50 77/1? HUMAN BODY. 



CHAPTER IV. 

 WORK AND WASTE THE BLOOD. 



SECTION I. 1. The body, like not-living machines, 

 tends to wear out by use. Even the enamel of the teeth, 

 which is the hardest substance found in it, at length gives 

 way. 



2. The soft parts wear out very rapidly. In every 

 movement of a muscle, in every action of the brain, some 

 of their particles are worn away. 



3. The body is always active. Even in sleep, the 

 breathing muscles, and the heart, and the muscles of the 

 digestive organs, are moving. The wear is, therefore, 

 constant. 



4. But the body differs from a not-living machine in 

 being able to repair itself. In early life, it does more than 

 this. It builds itself up, and grows. When it has got its 

 growth, it still repairs all waste, and may increase in 

 strength and endurance for many years. At length there 

 comes a period when it is not able to repair so much as it 

 once could ; it can not, therefore, endure so much wear, 

 the old can not be so active as the young; finally it is 

 unable even to supply the waste caused by the simplest 

 actions, such as breathing and digesting ; then life must 

 cease. 



SUGGESTION TO TEACHERS. The least stain of blood, on a glass slide, 

 under a microscope magnifying four hundred times, is sufficient to show 

 the red globules. It is not always easy to find white ones. 



