7 (E RJSASON WHY I 



" In human work. though laboured on with pain, 

 A thousand movements scarce our purpose gain ; 

 In God's one single can its ends produce; 

 Yet serves to second too some other use." POPE. 



invested the teeth, as it invests every other bone of the body, ac- 

 tion, necessary exposure, and irritation would have subjected the 

 animal to continual pain. General as it is, it was not the sort of integu- 

 ment which suited the teeth ; what they stood in need of was a strong 

 hard, insensible defensive coat, and exactly such a covering is given 

 to them, in the ivory enamel which adheres to their surface. 



44. Why are, the front teeth of the mouth sharp and the back 

 teeth broad and blunted? 



Because the office of the former is to cut and separate the food ; 

 while the purpose of the latter is to grind it to a pulp, by which it 

 becomes fitted for the process of digestion. 



45. What are the uses, distinct and mutual, of the bones 

 and muscles? 



The bones are to the body what the masts and spars are to a ship 

 they give support and the power of resistance. The muscles, 

 again, are to the bones what the ropes are to the masts and spars ; 

 it is to them that the bones are indebted for the preservation or the 

 change of their position. If the bones or masts are too feeble in pro- 

 portion to the weight which they are required to sustain, then a de- 

 viation from their shape or position takes place ; and, on the other 

 hand, if the muscles or ropes are not sufficiently strong and wel 1 

 bra^d, then insufficiency of support must necessarily result. 



46. /Jarly infancy affords an instance of both of the above-mentioned imperfec- 

 tions ttie bones being infirm, and the muscles small and destitute of true fleshy 

 fibres The disease called " Softness of the bones," is an illustration of what may 

 be c&~.c*~ a weak mast of the body, which must yield if its muscles be strongly 

 drawL f he state of muscular debility consequent on fever and many acute diseases, 

 >r eve. ./n sudden fright, is, on the other hand, an instance of the inability of the 



ones alone to preserve an attitude or execute motion, when the muscular system 



jveaUexied by disease. 



47. Why is the cylindrical form of the long bones of the body 

 advantaaes to structure of the human frame? 



The superior advaiilagea oi this arrangement are illustrated 



