PATHOGENIC FUNGI, MOLDS AND YEASTS 395 



the center of the nodule. The skin above becomes thinned 

 and softened. Serum is exuded from the surface and the 

 hair becomes loosensd and a crust holding the hair together 

 is formed. Finally the ulcers become crater-shaped and 

 contain a little creamy pus. The disease is not common but 

 has been recognized at several points in the United States, 

 particularly in Pennsylvania. 



The Genus Trichophyton.— Many different species of 

 this genus of molds have been described. Most of them were 

 isolated from eases of disease known as herpes tonsurans 

 and ringworm. In every case they are molds which grow 

 into the hairs, compactly filling the interior with a mass 

 of parallel hyphse extending longitudinally. These may 

 be readily seen by examining hairs from diseased areas 

 under the microscope after immersing for a few moments in 

 a warm solution of caustic potash. The organisms may be 

 grown readily upon suitable cultural media. Ringworms 

 have been described from horse, cat, cattle, sheep, swine and 

 dog. In many of these eases the disease has been known to 

 be transmissible to man. 



The Genus Microsporum. — Species of this genus are 

 closely related to members of the preceding genus of 

 Trichophyton and like them they invade the hair. In all 

 cases the organisms grow over the surface of the hair and 

 encase it for several millimeters from the skin with a deli- 

 cate white sheath. Four different species are known which 

 produce skin disease in man and some seven species have 

 been described from animals including the dog, the horse 

 and the cat. 



The Genus Achorion. — The organisms belonging to this 

 genus likewise closely resemble the preceding genera 

 Trichophyton and Microsporum. In man the Achorion 

 schonleinii is the common cause of favus, a disease of the 

 scalp characterized by the destruction of the hair and the 

 formation of a crust or shieldlike layer over the surface. A 



