CHAPTER XXIV. 



PATHS OF ENTRANCE OF PATHOGENIC ORGAN- 

 ISMS, OR CHANNELS OF INFECTION. 



A. The Skin.— If the skin is healthy there is no oppor- 

 tunity for bacteria to penetrate it. It is protected not 

 only by the stratified epithelium, but also in various animals 

 by coats of hair, wool, feathers, etc. The secretion pressure 

 of the healthy sweat and oil glands acts as an effective bar 

 even to motile bacteria. Nevertheless a very slight injury 

 only is sufficient to give normal surface parasites and other 

 pathogenies, accidentally or purposely brought in contact 

 with it, an opportunity for more rapid growth and even 

 entrance for general infection. Certain diseases due to 

 higher fungi are characteristically ''skin diseases" and rarely 

 become general— various forms of favus, trichophyton 

 infections, etc. A few disease organisms, tetanus, malignant 

 edema, usually get in through the skin; others, black-leg and 

 anthrax, quite commonly; and those diseases transmitted 

 by biting and blood-sucking insects, piroplasmoses, trypano- 

 somiases, spirilloses, scarcely in any other way. Defective 

 secretion in the skin glands from other causes may permit 

 lodgement and growth of bacteria in them or in the hair 

 follicles. "Pimples" and boils in man and local abscesses 

 occasionally in animals are illustrations. Sharp-edged and 

 freely bleeding wounds are less liable to be infected than 

 contusions, ragged wounds, burns, etc. The flowing blood 

 washes out the wound and the clotting seals it, while there 

 is less material to be repaired by the leukocytes and they are 

 free to care for invading organisms (phagocytosis). Patho- 

 genic organisms, especially pus cocci, frequently gain lodge- 

 ment in the milk glands and cause local (mastitis) or general 

 infection. 



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