DIGESTION AND USES OF FOOD. 167 



Thus we have seen how the digested food gets into the 

 blood, and how the blood is carried through all parts of the 

 body. The next question is as to what is done with this 

 blood. 



USES OF THE DIGESTED FOOD. The animal must be kept 

 warm, and therefore some food is required as fuel. The oxy- 

 gen of the air comes in through the lungs and unites with the 

 material in the blood, or with material such as fat formed from 

 the blood. To keep warm, therefore, food and fresh air are 

 necessary. Now you will understand why brisk walking, 

 running, or working in fresh air, even in cold air, will cause 

 you to become warm, especially if you have been well fed. It 

 is like starting a fire with plenty of good dry fuel and opening 

 the draughts. Then there is bone to be built up in one 

 place, muscle in another, wool or hair in another. If the 

 animal is growing, food is necessary ; even if it is not growing 

 food is necessary, for the old parts are constantly wearing away 

 and new parts being formed. If the cow is giving milk, the 

 material of the milk must be formed out of the material in 

 the blood. If the horse is doing hard work there must be 

 material in the blood to replace the muscle that is being worn 

 away, and also to be used up to produce the force or energy 

 that we see resulting in work. 



THE WASTE MATERIAL. In the burning up of food to 

 produce warmth, in the using up of food to produce work, in 

 the working over of material to form flesh, muscle, fat, bone, 

 wool, or milk, there will, of course, be more or less waste or 

 refuse material. How is this refuse material got rid of by the 

 animal ? In three ways : by the lungs, by the skin, and by 

 the kidneys. 



THE LUNGS. Animals breathe in pure oxygen and breathe 

 out carbonic acid gas and moisture. They should therefore 

 be able to get pure air and not be compelled to breathe over 

 again the air that has already come from their lungs, for it 



