Jacobson's Organ and the Respiratory Mechanism of Amphibians. 161 



the coutrol of a striated muscle 1 . By " means of tbe latter the 

 choana can be opened to permit the olfactory medium to enter the 

 nasal cavity from the mouth. Water is the usual medium of sniell- 

 Linder ordinary conditions air does not enter the nasal cavity either 

 in Inspiration or in exspiration (Bruner, 1913). 



In Necturus and Proteus the nasal cavity has a simple, almost 

 tubulär form and a typical organ of Jacobson is wanting (Seydel, 

 1895; Anton, 1911). According to Anton, Jacobson's organ is 

 probably represented by a lateral row of olfactory buds, which are 

 separated from the median part of the nasal cavity (true olfactory 

 Chamber) by a strip of indifferent epithelium on the floor of the 

 cavity. Muscles are wanting at the externa! naris, but a mechanical 

 choanal valve is present, which closely resembles the choanal valve 

 of the larval Salamander and the median valve of Siren (Bruner, 

 1913). On account of the presence of this valve in Necturus 

 the escape of the respiratory media from the mouth through the nasal 

 cavity is impossible. In fact the only fluid that can pass into the 

 nasal cavity from the direction of the mouth is the small amount 

 which lies between the valve and the actual opening of the choana. 



These conclusions, which are justified on anatomical evidences 

 are supported also by physiological observations. During branchial 

 respiration a very little water may escape from the external naris 

 of Necturus, but this amount can be readily accounted for by the 

 closing of the choanal valve. If the valve is cut away, water flows 

 freely from the external naris when the floor of the mouth is elevated. 

 During pulmonary inspiration water is first expelled from the nasal 

 cavity, and this is followed by a stream of air bubbles. I have 

 not been able to examine living specimens of Proteus, but on anatomi- 

 cal grounds I judge that the choanal valve of this form has a func- 

 tion similar to that just described. 



The olfactory medium in Necturus is respiratory water, which 

 passes through the nasal cavity into the mouth and is expelled 



1 As this paper goes to the printer, Professor Norris sends me a paper 

 Norris, H. W., On certain features of the anatomy of Siren lacertina, Anat. 

 Anz., Bd. XLIII, 1913) in which he has described two choanal muscles in Siren. 

 These he believes to be homologous with muscles previously observed by him 

 in Amphiuma. The latter are presumably the same muscles that open and 

 close the choana in Amphiuma. My description of the choanal apparatus of 

 Siren and' Amphiuma (Bruner, 1913) I believe to be correct. 



Morpholog. Jahrbuch. 48. 11 



