722 STRUCTURE OF EPIDERMIS. [BOOK n. 



granules of a peculiar nature. Hence this stratum is called the 

 stratum granulosum. 



The stratum above this consists of one or two or even more 

 layers of cells, elongated and flattened horizontally, the cell 

 substance of which is homogeneous and transparent, free from 

 granules and not staining very readily. In the middle of a cell 

 may frequently be seen a rod-shaped nucleus placed horizontally. 

 These clear transparent cells form a transparent seam, the stratum 

 lucidum, between the stratum granulosum and Malpighian layer 

 below and the horny layer above. 



435. The horny layer, which is as we have said of variable 

 but nearly always of considerable thickness, is formed of a number 

 of layers of cells which, differentiated already in the lowest layers, 

 have that differentiation completed as these pass upwards. The 

 upper, outer portion of this horny layer is continually being shed 

 or rubbed off in the form of flakes of variable size. Each flake 

 upon examination, as for instance after dissociation by maceration 

 or with the help of alkalis, is found to be composed of elements 

 which can no longer be recognized as cells, and which may be 

 spoken of as scales. Each scale is a flattened mass or plate in 

 which no nucleus can be seen, and which consists not of the 

 proteids and other constituents of ordinary cell substance ( 29) 

 but almost exclusively of a material called keratin. This is a 

 body, the exact nature of which has not yet been clearly made 

 out, but which has the general percentage composition of proteids, 

 from which it is a derivate, with the exception that it contains a 

 considerable quantity of sulphur (the keratin of hair contains as 

 much as 5 p.c.); this sulphur appears to be somewhat loosely 

 attached to the other elements of the keratin since it may be 

 removed by boiling with alkalis. 



The lowermost portions of the horny layer are composed of 

 elements which may still be recognized as cells, inasmuch as each 

 contains a nucleus, though this is obviously undergoing change 

 and on the way to disappear. Each cell is, however, flattened and 

 plate-like, and its substance already consists largely of keratin. 

 In passing upwards from the lower to the more superficial parts 

 of the horny layer such an imperfect cell loses its nucleus, and 

 becomes the wholly keratinous plate just described. The whole 

 horny layer consists of strata of elements, horny to begin with, 

 but becoming more completely so in the upper parts. Below, in 

 contact with the moist Malpighian layer, the horny layer is moist 

 but the superficial parts become dry by evaporation; and here 

 the strata delaminate from each other, the outer ones, as we have 

 said, being shed in the form of flakes, which seen in the dry 

 condition under the microscope have often the appearance of 

 irregular fibres. 



The mitotic changes seen in the cells of the Malpighian layer, 

 not only in those of the vertical layer but in the others as well, 



