BACTERIA OF THE MOUTH 139 



and other mammalia, the following description will deal only with the 

 phenomena of disease in the human subject. 



Many bacterial diseases are common to man and the higher animals, 

 and, as far as we know, there is no parasite of man that is without effect 

 for some animal or another. This fact represents an inestimable advantage 

 in the study of disease, since it enables us, by inoculating animals with 

 pure cultures of bacteria, to put pathological science upon an experimental 

 basis. The whole of our knowledge of micro-organisms in relation to 

 disease, including the promising science of sero-therapeutics, is the direct 

 result of experiments on animals. 



Bacteria in the Human Body. Every human being, even the healthiest, 

 carries about with him innumerable harmless messmates and guests in 

 the shape of metatrophic bacteria. All the cavities of the body that are in 

 communication with the exterior contain them. The intestine, the moist 

 surface of the mucous membrane of the mouth, nose, pharynx, and 

 female genitalia, abound with them (117). They do not penetrate the 

 tissues, but live quite harmlessly in the secretions or excretions, the nature 

 and composition of which determine the character of the bacterial ' flora ' 

 in each particular spot. Some species are constant inhabitants, others only 

 occasional visitors, each kind multiplying and thriving where it finds the most 

 suitable food, and serving perhaps as a protection against the invasion of 

 other and perhaps pathogenic species. 



Even the relatively dry skin of our bodies always holds spores and 

 vegetative cells. Their character depends largely upon the occupation of 

 the individual, and their number, as might be expected, upon the degree 

 of cleanliness observed. 



The bacterial flora of the mouth is very rich in species (118). About 

 fifty have been described, some as accidental intruders, others as indigenous 

 organisms. Although never absent, their numbers can be reduced very 

 much by keeping the mouth scrupulously clean. As we have already 

 seen (p. i), their chief forms were known to Leuwenhoek. 



Formerly all the bacteria met with in the mouth, whether cocci, rods, 

 vibrios or spirilla, were classed together under one name. They were 

 supposed to be different developmental stages of a single species, Lepto- 

 thrix buccalis, which commonly occurs in the form of long unbranched 

 cell-chains that sprout out from fragments of food, and from the contents 

 of the tonsillar follicles. Such a view is of course incorrect, and the name 

 L. buccalis must be used only as a collective designation for mouth bacteria. 

 Some of these, such as Bacillus maximus buccalis (Fig. 26) and a coccus 

 form (lodococcus), give the granulose reaction. Others, such as Vibrio 

 buccalis (Fig. 2,6, g\ Spirochaete dentium (/), and LeptotJirix innominata, 

 stain yellow with iodine. 



Most of the bacteria of the mouth have as yet resisted all attempts 



