THE SKIN 315 



over the gluteus maximus and on parts of the palm of the hand. The 

 subcutaneous fat helps in this matter. Under the heel the skin is very 

 thick, especially the fat and the epidermis, and constitutes a cushion to 

 relieve the jarring of the spine which else would occur, and with injurious 

 effect, in walking. As is familiarly seen in the causation of corns and of 

 calluses, especially on the hands, continued or oft-repeated pressure on 

 a portion of the skin gradually causes a thickening of the epidermis. 

 By this principle active manual occupations and sports soon lead to the 

 development of defences to abrasion and inflammation at just the spots 

 where they are needed. 



In cases of injury to the external parts of the body which are much 

 in contact, especially the fingers and the toes, the presence of the dead 

 epidermis not liable to inflammation prevents the injured parts from 

 growing together before they come into use again. 



Because of its structure and the materials from which it is made, the 

 skin is flexible and soft yet resistant to all but incisive blows. It has in 

 short in a lesser degree the well-known and unique properties of leather, 

 which of course is skin killed, hardened, and preserved. 



Besides these physical agencies there are many solid, liquid, and 

 gaseous chemical substances which come in contact with the skin 

 occasionally. The epidermis, however, especially the superficial parts 

 of it, is little more than a complicated network of keratin, which is one 

 of the most insoluble substances entering into the organism. It is un- 

 affected by most of the things which in a state of nature are apt to come 

 in contact with it. Many of these (for example, the alcohols) would 

 injure seriously the average living tissues, and alkalies and strong acids 

 attack it still more readily. In general, however, it is a very resistant 

 material and therefore one excellently adapted to intervene between an 

 organism and its varied environment. (See further as to the chemical 

 status of the albuminoids in the chapters on protoplasm, food, and 

 nutrition.) (Fig. 173.) 



Light is one of the agencies, chemiphysical in action, against too much 

 of which the pigment-cells of the epidermis protect the body-tissues. 



In addition to furnishing protection to a degree against physical and 

 chemical adversaries of its organism, the integument is a defence against 

 parasites, including the pathogenic bacteria except such as dwell within 

 its tissues. As long as the uncracked epidermis only is presented to 

 bacteria there is relatively little danger of infection, for its mass of minute 

 dead scales admirably serves as a filter against their passage, much as 

 does cotton in the bacteriological laboratory. Contrast with this the 

 prompt and vigorous invasion of mucous membranes by germs of disease. 

 These former do not have the dry, cool, filtering mass of scales which 

 make up the epidermis, but are living tissues rich in most of the conditions 

 the development of microorganisms requires, such as moisture, warmth, 

 and nutriment. 



The continual exfoliation of the dead epidermal squames protects the 

 body largely from the growth of parasites such as one sees sometimes on 



