352 ANATOMY FOR NURSES [Chap. XVIII 



Excretory function of the skin. — While sweat is an excretion, 

 its value lies not so much in the elimination of waste matter as in 

 the loss of body heat by the evaporation of water. This loss of 

 heat is necessary to balance the production of heat that is con- 

 stantly taking place. 



Less important functions of the skin. — The skin is to a slight 

 extent an absorbing organ. Soluble substances are very readily 

 absorbed if the epidermis is removed, but even when in solution 

 they are sparingly absorbed by the unbroken skin. Oily sub- 

 stances, if well rubbed in, are readily absorbed, especially in those 

 parts of the body where the epidermis is thinnest. 



Oxj'gen in small amount is also taken in through the skin, but 

 this gain to the body is counterbalanced by the carbon dioxide 

 which is thrown off. 



BODY HEAT 



From the standpoint of heat, production animals may be divided 

 into two great classes : — 



(1) Constant temperature animals, or those whose temperature 

 remains practically constant whether the surrounding air is hotter 

 or cooler than the body. The term warm-blooded is also applied 

 to this class. It includes human beings. 



(2) Changeable temperature animals, or those whose tempera- 

 ture varies with that of the surrounding medium. This class is 

 also described as cold-blooded. The human foetus is cold-blooded. 



The great difference between these two classes is in their reac- 

 tions to external temperature. A cold environment reduces the 

 temperature of the cold-blooded creature, reduces the metabolism 

 of all its tissues, and thus reduces its heat production. The 

 warm-blooded animal reacts in precisely the opposite way. Since 

 his temperature remains constant, his heat production must in- 

 crease in order to neutralize the effect of cold surroundings. 



Production of heat. — Heat in the body is produced by such 

 chemical changes going on in the tissues as are associated with 

 oxidation. Friction is a minor source of heat, i.e. that caused by 

 the movements of the muscles, the circulation of the blood, and 

 the ingestion of warm food. 



Where heat is produced. — Wherever metabolic changes are 

 taking place, there heat is set free. These changes take place 



