436 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



kidneys, is of two kinds — physical and vital. The for- 

 mer is exhalation, the latter excretion. The one is always 

 insensible, the other is sensible — sweat. 



Exhalation. — Water in a porous earthenware jar or 

 in a canvas bag, if hung up in the air in a hot, dry 

 climate, keeps cool, and even grows cooler. The water 

 soaks through and evaporates on the surface, but never 

 drips. So the tissues of the body are all permeable and 

 filled with water. Evaporation from the surface calls 

 the water to the surface as fast as it is evaporated. It 

 can not drip as sweat, because it is called there only 

 by drying. It is for this reason that in very arid re- 

 gions the suffering from heat is not very great, and 

 sunstrokes are unknown, although the thermometer in 

 the shade may run up to no to 120° F. Of course the 

 water which comes out and evaporates in this physical 

 way is nearly pure water. 



Excretion. — But besides this physical exhalation, 

 which would go on even if there were no special organs, 

 there is also the excretion of srveat. This is the product 

 of the sweat glands. It is not called out by evaporation, 

 but as if it were pushed by the specific activity of the 

 glands, and may exude in such quantity as to roll down 

 and drip from the skin. This also spreads and evap- 

 orates, and thus cools the surface. This is not pure 

 water like the other, but water containing many salts 

 with a little oil and a little C0 2 and a trace of urea. 



Structure of the Skin.— The skin, as already ex- 

 plained, is composed of two parts — viz., the dermis or true 

 skin and the epidermis. The dermis is a mass of felted 

 interlacing fibers. It is very strong, highly organized, 

 and full of blood vessels and nerves. The epidermis, 

 which alone concerns us here, is continuous with the 

 epithelium, and, as to its truly living parts, similarly 

 constituted. There is, however, this difference: The 



