RESPIRATORY APPARATUS OF 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 Yertebrata, 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 u* 
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, blow the size of this cavity may 
be made to vary considerably;—in the first place, by the 
