OUTPUT OF WASTE MATTER 137 



in the liver, with the separation of the 

 elements of water; hippuric acid is thus 

 ormed and appears in the urine. This is a 

 striking example of a synthesis in the living 

 body. 



70. Another organ by which excretion is 

 effected is the skin This structure not only 

 has a protective function, as it covers the 

 whole surface of the body, but it has also an 

 excretory function. Carbonic acid is to a 

 small extent eliminated. The sweat, consist- 

 ing of water holding a small amount of salts 

 in solution (chiefly chloride of sodium, and 

 phosphates), is separated by numerous long 

 tubular glands, the sweat or sudoriparous 

 glands, lined with epithelium. This fluid, at 

 certain temperatures, may at once pass into 

 a state of vapour or gas, thus taking up heat, 

 and cooling the surface, or, as in profuse 

 sweating, it may appear as sensible drops on 

 the surface of the skin. A kind of oily 

 matter is secreted by another set of glands 

 in the skin, the sebaceous glands, which some- 

 times open by ducts on the surface, or into the 

 little pouches from which hairs spring. 



