RESPIRATION. 



121 



right side of the heart, from all the various parts of the body 

 consists of three kinds of materials : 1st, the residue of the 

 arterial blood after nourishing the body, or that which is left 

 after the particles have been selected for the nourishment of 

 the textures; 2d, the old and dead atoms of flesh; and, 

 3d, the chyle, or digested food brought through the lacteals. 

 Neither of these three elements can nourish the body The 

 remnant of the arterial blood has lost much, if not all, of its 

 life-giving qualities. The dead particles would be poisonous 

 if carried round again; and the new chyle is not yet pre- 

 pared to furnish nutriment. They must, therefore, all be sub- 

 mitted to some process by which the first shall be strength- 

 ened, the second carried out of the body, and the third 

 perfected, before this blood can be used again to nourish the 

 body. All this is done by means of the air*in the lungs. 



265. The lungs of man are placed within the chest, 

 at the upper part of the trunk. They are organs of ex- 

 ceeding delicacy in their FIG. XV. Bones of th* Cheat. 

 structure, and would not 

 bear with impunity any 

 exposure to external in- 

 jury. They are therefore 

 protected with great care. 

 They are covered on all 

 sides with a bony frame- 

 work, which prevents all 

 contact with external ob- 

 jects. The bones which 

 compose the walls of this 

 chest are so arranged, and 

 fixed with joints and mus- 

 cles, that they admit of 

 very free motion, and al- 

 low to the internal cavity 

 great range of expansion 



and contraction, for ad- Q Breast -bone. b t b. Back-bone, 

 mitting and expelling air. c , c, c, c. Ribs. 



n 



