VOMERO-NA8AL ORGAN 271 



forms in which the external nose grows out to form a longer or 

 shorter tru ik or proboscis, at the distal end of which the nostrils 

 open (e.fj. Shrew, Mole, Pig, 1 Tapir, Elephant). By means of its 

 abundant nerve-supply, the proboscis serves as a delicate organ 

 of touch and may even give rise to a prehensile apparatus 

 (Elephant). In the Ape Ncisulis the peculiar and grotesque 

 external nose, with downwardly directed nostrils, cannot be 

 direct!} 7 compared with the human external nose. 



VOMERO-NASAL (jACOBSON's) ORGAN. 



By the term " Jacobson's organ " is understood a paired acces- 

 sory nasal cavity which in an early embryonic stage becomes 

 differentiated from the nasal chamber, and which is supplied by 

 the olfactory and trigeminal nerves. 



This cavity is first met with in Amphibians, but is wanting in 

 Proteus and Necturus. In the Iarva3 of Anura and Myctodera a 

 small gutter-like medio-ventral outgrowth of each nasal cavity 

 is formed, and in most Urodeles this later undergoes a relative 

 change of position, so as to be situated laterally (Fig. 200, A-D): 

 at its blind end a gland is developed. The accessory nasal 

 chamber of Csecilians 2 (p. 264) is developed in a similar manner 

 (E), and a large gland is in connection with it. 



The vomero-nasal organ of the Amniota is also developed in the 

 medio-ventral part of the nasal chamber, close to the septum nasi. 

 It loses its primary connection with the former, but in Hatteria, 

 Lacertilia, and Ophidia retains its median position, lying between 

 the floor of the nasal cavity and the roof of the mouth. It is lined 

 by an olfactory epithelium and communicates in front with the 

 mouth through the nasopalatine canal. A papilla usually extends 

 into its cavity from the floor (Figs. 194 and 260, F). These organs 

 are not present in Crocodiles, Chelonians, and Birds, but more or 

 less distinct indications of them have been described in the embryo 

 in some cases. 



Amongst Mammals, Jacobson's organ is well marked in Mono- 

 tremes (Fig. 200, G), in which it is much more highly developed 

 than in most Lizards (except Monitors)/ 5 It contains a well-marked, 



1 The external nose may be further supported by a median pre-nasal bone 

 (e.y., Mole, Pig). In Chrysochloris it is capped by a horny shield, and in 

 Condylura its Hat disc is provided with numerous tactile appendages. 



A curious apparatus exists in the Gymnophiona in connection with the 

 nasal, cavity and orbit. It consists of a retractile tentacle, a fibrous capsule witli 

 muscles, and a large gland, opening near the snout. Its function is not thoroughly 

 understood, and the same is true of the tentacle-like "balancers" of larval 

 Urodela and Aglossa. 



'' In the Australian Bat, Miniopterus, it is even larger relatively than in 

 Monotremes. 



