80 LIVESTOCK ON THE FARM 



In this sense the circulatory system is similar to a stove pipe 

 and chimney which take the smoke and gas from the stove or 

 furnace out to the exterior. The blood carries both solid and 

 gaseous waste products from the cell. The gaseous product 

 or carbon dioxide is brought to the lungs and there expelled 

 with the same air which brings in the oxygen. 



The solid waste products such as urea are taken by the 

 blood from the cell to the kidneys. These act as a sieve and 

 strain out these substances and then dissolve them in some 

 water. This solution is sent down to the bladder where it is 

 stored for periodic expulsion as urine. This is done by means 

 of contraction of the muscles around the body and the relaxa- 

 tion of the muscle about the neck of the bladder. If this blad- 

 der muscle fails to work or becomes exhausted the animal will 

 drip urine constantly. 



Body Temperature. — Water is also used for regulating body 

 temperature. When the work is hard or when the weather is 

 hot an animal perspires or sweats. When the animal works 

 there is considerable energy expended within the body and 

 this results in heat. On a hot day also there is a surplus of 

 heat in the body. All this must be taken out so that the body 

 or rather the protoplasm in the cells may not be cooked. 

 The difference between a normal cell and an overheated cell 

 is much the same as that between a raw egg and a boiled egg. 

 Overheated cells mean death to an animal. They are a 

 source of danger, also, in case of a high fever. 



The animal body maintains practically a constant tem- 

 perature. Then when it has more heat than it can get rid 

 of by other means it sends water to the surface in the form of 

 perspiration or sweat to be evaporated on the surface. This 

 takes up a great deal of heat and helps to keep the body tem- 

 perature at the right point. 



AIR OR OXYGEN 



Nothing has thus far been said of air which furnishes the 

 oxygen needed by the body. Air is not usually considered a 

 food and it is not a drink. 



How long can an animal live without food or water? Sev- 



