3i6 



NORMAL HISTOLOGY. 



the correspondence of its structure with that of the mucous membranes, with 

 which it is directly continuous at the orifices on the exterior of the body, 

 emphasizes the close relation of the skin to the alimentary and other mucous 



tracts. 



This general investment, the tegmentum commune, includes the 

 skin proper, with the specialized tactile corpuscles, and its appendages the 

 hairs, the nails and the cutaneous glands. Its average superficial area is 

 approximately one and a half square meters. 



The skin (cutis), using the term in a more restricted sense as applied 

 to the covering proper without its appendages, everywhere consists of two 

 distinct portions, a superficial epithelial and a deeper connective tissue 

 stratum, which are derivatives of the ectoderm and the mesoderm respec- 

 tively. The former, the epidermis, is devoid of blood-vessels, the capillary 

 loops never reaching farther than the subjacent cadum, as the outermost 

 layer of the connective tissue stratum is called. The thickness of the skin, 

 from . 5-4 mm. , varies greatly in different parts of the body, being least on 

 the eyelids, penis and nymphae, and greatest on the palms of the hands and 



soles of the feet and on the shoulders 

 and back of the neck. Of the entire 

 thickness, the proportion contributed 

 by the epidermis is in most localities 

 about . i mm. 



The Connective Tissue Stra- 

 tum. The connective tissue stratum, 

 usually much the thicker portion of the 

 skin, includes two layers, the corium and 

 the tela subcutanea, which, however, 

 are so blended with each other as to be 

 without sharp demarcation. 



The corium or derma, the more 

 superficial and compact of the connec- 

 tive tissue layers, lies immediately be- 

 neath the epidermis from which it is 

 always well defined. With the excep- 

 tion of within a few localities, as over 

 the forehead and the external ear, the 

 outer surface of the corium is not even 

 but beset with elevations, ridges, or 

 papillae, which produce corresponding 

 modelling of the opposed under sur- 

 face of the overlying epidermis. The 

 best developed papillae are on the flexor surfaces of the hands and feet, where 

 they attain a height of . 2 mm. or more and are disposed in the closely set 

 double rows that underlie the cutaneous ridges on the palms and soles. The 

 patterns formed by the cutaneous ridges remain throughout life unchanged 

 and, as seen in imprints of the fingers, are so distinctive for each individual 

 that they afford a reliable and practical means of identification. The mark- 

 ings of the two hands are symmetrical and sometimes identical. The papillae 

 afford favorable positions for the lodgement of the terminal capillary loops 

 and the special organs of touch; they are accordingly grouped as vascular 

 and tactile. 



The corium is subdivided into an outer papillary stratum, containing 

 the papillae, and a deeper retictdar stratum^ composed of the closely inter- 



FIG. 361. Portion of corium from palmar sur- 

 face of hand after removal of epidermis ; each 

 range includes a double row of papillae, which 

 underlie the surface ridges and the openings of 

 the sweat-glands; the latter appear as dark points 

 along the ranges of papillae. X 5. 



