1907.] PUBLIC DOCUMENT No. 73. 165 



from a week to ten days. The severe localized form of dermatitis is 

 perhaps more prevalent in May and June, as it is then that the cater- 

 pillars are reaching their maturity. The form of dermatitis in which 

 the rash is scattered is common when the moths are flying in large num- 

 bers (July), as well as earlier, in the season of the caterpillars. A 

 dermatitis may be acquired at any season of the year by the handling 

 of large numbers of nests or cocoons. The wearing of an undergarment 

 which had been packed away a year since its contamination with nettling 

 hairs has often been sufficient to produce dermatitis. 



The material on which the histological study of the dermatitis is based 

 consists in part of a piece of skin excised from the upper arm of the 

 author twenty hours after inoculation with the nettling hairs of a cater- 

 pillar collected early in March. The tissue after fixation in Zenker's 

 fluid was imbedded in paraffin and serial sections made. Without serial 

 sections it would be difficult, except perhaps by accident, to find the 

 nettling hairs in the tissue. Material for supplementary study was fur- 

 nished by lesions produced experimentally in animals. 



The lesions in the human skin may be summed up as follows. The 

 nettling hairs are found imbedded at various depths; some penetrating 

 in a direction oblique to the surface are situated superficially in the 

 epidermis, others have penetrated the entire thickness of the epidermis, 

 and others have passed through the epidermis and have penetrated the 

 corium for nearly their entire length. There is necrosis of the epider- 

 mis around the nettling hair, and in most instances there is exudation 

 of fluid into the epidermis, so that a tiny vesicle is formed. The con- 

 tents of the vesicle consists of clear fluid, in which are disintegrating 

 epithelial cells, a few large mononuclear cells (phagocytes), and numer- 

 ous eosinophiles, which in many instances are completely disintegrated. 

 The latter cells are found constantly, and appear to be the earliest cells 

 to invade the epidermis about the foreign hair. Many occur with irregu- 

 lar pseudopodia-like processes, as though fixed when in amoeboid motion. 

 The disintegration of these cells forms masses of eosin staining granules, 

 which are found in the spaces in the epidermis. In certain instances 

 the space in the epidermis occupied by the exudate appears continuous, 

 with a dilated lymphatic. There is marked inflammatory reaction in the 

 corium, as shown by the presence of large collections of cells about the 

 blood vessels. These collections of cells consist of large mononuclear 

 (or transitional) cells which are often phagocytic, of lymphoid cells, 

 and of numerous eosinophiles. An occasional polynuclear leucocyte is 

 found. 



Thus the process in the human skin consists of the penetration of 

 the nettling hairs into or through the epidermis, the necrosis of con- 

 tiguous epidermal cells, and exudation into the epidermis, a constant 

 element in which are numbers of eosinophile cells. There is undoubt- 

 edly local congestion and edema of the corium, but this did not appear 

 to any marked degree in the sections studied. 



