vm THE FUNCTIONS OF THE BODY 153 



there may be a slow diffusion outwards ; in some other 

 cases they may be in part utilised in forming skeleton ; 

 but in most animals their speedy removal is a condition 

 of health. It is interesting to notice that in the winter- 

 sleep of hibernating mammals like the hedgehog the 

 ordinary excretory function of the kidneys is in abey- 

 ance. 



Throughout the animal kingdom there is great variety 

 in the method of eliminating nitrogenous waste-pro- 

 ducts. In the Protozoa there are often contractile 

 vacuoles which accumulate fluid and burst to the ex- 

 terior ; in the Sponges and Ccelenterates the continual 

 washing of the body inside and out with water secures 

 the elimination of nitrogenous waste by diffusion. In 

 most animals, however, there are kidneys of some sort, 

 filters introduced into the blood-stream, which eliminate 

 the poisonous waste. In the way in which they get rid 

 of the injurious substances but do not let others, equally 

 or more diffusible, pass out, they give us a glimpse of 

 the intricacy of vital processes. 



In mammals the kidnevs filter out the nitrogenous 



*/ 



waste-product called urea, and some other substances, 

 chiefly salts. They also allow of the passage of a large 

 quantity of water and a minute quantity of carbonic 

 acid. They may be helped by the skin, which, in the 

 perspiration, gets rid of much water, some carbonic 

 acid, and some saline matter. But the chief use of the 

 skin is to protect and envelop, and to help in the regula- 

 tion of the body-temperature by alterations in the size 

 of its blood-vessels. 



8. Organs of Internal Secretion.- In Vertebrate 

 animals there are a number of inconspicuous organs- 

 glands of internal secretion which are of fundamental 

 importance. They produce substances which are carried 

 away and distributed by the blood, and exert an in- 

 dispensable regulatory influence on the activity of other 

 parts of the body. Thus, in our own body, there is the 

 small thyroid gland on each side of the windpipe below 

 the larynx, which makes a secretion indispensable to the 

 continued health of body and mind. Children in whom 



