THE SKIN. 271 



tive tissue gradually merge into each other, the boundary be- 

 tween them being only an artificial one. 



Commencing with the epidermis, we will describe in detail 

 the minute structure of the different tissues and organs of the 

 skin, omitting only the lymphatics. 



Description of the different layers. The epidermis is 

 generally subdivided into several layers, with specially distinc- 

 tive names for each layer ; but though such a division has 

 some practical value, histologically it 

 is incorrect, as the cells of the lowest 

 layer are transformed, at some period 

 of their existence, in their movement 

 toward the free surface, into the cells 

 of the other layers. Examination with 

 high powers also shows that the chan- 

 ges in the molecular constitution or 

 chemical condition of the cells of the 

 epidermis changes which produce 

 differences in their appearance are 



quite gradual. Consequently, sharply defined layers are not 

 found. For practical reasons, however, it is well to adopt the 

 usual classification. In Pig. 115 these layers are shown. 



Another division is into Malpighian and corneous layers 

 only, the former comprising the rete and the granular layer, 

 and the latter the stratum lucidum and corneous layer. The 

 Malpighian layer, as compared with the corneous layer, pre- 

 sents a more or less dark, granular appearance, while the latter 

 is homogeneous, and its cells have a lamellar arrangement. 



The rete MalpigTiii consists of nucleated corpuscles, rich in 

 protoplasm, granular in appearance, and disposed more or less 

 in parallel strata, the elements of the different layers differing 

 somewhat from each other as regards their size and shape. The 

 lowest layer consists of columnar-shaped cells arranged pali- 

 sade-like, with their long axes more or less perpendicular to the 

 surface of the corium. Where the papillae are well developed, 

 this perpendicular arrangement is not so marked. The base of 

 some of these bodies terminates in a pointed extremity, which 

 passes a short distance into the underlying corium. Each of 

 them has an oval nucleus. The cell-body consists of a small 

 quantity of slightly granular, shining protoplasm. The cor- 

 puscles of this layer are not united to each other by bands, as 



