138 INFECTION AND IMMUNITY. 



are vulnerable points in the defense which the 

 cutaneous surface represents. Although they are 

 protected somewhat by the flow of their excretions, 

 especially in warm weather, and although the en- 

 trance of germs is made more difficult by the con- 

 traction of the skin and consequent narrowing of 

 the orifices in cold weather, yet various incidents 

 may lead to the introduction and retention of 

 virulent micro-organisms in these structures. 

 When this occurs there is little difficulty in the 

 way of their producing necrosis of the epithelium, 

 invading the surrounding tissue and causing a 

 pustule, boil, carbuncle, cellulitis, or even a gen- 

 eralized infection. The secretion of the sebaceous 

 glands appears to be not germicidal. On the other 

 hand, the acid nature and certain salts found in 

 perspiration render this fluid antagonistic to the 

 development and virulence of certain micro-organ- 

 isms. 



The serous exudate, and the crust which forms 

 subsequent to an abrasion, antagonize infection. 

 The serum itself contains germicidal substances, 

 while the crust mechanically prevents microbic 

 invasion. 



Soluble poisons such as aconite and bacterial 



Connective . * 



Tissue, toxins are not absorbed through the unbroken skin. 

 Even after germs penetrate the epidermis, the 

 subcutaneous connective tissue is often an obstacle 

 to their further extension. The subcutaneous in- 

 jection of some micro-organisms (e. g., cholera) 

 is tolerated better by animals than one given into 

 the abdominal cavity or blood vessels. We are 

 also familiar with the benign course of lupus 

 compared with visceral tuberculosis; the same is 

 true of cutaneous and visceral glanders. This re- 



