22O Marsh. 



muscles, by means of which the cavity of the chest or thorax 

 is continually being altered in size and capacity, and alter- 

 nately made larger and smaller, keeps up a regular series of 

 inspirations and expirations, during the former of which 

 pure air passes into the lungs and mixes with that which 

 is already there, whilst during the latter, a part of the air 

 in the lungs (which has been robbed of some of its oxygen 

 by the blood in the lungs) is driven out. But in frogs and 

 toads there are neither ribs nor diaphragm ; they can only 

 breathe, therefore, by swallowing air. 



Watch that large toad by your side ! It keeps its mouth 

 closed, but continues to gulp down air, as it were. The 

 throat is alternately contracted and enlarged ; when it is 

 enlarged the tongue is depressed, the cavity of the mouth is 

 made larger, and air passes through the nostrils into the 

 mouth. The mouth is still kept closed when the muscles of 

 the throat contract, and as this contraction lessens the 

 mouth capacity, we should naturally expect the air to be 

 forced back through the nostrils again. But the nostrils 

 are each provided with a valve, and although these valves 

 allow the air freely to enter the mouth from outside, they do 

 not allow it to pass out again. In a somewhat similar 

 fashion the entrance to the alimentary canal is provided 

 with a valve. When, therefore, the mouth capacity is 

 lessened by muscular contraction, the valves at the nostrils 

 prevent the outward passage of the air, whilst that at the 

 entrance to the ossophagus prevents its escape into the 

 alimentary canal. Only one channel is open to it, and down 

 this it is accordingly driven into the lungs. The mouth is 

 afterwards opened to let the effete air escape, but closed 

 again to take another inspiration. There is therefore a con- 



