46 CLASSIFICATION 



are continually provided with fresh pure air, and get rid of impure 

 air. Without going into unnecessary detail, it may be pointed out 

 that the walls of the chest are movable, so that the size of that 

 region is alternately increased . (when air is breathed in) and 

 diminished (when air is breathed out). During the former process, 

 the breast-bone and ribs are moved on the joints by which the 

 latter are hinged to the backbone, in such a way as to increase 

 the volume of the chest from side to side and from front to 

 back. The general nature of these movements can be easily 

 noted by the reader in himself. At the same time the muscular 

 midriff or diaphragm, which separates the chest from the abdomen, 

 and in a state of rest is convex towards the former, flattens out 

 as a result of its own contraction, and thus increases the size of 

 the chest in the direction of its length. As the thorax is, so to 

 speak, an air-tight box, the contained lungs are obliged to expand 

 as it increases in volume, the result being that air passes into their 

 larger air-passages. The reverse process to the one described, 

 whereby diminution of volume is effected, is largely due to 

 the elasticity of the thoracic walls, while at the time the dia- 

 phragm ceases to contract, and resumes its normal curved shape. 

 Air consequently passes out from the larger air-passages. Purifi- 

 cation of the air contained in the smaller air-passages (bronchial 

 tubes) and their blind endings is brought about by diffusion. A 

 few words are necessary on the direction taken by the air. The 

 proper course for this (fig. 8) is through the nostrils, into the 

 nasal cavities, and thence by a special aperture (posterior nares) 

 into the pharynx, on the floor of which is a slit-like opening, 

 the glottis, leading into the windpipe. Of course breathing 

 can also be effected through the mouth, but the primary use 

 of this is as a food-passage. 



In front of the glottis there is an elastic flap, the epiglottis 

 (fig. 8), which, when food is swallowed, folds back over the 

 glottis and forms a sort of bridge to the gullet. The wind- 

 pipe (trachea), into which the glottis leads, is a good -sized 

 tube (fig. 22), with its walls stiffened by hoop-like pieces of 

 gristle and so prevented from collapse. It can easily be felt 

 in the front of the neck, from which it runs into the thorax, 

 there bifurcating into a bronchus for each of the lungs. Either 

 bronchus when traced is found to divide repeatedly to form 

 smaller and smaller air-passages, the smallest of which end, as 



