THE SKIN 223 



to care for invading organisms (phagocytosis). Pathogenic 

 organisms, especially pus cocci, frequently gain lodgment in 

 the milk glands and cause local (mastitis) or general infection. 



B. Mucosae directly continuous with the skin and lined with 

 stratified epithelium are commonly well protected thereby and 

 by the secretions. 



(a) The external auditory meatus is rarely the seat of even 

 local infection. The tympanic cavity is normally sterile, 

 though it may become infected by extension through the 

 Eustachian tube from the pharynx (otitis media). 



(6) The conjunctiva is frequently the seat of localized, 

 very rarely the point of entrance for a generalized infection, 

 except after severe injury. 



(c) The nasal cavity, on account of its anatomical structure 

 retains pathogenic organisms which give rise to local infec- 

 tions more frequently than other mucosae 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 epi- 

 thelium 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 tonsilitis, "quinsy"), and articular rheumatism is 

 not an uncommon sequel. The diphtheria bacillus charac- 

 teristically seeks these structures for its development. 

 Tubercle and anthrax organisms occasionally enter here. 



(/) 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. 



