=Upper Respiratory Tract.=--The oral cavity and pharynx are seldom free from bacteria. Miller has studied over one hundred species that he has found under various circumstances in the human mouth. Some of these are pathogenic; others are apparently absolutely innocent. Many of the forms which grow in saliva will not grow in ordinary media. (See Plate III, illustrating infection of the teeth.) Miller has also shown that many forms of dental caries are but expressions of bacterial invasion even of those apparently most solid structures, the teeth; and of late we have been taught more fully that such invasion may extend far beyond the confines of the teeth alone, and may spread to various, even to distant parts, and produce possibly fatal mischief. Abscesses in the brain and extensive septic infections have been traced to invasion along the line of the dental tubules. One of the most virulent of all the common inhabitants of the mouth is the pneumococcus of Fränkel, known also as the micrococcus lanceolatus of Stebernrg. In virulence it is a variable organism, but it is present in a virulent state in only 12 or 15 per cent. of cases of infection due to it. This is the organism which is the cause of lobar pneumonia, and frequently of bronchopneumonia, as well as of numerous phlegmons and other inflammations of the throat, and which, getting into the general circulation through the tonsils or other possible ports of entry about the mouth, causes serious septic and inflammatory disturbances in widely distant regions. Aside from dental caries, a widely opened port of entry is often afforded by those ulcerations around the margins of the gums which are produced by accumulations of tartar. Disease in the antrum of Highmore, for instance, and many other local destructions, are frequently caused in this way. The next most common port of entry is the _tonsils_, faucial, lingual, and pharyngeal, which contain a variety of crypts which are often filled with secretions or retentions loaded with bacteria. One of the most common sources of an involvement of the cervical lymph nodes in tuberculous disease is an infection springing first from the tonsils or the teeth. In spite of the fact that myriads of bacteria are swept into the nasal cavities with the air we breathe, few are seen in the nose. A peculiar capsule bacillus, closely allied to that described by Friedländer, has been found in a number of cases of ozena, while the pneumococcus of Fränkel is also often found there, and is known to produce abscesses of the brain. One specific organism--namely, that of _rhinoscleroma_--concerns the nose almost solely, its first ravages being met with in this location.