THE SKIN. 357 



average length of three-fourths of an inch, makes 

 the united length approximately twenty-eight miles. 

 This vast secreting surface is constantly secreting 

 moisture, either as insensible or sensible perspiration. 

 The amount of this perspiration within a given time 



Fig. 249. Under surface of the epidermis, separated from the cutis 

 by boiling. The sweat glands may be traced for a considerable part 

 of their length; a, Sweat gland ; b, longitudinal ridge; c, depression, 

 d, cross ridge (Bohm and Davidoff). 



varies considerably, but in the average person in 

 good health it is estimated at about two pints every 

 twenty-four hours. 



The excretory ducts open on the surface of the 

 skin by numerous sweat pores along the crests of the 

 epidermal ridges. These pores may be seen with a 

 low magnification or ordinary hand lens. The duct 

 is spirally twisted in the stratum corneum and enters 

 the dermis between two dermal papillae; that is, at 

 the apex of an epidermal papilla. In the dermis it 

 takes a sinuous or nearly straight course and pene- 



