90 



GENERAL ANATOMY. 



to the fact that these parts are exposed to intermittent pressure, but that this is 

 not the only cause is proved by the fact that the condition exists to a very consid- 

 erable extent at birth. The more superficial layer of cells, called the horny layer 



FIG. 68. Microscopic section of skin, showing the epidermis and derma; a hair in its follicle : the erector 

 pili muscle : sebaceous and sudoriferous glands. 



(stratum corneuni), may be separated by maceration from the deeper layers, which 

 are called the rete mucosum, and which consist of several layers of differently 

 shaped cells. The free surface of the epidermis is marked by a network of linear 

 furrows of variable size, marking out the surface into a number of spaces of polyg- 

 onal or lozenge-shaped form. Some of these furrows are large, as opposite the 

 flexures of the joints, and correspond to the folds in the derma produced by their 

 movements. In other situations, as upon the back of the hand, they are exceed- 

 ingly fine, and intersect one another at various angles ; upon the palmar surface 

 of the hand and fingers and upon the sole of the foot these lines are very distinct 

 and are disposed in curves. They depend upon the large size and peculiar arrange- 

 ment of the papillae upon which the epidermis is placed. The deep surface of the 

 epidermis is accurately moulded upon the papillary layer of the derma, each papilla 

 being invested by its epidermic sheath ; so that when this layer is removed by 

 maceration, it presents on its under surface a number of pits or depressions corre- 

 sponding to the elevations in the papillae, as well as the ridges left in the intervals 

 between them. Fine tubular prolongations are continued from this layer into the 

 ducts of the sudoriferous and sebaceous glands. 



In structure, the epidermis consists of several layers of epithelial cells agglu- 

 tinated together and having a laminated arrangement. These several layers may 

 be described as composed of four different strata from within outward : (1) The 

 rete Malpiyldi, composed of several layers of epithelial cells, of which the deepest 

 layer is elongated in figure and placed perpendicularly on the surface of the 

 corium, their lower ends being denticulate, to fit into corresponding denticula- 

 tions of the true skin ; while the succeeding laminae consist of cells of a more 

 rounded or polyhedral form, the contents of which are soft, opaque, granular, and 

 soluble in acetic acid. They are often marked on their surfaces with ridges and 

 furrows, and are covered with numerous fibrils, which connect the surfaces of the 

 cells : these are known as prickle cells (see page 43). (2) Immediately superficial 

 co these is a single layer of flattened, spindle-shaped cells, the granular layer, Avhich 

 contain granules that become deeply stained in haematoxylin, and are composed of 

 a substance termed eleidin. They are supposed to be cells in a transitional 

 stage between the protoplasmic cells of the rete Malpighii and the horny 

 cells of the superficial layers. (3) Above this layer the cells become indis- 

 tinct, and appear, in sections, to form a homogeneous or dimly striated mem- 

 brane composed of closely-packed scales, in which traces of a flattened nucleus 



