Skin and Mucous Membranes.
—Of all possible sources of infection, the skin itself is probably the most fertile. It is exposed to contamination by air and by everything which may come in contact with the body, and there is perhaps no organism met with in disease which may not be found upon its surface or within its recesses. In fact, these recesses, such as the crevices beneath the nails, the spaces between the toes, and the various pockets like the tonsils, the axillæ, etc., are those most commonly inhabited by microörganisms.
Bacteria may penetrate the skin by means of three different routes, namely, the sweat glands, the hair follicles, and the sebaceous glands, by means of their regular openings. The hairy appendages of the skin are even greater sources of danger than the skin itself, since a direct path of infection into the depths of the skin is afforded by their follicles. Experimentally it has been shown that when bacteria are rubbed into the skin where there are no follicles, there is freedom from infection, whereas the reverse is equally true, and it is clinically generally recognized that furuncles and carbuncles form almost exclusively in those parts provided with hair and sebaceous glands.
The mucous membranes are in constant contact with microörganisms and furnish conditions in many respects favorable for their rapid development. Nevertheless, the latter is interfered with and often inhibited by certain mechanical and chemical influences which afford protection. The conjunctiva is an extremely exposed membrane, which harbors, however, but a relatively small number of bacteria under ordinary circumstances. The tears before escaping from the conjunctival sac are sterile, and are probably saline enough to act as an antiseptic bath for the cornea. Moreover, by free escape of secretion through the nasal duct the conjunctival sac is kept constantly irrigated, to which is mainly due its ordinary healthy condition, as it is well known how commonly lesions follow obstruction to the lacrymal duct. The horrible results of Egyptian ophthalmia, i. e., the pyogenic form of conjunctivitis, are familiar to travellers in Egypt. Howe and others have shown that this disturbance is due to flies, which are carriers of infection, and are attracted toward the eyes of infants, while the superstitious notions of the parents restrain their children from instinctive protection of the eyes when thus irritated. There is probably no greater common carrier of pyogenic infection than the common house-fly, and nowhere is this agency more demonstrated than in the hot climates of the Orient.
PLATE III
FIG. 1
Artificial Dental Caries in Cross-section. Tubules Filled with Bacteria. (Miller.)
FIG. 2
