MICROBIAL DISEASES OF MAN AND ANIMALS. 649 



being deep-seated, tense, opaque, covered with a tough skin. There are 

 many other points of distinction, and any one familiar with the two 

 diseases can hardly fall into error when dealing with typical cases at 

 whatever stage they are encountered. To the layman's eye, however, the 

 two are often indistinguishable. 



Scarlet fever, measles, German measles, and Duke's disease are often 

 likewise confused by the laity and even by physicians who have not 

 had opportunities for extensive study. 



German measles is clinically related to true measles somewhat as 

 chickenpox is to smallpox, i.e. they are wholly distinct diseases yet 

 show characteristics easily confused on superficial consideration. Dukes 

 disease is perhaps not a distinct entity; much has been said on this 

 point and a satisfactory decision will probably never be reached 

 until the causative agents have been found. It may be described 

 briefly for clinical purposes as a scarlatiniform variety of German 

 measles. 



Scarlet fever has an average incubation period of about five days, 

 but varies somewhat widely from this. The prodromes are those usual 

 to all these infections headache, fever, and sore throat, but the latter' 

 is especially severe. Within twenty-four hours the rash appears usually 

 on the chest first, a bright scarlet superficial flush, extending rapidly 

 over the body. 



In measles the incubation period is longer, averaging nine days, almost 

 without any variation. The prodromes, headache, fever, and sore throat 

 are accompanied by very marked coryza and photophobia, catarrh and 

 "cold on the chest." 



The rash appears about the fourth day, appearing on the face and 

 back but rapidly extending. It is darker, bluer, and deeper than the 

 scarlet fever rash. Koplik's spots appear on the buccal membrane early 

 in the disease. 



In German measles the prodromes are so indefinite that it is difficult 

 to determine their length; very commonly the rash is the first thing no- 

 ticed. It appears on the face, chest, back, and arms as a light subcuticu- 

 lar mottling (measles type) or a more uniform pink flush (scarlatini- 

 form or Dukes type) ; with this rash the eyes show some injection and 

 slight photophobia develops. The attack passes off quickly, without 

 complications. 



