The Mechanisrn of Pulmonary Respiration in Amphibians with Gill Clefts. 69 



Finally I must mention a small muscle, the submentalis of Crypto- 

 branchus, which aids in closing the mouth in this form. Fischer 

 (1864, p. 47) states that this muscle prevents the outward rotation 

 of the lower margin of the mandible, but he is probably in error 

 when he adds that such a rotation would otherwise follow from the 

 contraction of the masseter muscle. A rotation of the mandible, such 

 as Fischer mentions, is easily produced by depression of the anterior 

 median portion of the mandible, and since the two halves of the 

 mandible are joined at the rostrum only by soft tissues, such a de- 

 pression would be very likely to occur during inspiration if it were 

 not prevented by contraction of the submentalis muscle. By such 

 action, the submentalis prevents the Separation of the jaws and thus 

 assists in the act of inspiration. 



The Closing of the Gill Clefts. 



During branchial respiration the gill clefts are closed in order 

 to prevent the entrance of water from the outside when the floor 

 of the mouth is depressed. During pulmonary respiration they are 



Fig. 1. 



Ttiton alpestris. Transverse section of an 18 mm. larva. hy hyoid areh, with valve-like foldatove; 

 I first gill cleft; oc opercnlar Chamber. 



closed to prevent the escape of air from the pharynx and the entrance 

 of water from the outside. The closing of the clefts is due primarily 

 to the action of the constrictor muscles of the gill arches, but these 

 may be assisted by the posterior part of the mylohyoid. The former 

 were long ago observed by Cüvier (1810), Martin-Saint-Ange (1831) 



