Infectious Agents. 6i 



albumens and in the absence of oxygen (Wiener, Hueppe). From 

 such considerations it may easily be appreciated that virulence is 

 a very variable property.* 



It is also essential for the development of an infectious dis- 

 ease that the infectious agent should be afforded a suitable path 

 of entrance or atrium to the tissues. Mere contact with a micro- 

 organism does not necessarily result in disease; there are often 

 pathogenic microbes upon the surface of the skin and mucous 

 membranes, the host in no wise suffering in consequence. The 

 virulent germs of tetanus and of spreading gangrene are very fre- 

 quently present in the intestinal canal of herbivora and omnivora, 

 but without inducing pathological results as long as the mucous 

 membrane is intact, their toxines being neutralized and destroyed 

 by the digestive juices. So, too, pyogenic and putrefactive micro- 

 organisms are found in large numbers in the intestinal contents 

 and externally upon the skin in healthy human beings and animals, 

 becoming ' pathogenic only in case of introduction into the lymph 

 and blood through some tissue lesion. 



The ordinary places through which microorganisms gain ac- 

 cess to the tissues are the external skin with its gland pores, the 

 digestive and respiratory tracts, the conjunctival mucous sur- 

 faces and the uro-genital passages. The protective epithelium of 

 the skin and mucous membranes interferes with the penetration 

 of most bacteria into the tissues and with their toxic action, partly 

 because of the impenetrable barrier afforded (the horny epithelial 

 layer), partly because the secretions of normal mucous membranes 

 may wash off and destroy the microorganisms and dilute 

 their toxines to such an extent that they are rendered inert or 

 neutralized. This protective means is not an absolute one against 

 some of the bacteria. Some may directly or by growth-extension 

 penetrate the unaltered skin or mucous membranes; or, having 

 gained entrance to sebaceous and sweat glands, especially the 

 glandular ducts or lymph follicles (which are open upon the 

 surface as in the pharynx and intestine), may be carried into the 

 deeper structures by leucocytes (glandular or follicular infec- 

 tion). Many microbes are provided with means of motility (flag- 

 ella) and are thus able to penetrate into canals, or from a surface 

 may find their way into slight depressions or inappreciable defects 

 in the epithelium, where, after local increase, their toxic metabolic 



'See further Kitt, Bakterienkunde fur Tierarzte, M. Perle, publisher, Wien, 

 1903, 4th edition. 



