112 EXCRETION. 



are well marked and regular, particularly upon the palmar 

 surfaces of the last phalanges, where they are in the form 

 of concentric curves, so easily observed with the naked eye 

 that further description is unnecessary. These lines are 

 formed by the more or less regular arrangement of the papil- 

 lae of the true skin. 



Extent and Thickness of the Skin. Sappey has made a 

 number of very careful observations upon the extent of the 

 surface of the skin. Without detailing the measurements 

 of different parts, it may be stated, as the general result of 

 his observations, that the cutaneous surface in a good-sized 

 man is equal to about twelve square feet ; and in men of 

 more than ordinary size it may extend to fourteen, fifteen, 

 or even eighteen square feet. In men of medium size, in 

 France, the surface does not exceed ten square feet ; and in 

 women, it is ordinarily from six to eight. 1 When we con- 

 sider the great extent of the cutaneous surface, it is not sur- 

 prising that the amount of secretion, under certain conditions, 

 should be enormous. Indeed, under all circumstances, the 

 amount of elimination is very considerable, and the skin is 

 really one of the most important of the glandular structures. 



The thickness of the skin varies very much in different 

 parts. Where it is naturally exposed to constant pressure 

 and friction, as on the soles of the feet or the palms of the 

 hands, the epidermis becomes very much thickened, and in 

 this way the more delicate structure of the true skin is pro- 

 tected. It is well known that the development of the epi- 

 dermis, under these conditions, varies in different persons, 

 with the amount of pressure and friction to which the sur- 

 face is habitually subjected. The true skin is from -^ to $ 

 of an inch in thickness ; but in certain parts, particularly the 

 external auditory meatus, the lips, and the glans penis, it 

 frequently measures not more than y^- of an inch. 2 



1 SAPPEY, Traite d' anatomic descriptive, Paris, 1852, tome ii., p. 447. 



2 POUCHET, Precis d'histologie humaine, Paris, 1864, p. 329. 



