ANIMALS. 117 



2. To repair Waste or supply Mechanical Force. 

 The particles of . matter of which the bodies of animals 

 are composed do not remain fixed and permanent. They 

 are constantly being removed and replaced by new par- 

 ticles. After a time the particles become old, and no 

 longer useful, and new particles are formed from the 

 food to take their place. 



A constant exchange is thus going on in all parts of 

 the body, so that after a number of years a new body is 

 formed, no part of the old remaining. The only excep- 

 tion to this is the enamel upon the teeth, which is believed 

 to remain without change. 



This wasting, or wearing away, of the substances of 

 the body is increased by work or muscular exertion. 

 The body is never at rest. When asleep, or when per- 

 fectly quiet, the heart still beats to force the blood 

 through the body, and the lungs are regularly ex- 

 panded to draw the breath. Every movement requires 

 force, and this force is supplied either by food, or by 

 particles of the body which must be replaced by the 

 food. 



The force that drives a train of cars comes from the 

 consumption, or burning, of fuel under the boiler ; so 

 the force to produce motions of the body is obtained by 

 chemical processes, which either consume the elements of 

 food or particles of the body which have been formed 

 from food. 



3. To supply Heat. The temperature of the bodies 

 of animals, when in a healthy condition, is from 98 to 

 100. It cannot be allowed to vary many degrees from 

 this point, for any considerable time, without causing 

 death. In cold and hot climates, in winter and in sum- 

 mer, it must be kept constantly the same. When the 



