274 THE SKIN 



reaches the epidermis, which it enters in the interval between the dermal 

 papillae (interpapillary portion of the epidermis). Its lining epithelium 

 is continuous with that of the stratum germinativum, and in its course 

 through the epidermis the wall of the duct consists solely of the sur- 

 rounding epidermal cells. The stratum granulosum and adjacent portion 

 of the horny layer in the immediate neighborhood of the duct is in- 

 vaginated into the stratum mucosum, which is thus considerably thinned 

 by the passage of the duct. 



The sweat glands are abundantly supplied with capillary blood- 

 vessels and small non-medullated nerves, which form plexuses about 

 the walls of the coiled portion of the gland, and from which terminal 

 fibrils penetrate the basement membrane and end in contact with the 

 secreting cells. 



Development. The sudoriparous glands first appear in the embryo 

 during the fifth month as solid columnar ingrowths from the stratum 

 germinativum of the epidermis. These processes grow inward through 

 the primitive corium to its junction with the looser subcutaneous tissue. 

 Here the cell columns become thickened and convoluted, and at about 

 the same period their lumen appears. The glandular lumen is not at 

 first connected with the free surface, but as the cells of the germinal 

 layers of the epidermis gradually replace those which are more super- 

 ficial the epidermal portion of the duct is formed. At about the seventh 

 month the lumen of the duct opens upon the epidermal surface. 



The membrana propria of the fundus and dermal portion of the 

 duct is derived from the surrounding connective tissue elements of the 

 mesenchyma. 



THE NAILS 



The nails are produced by a peculiar modification of the epidermis 

 by which the stratum lucidum becomes greatly thickened (Bowen, 1899) 

 while the horny layer (eponychium of the embryonic nail) is at the same 

 time wanting. The nail is divisible into the nail body and nail root; 

 the former comprising the exposed, the latter the hidden portion of 

 the organ. The root of the nail is overhung by a fold of the skin, the 

 thickened horny layer at the margin of which forms an adherent border, 

 the eponychium of the adult nail. 



The nail groove or sulcus is included between the overhanging skin 

 and the root of the nail. It is deep at its proximal end but is shallow 

 at the lateral margins of the nail. The distal or free border of the nail 

 projects over the skin at the tip of the finger and the thickening of 



