198 GENERAL BIOLOGY OF MICRO-ORGANISMS 



lorum. The human ovum also seem occasionally to be infected 

 with Spirochata pallida in this way. It may also become in- 

 fected with the same organism derived from the seminal fluid. 

 The developing fetus is sometimes invaded by pathogenic micro- 

 organisms introduced through the placental circulation. The 

 organisms of tuberculosis, small-pox, typhoid fever and the 

 pyogenic cocci are known to be transmitted, somewhat uncom- 

 monly to be sure, in this way. As a rule the germ must be circu- 

 lating in the blood of the mother in considerable numbers, or 

 there must be actual infectious lesions of the placenta before 

 placental transmission occurs. After birth non-pathogenic mi- 

 crobes gain access to the entire surface of the body and penetrate 

 the various canals opening to the exterior to certain normal 

 limits. Pathogenic germs may be introduced with the food and 

 drink, which is the common natural mode of infection with cholera 

 and typhoid fever in man and with tuberculosis in hogs and cattle. 

 The barrier presented by the activity of the gastric juice is fre- 

 quently passed in safety by the ingested microbes. Inhalation 

 is probably the most common way in which tuberculous infection 1 

 reaches the lungs in man, although there is conclusive evidence 

 that tuberculosis in this location may be derived from the alimen- 

 tary tract through the blood stream. Experimentally, guinea- 

 pigs are much more susceptible to infection with tubercle bacilli 

 by inhalation than by ingestion. Mere application of the in- 

 fectious agents to the epithelial surface of the skin or mucous 

 membranes results in infection in many instances and, indeed, 

 infection by ingestion and inhalation may be regarded as examples 

 of this. The mucous membranes of the urethra and the eye, and 

 also of the rectum in young children, are especially susceptible to 

 infection with the gonococcus. The unbroken skin may be infected 

 with staphylococci, which seem to penetrate through the hair fol- 

 licles and sebaceous glands, giving rise to boils and carbuncles; but 

 to most microbes the uninjured skin presents an effective barrier. 



1 McFadyean, Journal Royal Institute of Public Health, 1910, Vol. XVIII, pp. 

 703-724. 



