APPARATUS OF BESPIBATION. 



73 



mals. In birds, some of these air tubes open into large 

 membranous pouches, which proceed as far as the limbs, and 

 conduct the air even into the interior of the bones. Thus 

 respiration becomes more active in this class of animals. 



138. Mechanism of Respiration in Man, By the 

 movements of inspiration and expiration performed by the 

 walls of the thorax or chest, the air is constantly renewed 

 in, and expelled from, the lungs. The walls of the chest 



Fig. 50. Thorax of Man.* 



are moveable, and more especially one, which is not seen, 

 the diaphragmatic wall ; dilated by a muscular effort, the 

 air rushes into the cavity through the nostrils and tra- 

 chea, pressed on by the whole weight of the incumbent 



* On the left side the muscles have been removed. The arch forming the 

 diaphragm towards the interior of the chest is seen on the left side g, and on 

 the right side a dotted line marks the extent of the ascent of the same muscle 

 on the right side ; h, pillars of the diaphragm attached to the lumbar ver- 

 tebrae ; i, elevator muscles of the ribs ; d, collar bone. 



