THE OUTER SKIN. :\'l\ 



cells. The lowermost layer has quite high cylindrical cells 

 lying beside one another. The prickles directed downward, 

 with fine fibres extending between them from the corium, as 

 well as the presence of cement substance, provide a means of 

 joining the epidermis firmly to the corium. The stratum 

 spinosum, whose cells have been described above, consists of 

 many rows of cells which fill up the free spaces between the 

 papillae. Above this is the stratum granulosum, consisting 

 usually of two or three rows of flattened cells. These pos- 

 sess refractive granules, which indicate the beginning of the 

 process of cor nifi cation. They are spoken of as keratohyaline 

 granules, and are regarded by some authors as modified cell 

 protoplasm, arid by others as a product of the dying nucleus 

 of the cell. This latter view finds some support in the fact 

 that often the development of keratohyaline granules is accom- 

 panied by a poverty of the nucleus in chromatin and its final 

 disintegration. 



Above the stratum granulosum there is a refractive layer, 

 the stratum lucidum, which consists of two or three layers of flat 

 cells. These possess disintegrating nuclei, and contain a homo- 

 geneous substance called eleidin, which is derived from the 

 keratohyaline granules. The latter increase in size and coalesce 

 to form a semifluid substance, which develops new staining re- 

 actions. Keratohyaline stains with ha3matoxylin, while eleidin 

 is colored by" eosin or nigrosin. In this layer the boundaries 

 of the cells are often not distinct. The stratum lucidum is 

 often wanting in places where the epidermis is thin. It forms 

 a direct transition to the horny layer. 



The cells of the horny layer (stratum corneum) are like thin 

 scales and show no remains of nuclei. The whole cell is made 

 up of keratin, which, as opposed to eleidin and keratohyaline, 

 can be dissolved neither in trypsin nor in pepsin. In sections 

 treated with osmic acid the horny layer shows on its upper and 

 under surfaces, as well as on its sides, a black boundary, which 

 is due to impregnation of the dried layer with fat. The 

 middle part of the horny layer cannot be blackened by osmic 

 acid. 



21 



