xii. 1 6 RESPIRATION 333 



from water, makes it necessary to possess more complicated methods 

 of self-maintenance. 



16. Respiration in the frog 



The lungs of the frog are paired sacs, opening to a short laryngeal 

 chamber, which communicates with the pharynx by a median aper- 

 ture, the glottis. The glottis and laryngeal chamber are supported by 

 the arytenoid and cricoid cartilages. The arytenoids guard the open- 

 ing of the glottis and are moved by special muscles. During breathing 

 the mouth is kept tightly closed, the lips being so arranged as to make 

 an air-tight junction. Air is sucked in through the nostrils by lowering 

 the floor of the mouth by means of the hypoglossal musculature, and 

 can then either be breathed out again or forced into the lungs by 

 raising the floor. The external nares are closed by a special pad on the 

 anterior angle of the lower jaw, supported by the mento-Meckelian 

 bones. This pad is thrust upwards and pushes the premaxillaries 

 apart, so altering the position of the nasal cartilage that the nostrils 

 are closed. This is a special mechanism, found only among anurans. 

 In urodeles the nostrils are closed by valves provided with smooth 

 muscles. Such valves are present in the frog but are said to be func- 

 tionless. 



The movements of the floor of the pharynx are not continuously of 

 the same amplitude. After a period of relatively slight movements the 

 nostrils are kept closed while the throat is lowered. Air is thus drawn 

 from the lungs and then again returned to them once or twice before 

 the nostrils are reopened. The whole procedure presumably ensures 

 the maximum gaseous interchange for the minimum water-loss. 



This method of taking in air is clearly derived from the movements 

 of the floor of the mouth of fishes, by which water is passed over the 

 gills. In amphibian larvae water is pumped in this way and there is 

 direct continuity between the mechanism of larva and adult. The 

 basic rhythmic mechanism, centred on the nerve-cells of the medulla 

 oblongata, is no doubt the same throughout, but the anurans have 

 improved upon it by the addition of special features, requiring intri- 

 cate coordination of the muscles of the larynx and the apparatus for 

 closing the nostrils. 



The skin is very vascular, and especially so in the buccal cavity. It 

 plays a large part in respiration, actually serving to remove more 

 carbon dioxide than do the lungs. There is, however, little power to 

 vary the amount of exchange through the skin, which is therefore 

 constant throughout the year. There is considerable regulation of the 



