The Mechanism of Pulmonary Eespiration in Amphibians with Gill Clefts. 79 



of tke rn. dorsalis choanae is a sornewhat stronger muscle, m. late- 

 ralis choanae, whose fibres arise in part frora tlie palatine portion 

 of the maxillaiy bone, in part from the anterior end of the pterygoid 

 cartilage. The fibres run forward and mesad and insert on the 

 postero-lateral surface of the antorbital cartilage. 



Since Amphiuma is provided with two mechanisms for closing 

 and opening the nasal passage, a question arises in regard to the 

 function of each. On this point the following inay be said: During 

 acquatic respiration the muscles of the nasal Vestibüle are inactive 

 and the introductory passage remains wide open. When the floor 

 of the mouth is elevated the choana is usually closed. Occasionally 

 it may be kept open to permit the escape of water through the nasal 

 cavity. In pulmonary breathing the muscles of the nasal vestibule 

 are active, the constrictor muscle contracting during inspiration, as 

 it does in the higher urodeles. In expiration both choana and nasal 

 vestibule are open and the air passes through the nasal cavity. 

 Evidently the muscles of the nasal vestibule may also be used to 

 protect the nasal cavity, for example, during the process of burrowing. 



The Closing of the Choana in Cryptobranchus. 

 During pulmonary respiration the nasal passage is closed by 

 the smooth muscle mechanism which lies at the inner end of the 

 short nasal vestibule. In acquatic bucco-pharyngeal respiration the 

 nasal vestibule remains wide open, but the loss of water through 

 the nasal cavity is prevented by the closing of the choana. This 

 is effected by the ceratohyal whose broad anterior portion is pressed 

 against the oval opening when the floor of the mouth is elevated. 

 An important part in the process is played by a cushion-shaped 

 swelling which lies on the lateral side of the opening and was 

 described by Fischer (1864, p. 114) as a valve. It receives the 

 lateral border of the ceratohyal when the floor of the mouth is 

 elevated and thus aids in closing the choana. This method of closing 

 the choana is possible in acquatic respiration because the oscillations 

 of the floor of the mouth do not have great amplitude and the hyoid 

 is never far removed from the choana. During pulmonary respiration 

 the entire floor of the mouth is strongly depressed in aspiration 

 and the choana remains open until a late stage of inspiration, if it 

 is closed at all. 



