604 INFECTION AND IMMUNITY. 



tion and then a gradual return to normal within 

 a week or two. 



2. Negative reactions show the same phenomena 

 as the control site. The swelling lasts only 

 twenty-four hours, and the areola is under 5 mm. 

 in diameter. 



3. The positive reaction: (a) Incubation period, 

 which lasts from three to twenty-four hours. In 

 most cases the reaction is fully developed in 

 twenty-four hours. Those developing later than 

 twenty-four hours v. Pirquet calls "torpid." These 

 torpid reactions occur more frequently in older 

 than in young children and in clinically unsus- 

 pected cases. It occurs in manifest tuberculosis 

 only exceptionally. 



(b) Development: The inflammatory reaction 

 begins usually with a slightly raised areolar red- 

 dening which spreads from the scarification site 

 and increases rapidly in diameter and height. The 

 diameter of the papule is on an average about 1 

 cm. but may reach 3 cm. Small vesicles may form 

 on the surface of the papule. The color differs 

 with the normal coloring of the skin; usually it 

 is of a deep red color. Very pale papules some- 

 times develop in cases of fatal tuberculosis (cach- 

 ectic reaction). The border of the papule is some- 

 times sharp, sometimes irregular and at times 

 small papules may be found surrounding it. 



(c) Eetrogression. The maximum development 

 is usually reached in forty-eight hours and after 

 this the swelling declines and the red color changes 

 to a violet, then to a yellow color and finally 

 becomes brown. The swelling disappears usually 

 in from five to eight days and the pigmentation 



