisms of the human mouth with great zeal. He describes
among others Streptococcus contimwsus, Staphjlococcus medius,
Staph ijlococcus magnus, Coccus cumulus mmor (probably Sarcina or
INIicrococcus tetragenus), and a "gelatine-forming" organism,
Bacillus gelatogenes. (See page 22.)

Chromogenic Mouth-bacteria.

It needs scarcely to be remarked that many theories have been
proposed to account for the various colors presented by decayed
dentine. The best known is that of Watt,*'^ which attributes
these colors to the action of various mineral acids, supposed to
be concerned in the production of caries. Another view assigns
the chief role to articles of food, drink, etc., which do certainly
sometimes produce discoloration of the decayed as well as of the
healthy tooth-tissue.
Others, again, would have us believe that the color comes from
within and is one of the results of the vital reaction of the tooth-
substaiice itself. Black seeks to account for the discoloration
by the impregnation of the carious tissue with sulphides, while,
finally, the view has not been wanting in advocates that certain
bacteria which. possess the property of forming various coloring