RESPIRATORY MOVEMENTS OF THE GLOTTIS. 239 



air Iving nearest the exterior, in the trachea and large bronchi, 

 would fluctuate backward and forward, without ever penetrating 

 into the deeper parts of the lung, were there no other means pro- 

 vided for its renovation. There are, however, two other forces in 

 play for this purpose. The first of these is the diffusive power of 

 the gases themselves. The air remaining in the deeper parts of 

 the chest is richer in carbonic acid and poorer in oxygen than that 

 which has been recently inspired ; and by the laws of gaseous dif- 

 fusion there must be a constant interchange of these gases between 

 the pulmonary vesicles and the trachea, tending to mix them 

 equally in all parts of the lung. This mutual diffusion and inter- 

 mixture of the gases will therefore tend to renovate, partially at 

 least, the air in the pulmonary lobules and vesicles. Secondly, the 

 trachea and bronchial tubes, down to those even of the smallest 

 size, are lined with a mucous membrane which is covered with 

 ciliated epithelium. The movement of those cilia is found to be 

 directed always from below upward and, like ciliary motion 

 wherever it occurs, it has the effect of producing a current in tl.e 

 same direction, in the fluids covering the mucous membrane. The 

 air in the tubes must partici- 

 pate, to a certain extent, in Fig. 72. 

 this current, and a double 

 stream of air therefore is estab- 

 lished in each bronchial tube ; 

 one current passing from with- 

 in outward along the walls of 

 the tube, and a return current 

 passing from without inward, 



. SM AT, L BRONCHI AT, TUBE, showing ontward 



along the Central part Of its and iuward current, produced by ciliary motion. 



cavity. (Fig. 72.) By this 



means a kind of aerial circulation is constantly maintained in the 

 interior of the bronchial tubes ; which, combined with the mutual 

 diffusion of the gases and the alternate expansion and collapse of 

 the chest, effectually accomplishes the renovation of the air contained 

 in all parts of the pulmonary cavity. 



EESPIRATORY MOVEMENTS OF THE GLOTTIS. Beside the move- 

 ments of expansion and collapse already described, belonging to 

 the chest, there are similar respiratory movements which take place 

 in the larynx. If the respiratory passages be examined after death, 

 in the state of collapse in which they are usually found, it will be 



