JACOBSON'S OKGAN. 



179 



olfactory epithelium, from the floor of which a papilla arises : 

 it communicates with the mouth by a special aperture in front of 

 the internal nostrils. 



Jacobson's organs are not known in Crocodiles, Chelonians, 

 or Birds, but are very general in Mammals, being especially 

 well marked in Rodents, Ruminants, and Perissodactyles. They 

 here consist of two tubes lying at the base of the septum nasi, 

 enclosed by definite cartilages (ploughshare cartilages of Huschke) ; 

 they end blindly posteriorly, but open anteriorly into the mouth 



FIG. 149. THE LEFT SO-CALLED "TENTACLE" OF Coecttia oxyura. (Opened from 



the dorsal side.) 



Me, constrictor muscle ; Be, retractor muscle, which becomes tendinous at Be 1 , and is 

 inserted into the ridge or papilla, Pa ; D, the large gland, surrounded by the 

 constrictor, with its two ducts (a, b), which further forwards unite to form a 

 common duct (c) ; IS, AS, inner and outer fibrous tubes ; A, eye ; S, S, skull, 

 with the tentacular gland (TD) lying in the nasal cavity, showing through. At 

 t the ducts of the tentacular gland (Co) pass out from the skull, and after extend- 

 ing a short distance, pass into the tentacular sheath. 



by means of the incisive or naso-palatine canals (ducts of Stenson), 

 which extend through the palate behind the premaxillse. 



The structures present in Man which have usually been described as rudi- 

 ments of Jacobson's organ probably correspond with the remnants of a gland 

 in connection with the nasal septum (Gegenbaur). 



The physiological function of Jacobson's organ may consist in 

 bringing the food taken into the mouth under the direct control of 

 the olfactory nerve. 



THE SPOUTING APPARATUS (SO-CALLED "TENTACLE") OF GTMNOPHIONA. 



A very remarkable organ exists in Caecilians, which is closely related as 

 regards position both to the nasal cavity and to the orbit. 



It consists of a fibrous capsule lying in the orbit, and surrounded by strong 

 muscle (Figs. 148, Cg, 149, Me), which extends forwards in the form of a 



N 2 



