382 



THE SKIN AND ITS APPENDAGES. 



in concentric lamellae (Kolliker, 89). Here and there between the 

 cornified cells structures may be seen which probably represent the 

 remains of intercellular bridges. The thickness of the epidermis 

 varies greatly according to the locality, and is directly proportionate 

 to the number of its cell layers. As a rule, the stratum Malpighii 

 is thicker than the stratum corneum, but in the palm of the hand 

 and the sole of the foot the latter is considerably the thicker. 



The various layers of the epidermis are in close genetic relation- 

 ship to one another. The constant loss to which the epidermis is 

 subjected by desquamation is compensated by a continuous upward 

 pushing of its lower elements ; cell-proliferation occurs in the 

 basal cells and adjacent cellular strata of the stratum germinativum 

 (Malpighii), where the elements are often seen in process of mitotic 

 division. The young cells are gradually pushed outward, and dur- 

 ing their course assume the general characteristics of the elements 



composing the layers 

 through which theypass. 

 For instance, such a cell 

 changes first into a cell 

 of the stratum germina- 

 tivum ; then, when it 

 commences the forma- 

 tion of keratohyalin, into 

 a cell of the stratum 

 granulosum ; later, into 

 a cell of the stratum lu- 

 cidum, and finally into 

 an element of the stra- 

 tum corneum, where it 

 loses its nucleus, corni- 

 fies, and at last drops off. 

 The mesodermic por- 

 tion of the skin, the co- 

 rium, consists of a loose, 



subcutaneous connective tissue containing fat, the subcutaneous 

 layer, with the panniculus adiposus, and of the true skin, or 

 corium proper. The amount of adipose tissue in the subcutaneous 

 layer is subject to great variation ; there are, however, a few re- 

 gions in which there is normally very little or no fat (external ear, 

 eyelids, scrotum, etc.). To the subcutaneous connective tissue is 

 due the mobility of the skin. The corium may be compared to the 

 mucosa of a mucous membrane, and consists of two layers of a 

 deeper and looser pars reticularis, and of a superficial pars papillaris 

 supporting the papillae. The transition from the one to the other 

 is very gradual. Elastic fibers are present in the connective tissue 

 of both layers. 



The pars reticularis is made of bundles of connective-tissue fibers 

 arranged in a network, nearly all of the strands of which have a direc- 



Fibrils which 

 pass from one __ 

 cell to another. 



Nucleolus. 



Intercellular 

 bridges. 



Nucleus of -- 

 cell. 



Fig. 305. Prickle cells from the stratum Malpighii 

 of man ; X 4 8 - 



