688 STRUCTURE OF EPIDERMIS. [BOOK n. 



leucocyte, about 12/A by 6//., consists of a relatively large oval 

 nucleus lying in the midst of a coarsely granular cell-substance, 

 which stains readily with the ordinary staining reagents. The base 

 of the cell abutting on the dermis often shews fine processes 

 interlocking with corresponding processes from the dermis ; the 

 sides of the cells are in close contact, but merely in contact, no 

 cement substance existing between them. 



The rest of the cells of the Malpighian layer, much like each 

 other, are polygonal or irregularly cubical cells, resembling the ver- 

 tical cells just spoken of in so far that each consists of a coarsely 

 granular cell-substance in which is imbedded a relatively large 

 nucleus ; this however is spherical not oval. The surface of each 

 cell is thrown up into short ridges, radiating somewhat irregularly 

 from the centre of the cell and projecting at the surface and 

 edges, so as to give the cell somewhat the appearance of being 

 armed with a number of prickles. Hence these cells are often 

 called ' prickle cells.' The prickles of a cell do not interlock with 

 those of its neighbours but touch at their points, so that the 

 contact of two adjacent cells is not complete but carried out by 

 the points of the prickles only, minute spaces being left between. 

 Hence the whole Malpighian layer is traversed by a labyrinth of 

 minute passages, along which fluid can pass between the touching 

 prickles. 



In dark skins, as in that of the negro, pigment particles abound 

 in the lower Malpighian cells, especially in the vertical layer. In 

 such cases branched pigment-cells, connective tissue corpuscles 

 loaded with pigment granules, are to be seen in the dermis also ; 

 and occasionally similar branched cells may be seen in the epidermis 

 between the Malpighian cells. Leucocytes also not infrequently 

 pass out of the dermis and wander among the cells of the Mal- 

 pighian layer. 



The nuclei not only of the vertical but also of the other 

 polygonal cells may, not unfrequently, be observed in various 

 stages of karyomitosis. Throughout life the cells of this Mal- 

 pighian layer of the skin appear to be undergoing multiplication 

 by division ; the increase of population thus arising is kept down by 

 the cells passing upwards and outwards, and becoming transformed 

 into the cells of the horny layer. 



434. The line of demarcation between the Malpighian layer 

 and the horny layer is, as we have said, sharp and distinct. It is 

 furnished by two peculiar strata of cells, more conspicuous in some 

 regions of the skin than in others. The lowermost, innermost 

 stratum consists of a single layer or of two or three layers of cells 

 which are not unlike Malpighian cells, but are differentiated by 

 their form, being extended horizontally so as frequently to appear 

 fusiform in vertical sections, by the absence of prickles, by their 

 staining very deeply with certain reagents, such as osmic acid, and 

 especially by their cell substance being crowded with large discrete 



