NAILS AXD HAIR. 117 



The epidermis serves as a protection to the more delicate 

 structure of the true skin, and its thickness is proportionate 

 to the exposure of the different parts. It is consequently 

 much thicker upon the soles of the feet and the palms of the 

 hands than in other portions of the general surface, and its 

 thickness is very much increased in those who are habitually 

 engaged in severe manual labor. Upon the face, the eyelids, 

 and in the external auditory passages, the epidermis is' most 

 delicate, measuring from -^J-g- to -^-J-^ of an inch in thickness. 

 Upon the palm it is from -^ to -^ of an inch thick, and upon 

 the sole of the foot it measures from -^ to -J of an inch. 1 

 These variations depend entirely upon the development of the 

 horny layer. The thickness of the rete mucosum, although 

 it presents considerable variation in different parts, is rather 

 more uniform. 



There is constantly more or less desquamation of the epi- 

 dermis, particularly the horny layer, and the cells are regen- 

 erated by a blastema exuded from the subjacent vascular 

 parts. It is probable that there is a constant formation of 

 cells in the deeper strata of the horny layer, which become 

 flattened as they near the surface ; but there is no direct 

 evidence that the cells of the rete mucosum undergo trans- 

 formation into the hard, flattened scales of the horny layer. 



Physiological Anatomy and Uses of ike Nails and 

 Sairs. It is unnecessary, in this connection, to discuss very 

 minutely the anatomy of the nails and hairs. They are or- 

 dinarily regarded as appendages of the epidermis, produced 

 by certain peculiar organs belonging to the true skin ; and 

 an elaborate study of these parts belongs strictly to descrip- 

 tive and general anatomy. To complete, however, the 

 physiological history of the skin, it will be necessary to 



1 KOLLIKER, Manual of Human Microscopical Anatomy, American Edition, 

 Philadelphia, 1854, p. 146. Kolliker gives (he. cit.) accurate measurements of 

 the epidermis in many different portions of the skin, to -which the reader is re- 

 ferred for further information on this point. 



