482 THE TISSUES. 



Chemical Composition and Physical Properties of the Skin. 

 The general composition of the cuticle, as one of the horny tis- 

 sues, has already been indicated (p. 235). Its cell-walls are insoluble 

 in water, but concentrated alkalies and concentrated sulphuric acid 

 easily dissolve them ; and hence the skin, if wetted with these agents, 

 feels slippery and greasy. The cuticle contains less sulphur than 

 the hair and nails; and hence, perhaps, salts of lead, mercury, and 

 bismuth color the hair, but not the epidermis. Nitrate of silver 

 colors it violet or brownish black; the oxide, chloride, and black 

 sulphuret of silver being formed from the chloride of sodium and 

 the sulphur it contains. The tissue of the cuticle is, however, quite 

 unchanged, the microscope merely detecting minute dark granules 

 between its elements. But strong solutions of the iodide and of the 

 cyanuret of potassium remove the color, by dissolving away the 

 horny plates themselves. 



The horny structure of the epidermis permits no fluids, except 

 those which act chemically upon it, to pass through it ; while it 

 readily takes up gaseous matters, or easily vaporizable substances, 

 as ether, alcohol, acetic acid, ammonia, ethereal solutions of chloride 

 of iron, and alcoholic solutions of acetate of lead. It also gives all 

 these oft' by cutaneous evaporation. (Krause.) Water, ointments, 

 and even solid matter (sulphur, cinnabar), pass through the unin- 

 jured cuticle ; but here there is a mechanical intrusion, in and 

 through the sweat-ducts and hair sacs, to the stratum Malpighii, 

 which, on the contrary, is easily penetrated by fluids. Hence, also, 

 the very ready occurrence of absorption after the separation of the 

 horny layer and the superficial portion of the Malpighian, by a 

 blister. 



The corium affords gelatine on boiling, from the osteine contained 

 in its white fibrous tissue. It putrefies with difficulty, and not at 

 all when tanned. Its toughness and slight elasticity have already 

 been mentioned. 



Vessels of the Skin. 



The arteries in the subcutaneous areolar tissue give off' many 

 branches to the hair-papillae (p. 259), to the fat-lobules (Fig. 188), 

 and the smooth muscular fibres. More externally, they supply the 

 sweat- and the sebaceous glands (Figs. 138, 135), and the inner por- 

 tion of the corium ; and finally penetrate into its outer part, and 



