A STUDY OF THE SKIN AND THE KIDNEYS 251 



6. A COMPARISON OF EXCRETORY ORGANS 



Before making any study of the kidneys in various groups 

 of animals, it will be well to compare the different kinds of 

 excretory organs in man* We must first, however, distin- 

 guish two terms, namely 



Secretion and Excretion. Glands and their functions have 

 been referred to frequently in the preceding chapters. We 

 have defined a gland (p. 75) as an organ that secretes any 

 kind of a liquid. Now, the liquids made by glands are of 

 two kinds. Some, like the saliva, gastric and pancreatic 

 juices, are made from materials furnished by the blood, and 

 are of great service in the economy of the body. Such 

 fluids are called secretions. Perspiration and urine, on the 

 other hand, are composed of highly injurious wastes. They 

 are secreted by the glands of the skin and by the kidneys, 

 it is true ; but since these liquids are of no use to the body, 

 and are at once thrown off, they are called excretions. 



The Kidneys and the Skin. Attention has been called 

 to the fact that the work of the kidneys may at times be 

 performed to a certain extent by the skin. In structure, 

 likewise, the two organs are somewhat similar, if we com- 

 pare one of the kidney tubules with one of the sweat 

 glands. Each consists (1) of a region richly supplied with 

 blood vessels where secretion is carried on, and (2) of a 

 tortuous duct that carries off this secretion. On the other 

 hand, in the skin the glands are completely separated from 

 each other, while the tubules are closely massed together in 

 the kidneys. The most striking difference is seen, however, 

 when we compare the two excretions. The principal use of 

 the urine is to carry off the poisonous urea ; the perspiration 

 serves primarily to regulate the temperature of the body. 



The Lungs as Excretory Organs. If respiration ceases, 

 death ensues in five to ten minutes, and this is largely due 

 to the fact that the wastes of the body are not being prop- 

 erly removed. The lungs are therefore the most important 



