156 THE MORE CHRONIC INFECTIOUS DISEASES 



reaction in a tuberculous person. This refined pro- 

 duct is used in the conjunctival test by dropping a 

 small quantity into the eye. If tuberculosis be present 

 a congestion and discharge will appear in the conjunc- 

 tiva within forty-eight hours. 



It is claimed by many that all adults have some 

 tuberculosis in their body, acquired during childhood, 

 which has remained quiet or has healed completely, 

 but which has left their blood in such a condition that 

 a tuberculin reaction will appear. For this reason the 

 skin test may be positive in adults who are really not 

 suffering from their slight latent infection, and it is 

 therefore not reliable. It should only be used in chil- 

 dren. The supposed cause of the tuberculin test either 

 under or upon the skin, is the stimulation of the tuber- 

 culous disease by the introduced toxin, and the out- 

 pouring from the tubercles of more of their own poison. 

 No reaction of any sort follows the administration of 

 tuberculin to persons free from tuberculosis. 



Morphology and General Characteristics. The tubercle 

 bacillus is a true parasite, that is, it does not multiply 

 in nature outside the animal body. It is a rather 

 large organism, about y-(njTf inch wide and from 

 Wir no to sinnr inch long. It may be straight or slightly 

 bent, usually single, but also in pairs. It is non- 

 motile, and produces no spores. It stains with con- 

 siderable difficulty, owing to its thick cell wall. There 

 is much fatty and waxy matter in the tubercle bacillus 

 which gives it its resistant power. It grows upon 

 laboratory culture media very slowly. For this reason 

 it must be obtained in as pure a condition as possible. 

 Cultures are best made from the lesions in guinea- 



