52 LIVESTOCK ON' THE FARM 



different organs, systems, etc. Limbs, muscles, and all func- 

 tioning parts are, of course, of great importance. The diges- 

 tive system with its numerous glands, prepares the food. The 

 respiratory system brings in good air and takes out bad air. 

 This, then, supplies oxygen and takes out carbon dioxide. 

 The circulatory system embracing the heart and blood vessels 

 is simply a means of carrying digested food and the oxygen to 

 the different parts of the body, and of bringing back the waste 

 products so that they can be thrown off. The carbon dioxide, 

 as stated above, is thrown off by means of the lungs and the 

 urea goes out by way of the kidneys and bladder in the form 

 of urine. The urine is simply water with the urea dissolved 

 in it. The heart is a force pump which forces the blood 

 through tubes or the blood vessels. The blood is a body 

 tissue or a medium by means of which the food and waste 

 products are carried. The regulative system controls the tem- 

 perature of the body. In winter when the weather is cold 

 heat must be generated and the body kept warm, and in 

 summer when the weather is warm there is sometimes too 

 much heat in the body, because of the work done inside and 

 this must then be taken out and thrown off. This is done by 

 means of the evaporation of the water on the surface and also 

 by radiation and conduction. 



Processes that go on inside of the body, aside from digestion 

 and circulation, are called metabolism. 



The nervous system centers in the brain and thence per- 

 meates all parts of the body. The brain is the center of ex- 

 change between communications from different parts of the 

 body, and between the body itself and the exterior world. 

 The nerves are the wires carrying the messages. There is 

 perfect harmony between all the parts of a normal animal 

 body. If any one part wants anything of another part or 

 organ a communication is sent to "central" (the brain) and 

 it sends a message to the other part to do the necessary work. 

 For example: An animal increases the amount of work done. 

 This calls for more action in the cells and results in more waste 

 products. It also requires more oxygen. By the proper 

 messages sent back and forth the heart begins to beat faster 

 and the respiration is accelerated. 



