PHYSIOLOGICAL ANATOMY OF THE SKIN. 383 



in diameter. The deeper cells are thicker and more rounded than those of the super- 

 ficial layers. 



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 conse- 

 quently 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 manual labor. Upon the face, the eyelids, and in the exter- 

 nal auditory passages, the epidermis is most delicate, measuring from ^ to -^ 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 T ^ to % of an inch. These variations in thickness depend 

 entirely upon the development of the horny layer. The thickness of the rete rnucosum, 

 although it presents considerable variation in different parts, is rather more uniform. 



There is constantly more or less desquamation of the epidermis, particularly of the 

 horny layer, and the cells are regenerated by a blastema exuded from the subjacent vas- 

 cular 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 transformation into the hard, flat- 

 tened scales of the horny layer. 



Physiological Anatomy of the Nails and Hairs. It is unnecessary, in this connec- 

 tion, to discuss very minutely the anatomy of the nails and hairs. They are ordi- 

 narily 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 

 descriptive and general anatomy. To complete, however, the physiological history of 

 the skin, it will be necessary to consider briefly the general arrangement of the cuticular 

 appendages. 



The nails are situated on the dorsal surfaces of the distal phalanges of the fingers and 

 toes. They serve to protect these parts, and, in the fingers, they are quite important in 

 prehension. The general appearance of the nails is so familiar that it requires no special 

 description. In their study, anatomists have distinguished a root, a body, and a free 

 border. 



FIG. 105. Anatomy of the nails. CSappey.) 



A, nail in sift/ : 1, cutaneous fold covering 1 the root of the nail ; 2, section of this fold, turned back to show the root 

 of the nail ; 3, lunula ; 4, nail. B, concave or adherent surface of the nail: 1, border of the root ; 2, lunula and 

 root; 3, body ; 4, free border. 0, longitudinal section of the nail: 1, 2, epidermis; 8, superficial layer of the 

 nail; 4, epidermis of the pulp of the finger ; 5, 6, true skin ; 7, 11, bed of the nail ; 8, Malpighian layer of the 

 pulp of the finger; 9, 10, true skin on the dorsal surface of the finger ; 12, true skin of the pulp of the finger ; 

 13, last phalanx of the finger. 



The root of the nail is thin and soft, terminating in rather a jagged edge, which is 

 turned slightly upward and is received into a fold of the skin extending around the nail 

 to its free edge. The length of the root of course varies with the size of the nail, but it 

 is generally from one-fourth to one- third of the length of the body. 



The body of the nail extends from the fold of skin which covers the root to the free 

 border. This portion of the nail, with the root, is closely adherent by its under surface 

 to the true skin. It is marked by fine but distinct longitudinal striao and very faint 



