90 INFECTION 



wide-spread metastases in the heart valves and joints. The local syphi- 

 litic lesion is quickly followed by general infection. Chancroids alone 

 remain localized, although the initial lesion frequently spreads quite 

 rapidly by continuity of tissues. In rarer instances other microorgan- 

 isms, such as the ordinary pyogenic cocci, tubercle bacillus, and diph- 

 theria bacillus, may infect these organs and be transmitted by sexual 

 conjugation. 



5. There is considerable controversy of opinion regarding the suscep- 

 tibility of the placenta and the filtering properties it possesses for various 

 infectious agents. A study of this subject by Nee' low 1 would indicate 

 that the non-pathogenic bacteria do not pass from the mother through 

 the placenta to the fetus. Other pathogenic agents may, however, pass 

 through quite readily, for example, pregnant women suffering from 

 smallpox may be delivered of infants showing active lesions of prenatal 

 infection, and syphilitic infection of the fetus is a well-known condition. 

 Most controversy centers around congenital tuberculosis, and directly 

 opposing views for and against prenatal infections are held by several 

 authorities. Baumgarten is of the opinion that many children are 

 subject to antenatal infection, though the disease infrequently develops 

 in a few of them. 



The general subject of antenatal infection and pathology is a field 

 requiring considerable investigation and research. 



NORMAL DEFENSES AGAINST BACTERIAL INVASION 



When the large area of the body that is subject to traumatic injury 

 and accidental infection is considered, it is remarkable that, considering 

 the enormous numbers of various bacteria, infection does not occur more 

 frequently. 



Bacterial invasion of the tissues is of frequent occurrence, but in 

 health they do not usually cause infection, and tend to be destroyed very 

 soon after they enter the tissues. 



It may be well to discuss at this point the factors tending to prevent 

 invasion, and leave the consideration of the defensive mechanism, 

 whereby the body destroys bacteria after successful invasion and thus 

 prevents infection, for the chapter on Natural Immunity. 



Of the factors preventing bacterial invasion, the following are recog- 

 nized: 



1. The structure of the surface layer of epithelium. The epidermal 

 1 Centralb. f. Bakt., 1902, orig., xxxi, 691. 



