The Mechanisni of Pulmonary Respiration in Amphibians with Gill Clefts. 77 



Hitherto undescribed choanal muscles in Amphiuma. 



Since the expiratory currents pass freely through the nasal 

 cavity of Amphiuma, it is evident that the choana is not closed in 

 a purely mechanical way, as assumed by earlier authors. The ope- 

 ning lies at the posterior end of a short choanal canal, whose floor 

 s formed by a membraneous fold, the so-called valve of Cüviee. 

 The choana is closed bj^ Fio . g 



depression of the roof of 

 the choanal canal until it 

 meets the ventral fold. By 

 elevation of the roof of the 

 canal the choana can be 

 so widely opened that it 

 can not be closed by 

 the fold. 



The niechanism which 

 opens and closes the choana 

 includes the antorbital car- 

 tilage and two striated 

 muscles. The antorbital 

 cartilage (Figs. 9, 10, 11) 

 is a horizontal bar which 

 arises by a short fibrous 

 stalk from a remnant of 

 the trabecula cranii which 

 s exposed on the lateral 

 surface of the orbito-sphe- 

 noid bone. From its at- 

 tachment it extends for- 

 ward and laterad on the 

 dorsal aspect of the choana 

 and the choanal canal. The distal end of the cartilage is elevated 

 and depressed by the muscles above mentioned (Figs. 9 and 11). 

 One of these, which I shall name m. dorsalis choanae, arises from 

 the lateral wall of the cranium, chiefly from a slender process of 

 the parietal bone which lies between the frontal and the orbito- 

 sphenoid. From their origin the fibres of the muscle run ventro- 

 laterad to insert on the dorsal surface of the antorbital cartilage. 

 This muscle elevates the distal end of the cartilage and the roof 

 of the choanal canal and thus opens the choana. The antagonist 



Amphiuma tridactylum. Choanal region, viewed from 

 above. ac antorbital cartilaga; de m. dorsalis choanae; Ic 

 m. lateralis choanae; mx maxillare; n choanal canal, which 

 terminates in the choana heneath the antorbital cartilage 

 and the m. dorsalis choanae; nc cartilaginous nasal capsule ; 

 pa parietale ; pt pterygoid cartilage. 



