388 



HISTOLOGY 



The process of cornification presents a more elaborate picture in 

 sections of the palms and soles. Outward from the stratum germinativum 

 there is a darkly staining, coarsely granular layer, one or two cells thick, 

 which is followed by a clear, somewhat refractive band in which the cell 

 outlines are indistinct. This layer seems saturated with a dense fluid 

 formed by dissolution of the underlying granules. In haematoxylin 

 and eosin specimens, the granular layer or stratum granulosum is followed 

 by a pink and then by a bluish band, which are subdivisions of the clear 

 stratum lucidum. These' are followed by a very thick stratum corneum. 

 Except in the palms and soles, the granulosum is thin and the lucidum is 

 absent. Chemically the coarse granules of the stratum granulosum resem- 

 ble the horny substance keratin (from which they differ by dissolving in 

 caustic potash) ; they are therefore called kerato-hyalin granules. Their 

 diffuse product in the stratum lucidum is named eleidin. In the corneum 

 it becomes pareleidin, which, like fat, blackens with osmic acid, but the 

 reaction occurs more slowly. The pareleidin is not due to fat entering 

 the skin from oily secretions on its outer surface. Further information 

 regarding these substances is supplied by Pinkus (Keibel and Mall's 

 Human Embryology, vol. i). 



The color of the skin is due to fine pigment granules in and between 

 the lowest layers of the epidermal cells. Underlying cells of the corium 

 sometimes contain groups of finer pigment granules, but such cells are 

 absent from the palms and soles and are infrequent elsewhere. They 

 may be found in the deeply pigmented circum-anal tissue, and in the 

 eyelids. 



NAILS. 



The nails are areas of modified skin consisting of corium and epi- 

 thelium. The corium is composed of fibrous and elastic tissue, the bundles 



Nail. 



Corium. 



Stratum 

 germinativum. 



Nail wall. 



Nail groove. 



Bone of third 

 phalanx. 



FIG. 391. DORSAL HALF OF A CROSS SECTION OF THB THIRD PHALANX OF A CHILD. X 15. 

 The ridges of the nail bed in cross section appear like papillae. 



of which in part extend vertically between the periosteum of the phalanx 

 and the epithelium, and in part run lengthwise of the finger. In place of 

 papillae, the corium of the nail forms narrow longitudinal ridges, which 



