RESPIRATORY APPARATUS OP MAMMALS. 281 



prevent the entrance of anything injurious into the windpipe. 

 Thus if we attempt to breathe carbonic acid gas, which would 

 produce an immediately fatal result if introduced into the 

 lungs, the lips of this chink immediately close together, and 

 so prevent its entrance. The contact of liquids or of solid 

 substances, too, usually causes the closure of the aperture, so 

 that they are prevented from finding their way into the wind- 

 pipe ; but this does not always happen, especially when the 

 glottis is widely opened to allow the breath to be drawn- in 

 ( 193). 



329. The larynx, trachea, and bronchial tubes, to their 

 minutest ramifications, in all air-breathing Vertebrata, are 

 lined by a mucous membrane continued from the back of the 

 throat ; and this membrane, like the gills of aquatic animals, 

 is covered with cilia, which are in continual vibration. It is 

 obvious, however, that the purpose of this ciliary movement 

 must be here different from that which is fulfilled by the same 

 action on the surface of the gills ( 319); and it probably 

 serves to get rid of the secretion which is being continually 

 poured out from the surface of the mucous membrane, and 

 which, if allowed to accumulate there, would clog up the air- 

 cells, and in time produce suffocation. The vibration of the 

 cilia is observed to be always in one direction, towards the 

 outlet ; and it is in this direction, therefore, that the fluid is 

 gradually but regularly conveyed. The ciliary movement may 

 be seen to take place on the surface of the mucous membrane 

 of the nose ; but not on that of the pharynx, where it would 

 be continually interrupted by the passage of food. 



330. The constant renewal of the air in the lungs is pro- 

 vided for, in Mammals, by a peculiar mechanism, which accom- 

 plishes this purpose most effectually, though itself of the 

 most simple character. We must recollect that the thorax in 

 this class is an entirely closed cavity. It is bounded above 

 and at the sides by the ribs (the space between which is filled 

 up by muscles, &c.), and below by the diaphragm, which here 

 forms a complete partition between the thorax and abdomen. 

 The whole of this cavity, with the exception of the space 

 occupied by the heart and its large vessels (and also by the 

 gullet, which runs down in front of the spine), is constantly 

 filled-up by the lungs. Now the size of this cavity may 

 be made to vary considerably; in the first place, by the 



