(_a_) The external auditory meatus is rarely the seat even of local infection. The tympanic cavity is normally sterile, though it may become infected by extension through the Eustachian tube from the pharynx (_otitis media_). (_b_) The conjunctiva is frequently the seat of localized, very rarely the point of entrance for a generalized infection, except after severe injury. Those diseases whose path of entrance is generally assumed to be the respiratory tract (see “Lungs” below) might also be admitted through the eye. Material containing such organisms might get on the conjunctiva and be washed down through the lachrymal canal into the nose. Experiment has shown that bacteria may pass in this way in a few minutes. In case masks are worn to avoid infection from patients suffering with these diseases, the eyes should therefore be protected as well as the nose and mouth. (_c_) The nasal cavity on account of its anatomical structure retains pathogenic organisms which give rise to local infections more frequently than other mucosæ of its character. These may extend from here to middle ear, neighboring sinuses, or along the lymph spaces of the olfactory nerve into the cranial cavity (meningitis). Acute coryza (“colds” in man) is characteristic. Glanders, occasionally, is primary in the nose, as is probably roup in chickens, leprosy in man. The meningococcus and the virus of poliomyelitis pass from the nose into the cranial cavity without local lesions in the former. (_d_) The mouth cavity is ordinarily protected by its epithelium and secretions, though the injured mucosa is a common source of _actinomycosis_ infection, as well as thrush. In foot-and-mouth disease no visible lesions seem necessary to permit the localization of the unknown infective agent. (_e_) The tonsils afford a ready point of entrance for ever-present _micrococci_ and _streptococci_ whenever occasion offers (follicular tonsillitis, “quinsy”), and articular rheumatism is not an uncommon sequel. The diphtheria bacillus characteristically seeks these structures for its development. Tubercle and anthrax organisms occasionally enter here. (_f_) The pharynx is the seat of localized infection as in _micrococcal_, _streptococcal_ and diphtherial “sore throat” in human beings, but both it and the esophagus are rarely infected in animals except as the result of injury. (_g_) The external genitalia are the usual points of entrance for the venereal organisms in man (gonococcus, _Treponema pallidum_, and Ducrey’s bacillus). The bacillus of contagious abortion and probably the trypanosome of dourine are commonly introduced through these channels in animals.