THE SKIN 323 



in dermatology as seborrhea and popularly as dandruff. Sometimes 

 the secretory product is along with its over-abundance abnormally oily 

 (seborrhea oleosa), while at other and more frequent times the excessive 

 sebum is thicker in consistence than usual (seborrhea sicca). The cause 

 of this hyper-secretion is not known, but it seems to be related to some 

 lowered tone of the nervous system or the blood. It is most abundant 

 in those regions where the sebaceous glands are most common. 



Respiration is another subsidiary function of the skin. It appears 

 that under normal conditions about one-half of one per cent, of the body's 

 total respiration is carried on directly through the epidermis. When the 

 lungs are thrown out of function in large part the respiratory action of the 

 skin may be largely increased, although not to an extent which makes 

 it ever life-saving to the human organism. The subject has already 

 been discussed in the chapter on respiration (see page 129). 



Absorption of certain substances through the skin takes place under 

 the necessary conditions. The process is more important therapeuti- 

 cally, by artificial means, than as a spontaneous physiological process. 

 It is not apparent that naturally any substance save perhaps oxygen in 

 very small amounts passes inward through the intact epidermis, so 

 perfect is the protective structure and function of the integument. 



When the body is immersed in water, especially if the latter be warm, 

 it is likely that a very small amount finds its way into the circulation. 

 Thus, ship-wrecked persons have often relieved themselves, somewhat, 

 of the feeling of thirst by immersing their bodies even in the salty ocean- 

 water. This, however, is no proof that any valuable amount of the 

 water enters the body, for in the first place, no one has ever saved him- 

 self from dying of thirst (that is from loss of water from the blood and 

 body-tissues) by thus surrounding the body with water. This loss of 

 water normally occurs half at least by evaporation from the skin, and 

 immersion in water would check this evaporation and so give relief to 

 some extent. 



The sort of substances which may be made artificially to pass into and 

 through the normal skin (including the epidermis) are mostly such as 

 are already there, namely fats. These even when only placed upon the 

 epidermis are absorbed, and unlimited amounts may be made to pass 

 inward by inunction. Naturally also substances soluble in fats likewise 

 readily enter the organism. This fact is taken advantage of in medi- 

 cating not only the skin itself, but the entire organism. Thus, a person 

 is readily salivated by the inunction of mercurial ointments. Similarly, 

 alcoholic extracts and ethereal solutions, being soluble in the sebum and 

 body-fats, may be readily made to enter the unbroken skin. 



It is the oiled epidermis which prevents the passage inward of most 

 substances, for the mucous membranes and the naked tissue-protoplasm 

 readily absorb a large variety of materials. The subject of absorption 

 through the skin has not nearly the importance or popularity now that it 

 had formerly when it was supposed by many that the organism received 

 much benefit by absorbing the substances, for example, dissolved in 



