Jacobson's Organ and the Respiratory Mechanism of Ainphibians. 159 



The ordinary medium of smell is air, which passes in and out 

 through the nasal cavity both in pulmonary and in bucco-pharyngeal 

 respiration. The latter is the chief means of renewing the olfac- 

 tory medium; degeneration of the lungs is not followed by de- 

 generation of the olfactory organ. Some adult Salamanders [Am- 

 blystoma) may resort to acquatic oral respiration, in which water 

 passes through the nasal cavity, but it is not certain that water 

 is used for olfactory purposes. 



In the Anura [Rana] the choana is similar to that of the higher 

 urodeles. The external naris is opened and closed by a System of 

 levers under the control of striated muscles. The complicated ol- 

 factory organ includes a true olfactory Chamber, a maxillary cavity 

 and three anterior bliud sacks, a superior, a middle and an inferior 

 one, the last containing Jacobson's organ. Through the maxillary 

 cavity this organ is easily reached by the outgoing respiratory cur- 

 rents (G-aupp, 1904). Air is the usual medium of smell. Water 

 does not enter the nasal cavity under ordinary conditions. 



In the larval frog and Salamander the olfactory organ gradually 

 developes in preparation for adult life, but the arrangements for Con- 

 trolling the respiratory media differ entirely from those of the adult 

 (Brüner, 1913). In both frog and Salamander, the mechanism which 

 is used to close the external naris of the adult during inspiration 

 reaches a functional condition in a late stage of larval life. Schulze 

 (1892) has pointed out the fact that the choana of larval Pelobates 

 is closed during acquatic »expiration« by certain fringes which 

 Surround the opening, and an examination of the frog larva shows 

 the presence of similar valves, which effectually prevent the passage 

 of the respiratory media from the mouth into the nasal cavity. 

 The medium of smell in the larval frog is respiratory water 1 , which 

 passes through the nasal cavity into the mouth, and thence through 

 the gill clefts and peribranchial Chamber and out through the 

 spiracle. Pulmonary air is taken by the open mouth and does not 

 enter the nasal cavity. 



In the larvae of lungbearing Salamanders [Amblystoma) the 

 olfactory process is similar to that of the frog larva. Respiratory 

 water is taken through the nostrils and is the only medium of smell. 

 A fold of mueous membrane on the median side of the choana 



1 In the revised edition of Ecker and Wiedersheim's Anatomie des 

 Frosches, III. Abt., 1904, it is assumed that air is the only medium o emell 

 in the frog larva (p. 672). 



