7. =Fetal Infection.=--It is only in a very limited class of cases that infection can be transmitted from mother to fetus, but there are instances of this kind in which the surgeon is deeply concerned. As Welch has stated, syphilis is the only infection capable of direct transmission through the ovum or spermatozoön; but intra-uterine infection may occur in many ways, and many diseases may be thus transmitted. The placenta is usually regarded as a perfect filter; nevertheless, it is occasionally passable to microörganisms. These may be caused by preëxisting lesions in the placenta or by the virulence and activity of bacteria. It is known that in animals the bacilli of chicken cholera (inoculated into the mammalia), of symptomatic anthrax, and the pyogenic cocci, frequently traverse this barrier. In mankind infection _in utero_ has been observed in smallpox, measles, scarlatina, relapsing fever, syphilis, tuberculosis, croupous pneumonia, typhoid fever, anthrax, and surgical sepsis. SOURCES OF INFECTION. That the effects of bacterial invasion may be anticipated and guarded against most effectually it is necessary that the practitioner should be thoroughly familiar with the sources from which they come, and the localities in and about the body which they most commonly inhabit or where they are met with in largest numbers. =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.