50 ORGANS OF RESPIRATION. 



enlarge and diminish the size of the cavity alternately ; the lungs 

 follow these motions and dilate, and contract in consequence ; 

 now, in the first case, (when the thorax dilates,) the air, pressed 

 hy all the weight of the surrounding atmosphere, is forced 

 into the chest, through the mouth or nostrils, and trachea, and 

 tills the pulmonary cells, in the same way that water mounts in 

 the body of a pump when the piston is raised. In the second 

 case, (in the act of expiration), the air contained in the lungs, is 

 on the contrary, compressed, and partially escapes by the route 

 which served it for entrance. 



, 13. 18. The cavity of the thorax, (Fit/. 13. T.) is 



formed principally by the ribs, which are 

 attached, posteriorly, to the spine or verte- 

 bral column, and in front to the bone of the 

 sternum; the spaces which exist between the 

 \wv*m>i>^i- r ' hs ' are fi" e d U P by muscles, and below, 

 this species of chamber is separated from 

 the belly by a fleshy partition called the 



19. Inspiration, or the enlargement of the 

 chest, is produced in two ways; 1st. by 

 the elevation of the ribs; 2nd. by the mus- 

 / cular contraction of the diaphragm, which, 

 in a state of repose, rises into the chest in the form of an arch, 

 and which in contracting, is lowered down. Expiration, or con- 

 traction of the chest, on the contrarv, is produced by the depres- 

 sion of the ribs, and relaxation of the diaphragm. We observe 

 many degrees in the extent of these movements ; and in ordinary 

 respiration, the quantity of air received into, or expelled from the 

 lungs, does not much exceed one seventh part of what these 

 organs are capable of containing. The number of respiratory 

 movements varies in different individuals according to the age ; 

 in adult age, we count about twenty inspirations a minute, in 

 infancy they are much more frequent. 



20. We have seen that it is by the nose or mouth, the pharynx, 

 the larynx, the trachea, and the bronchiae, that the air enters into 



Explanation of Fig- 13. A vertical section of tho trunk, !o show the 

 position of fie diaphragm (<].} which separates the ahdo tien (A.) from ihe 

 tiiorax (r.) into which ii ri^es up like an arch, CD, vertebral column, c, ribs 



18. [!<>w is the cavity of the thorax formed? What separates it from 

 '.he belly ? 



19. How is inspiration produced ? How is inspiration effected? How 

 many inspirations does an adult take in a minute ? 



20 Ho A does the air act upon the blood ? 



