CAUSING CHRONIC INFECTIONS. 93 



sions and ascitic fluid the number of tubercle bacilli 

 is always small; so to detect them the inoculation of Exudates 

 guinea-pigs with the fluid is often practised. If 

 tubercle bacilli are present in the fluid injected, the dis- 

 ease develops in the animal after a period of three to 

 six weeks. 



The tubercle bacillus may cause infection by 

 entering the body in the following ways : 



Hereditary transmission, long believed to be a 

 common occurrence, has not been proven among 

 human beings. In very rare instances the bacilli may 

 pass from the mother to the child in the uterus, but 

 this depends upon some injury or disease of the 

 placenta. 



Respiratory: This is the most common way that 

 infection takes place. The sputum of consumptives is 

 the direct carrier of the infection. Deposited in 

 houses, on floors and streets, the bacilli become incor- 

 porated with the dust which is breathed in by those in 

 close contact with the patients. The careless disposal 

 of sputum is responsible for the greatest number of 

 infections. 



Intestinal : This is more common in children than 

 in adults. The bacilli gain entrance through the milk 

 from tuberculous cattle or food infected by consump- 

 tive people. The habit children have o>f putting every- 

 thing into their mouths, is responsible for many in- 

 fections, particularly in houses where consumptives 

 are living. The bacilli resist the action of the acid in 

 the stomach, and in the intestine may penetrate the 



