46 VERTEBRATE RESPIRATION 



Species of frog but the essential mechanism is the same and 

 involves co-ordinated activity of the nostril valves, glottis, and 

 hyoid apparatus. After periods of intense activity, frogs may 

 show continuous pulmonary ventilation but at rest it is more 

 usual to observe bucco-pharyngeal movements interrupted 

 periodically by the pulmonary movements (fig. 14.) Evidently 

 these movements bring about renewal of the air in the bucco- 

 pharyngeal cavity which is then pumped in and out of the lungs 

 until a good proportion of its oxygen has been removed before 

 its expulsion. In this way maximum use is made of the oxygen 

 and water loss is kept to a minimum. 



The mechanism of the nostril valves in frogs is different from 

 that of most other Amphibia where they are controlled by 

 sphincter muscles. When the mouth is closed firmly, pressure of 

 the lower jaw between the premaxillae is transmitted to the nasal 

 bones which move in such a way that the external nostrils are 

 closed. This can be demonstrated quite easily in a live frog by 

 opening the mouth and pressing with a seeker on the inside of the 

 upper jaw just behind the premaxillae. The internal nostrils are 

 also visible in the roof of the mouth as quite large and widely 

 separated openings and it has been suggested that they are closed 

 by the action of the anterior horns of the hyoid plate. The dis- 

 tance separating these horns is similar to that between the 

 internal nostrils which they will occlude when the hyoid is 

 raised. The whole of the external rim of the buccal cavity is 

 firmly closed when the jaws are pressed together and it is evident 

 that this cavity may become completely air-tight when air is 

 forced into the lungs. The glottis is supported by cartilages and 

 its slit-like aperture is controlled muscularly. 



Many Amphibia are able to live beneath the surface for very 

 long periods. For example, the clawed toad, Xenopus, can 

 remain under water for two weeks or more and remain alive. 

 Under these conditions, oxygen is mainly obtained through the 

 skin which is profusely supplied with blood derived from the 

 pulmo-cutaneous arch. There is no doubt that cutaneous 

 respiration plays a very important role in several of the lower 

 tetrapods, and in frogs it has been shown that the amount of 



