THE THORAX AND RESPIRATION. 241 



enough ; and yet the interest might be increased 

 by observing the manner in which the textures of 

 the ribs are accommodated to variations in the 

 mode of respiration, or to the necessity of the an- 

 imal expressing the air from the lungs in diving. 

 We have seen how the thorax is expanded in 

 birds to the whole extent of the body, for obvious 

 reasons ; and the counterpart of that is presented 

 where the animal, instead of being buoyant in the 

 atmosphere, has to dive into the water and crawl 

 at the bottom — not at great depths, but yet under 

 water, in shallow pools and marshy places. The 

 frog has no ribs ; and its mode of respiration 

 shows a complete change from that of animals 

 which breathe with a diaphragm. It has the 

 power of compressing its body, and expelling the 

 air from the lungs ; and were it not for this, the 

 animal would remain on the surface of the water 

 as when cruel boys blow them up with a straw. 

 The crocodile and other saurian reptiles have their 

 ribs accommodated so as to produce a similar 

 effect, and for a similar purpose. Instead of the 

 arched form of the ribs, which we have described 

 as capable of a slight change of figure only, they 

 have ribs composed of distinct pieces, and jointed 

 in such a manner as to enable them to compress 

 the chest into a smaller volume. 



We have a sort of exposition of the uses, if 

 not the necessity, of respiration to the voice, in 

 observing by what substitutes sound is producecf, 

 22 



