EYE 207 



A Jacobson's organ is first met with in Amphibians. In young 

 Triton larvae a small gutter-like medio-ventral outgrowth of each 

 nasal cavity arises, with which the ventral branch of the olfactory 

 nerve comes into relation. This outgrowth later undergoes a re- 

 lative change of position, and comes to be situated laterally towards 

 the upper jaw (Fig. 165 A D). At its blind end a gland is developed. 

 In Siren the primary median position is retained, and in the Axolotl 

 it does not extend so far laterally as in the adult Triton. The acces- 

 sory nasal chamber of Coecilians ] (p. 200) is developed in a similar 

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

 also be little doubt that this cavity is represented in Anura, 

 although its relative position is somewhat different to that seen in 

 Urodeles. 



The Jacobson's 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 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 corresponding 

 naso-palatine canal (p. 100). In Lacertilia and Ophidia a papilla 

 extends into its cavity from the floor (Figs. 161 and 165, F). 



These organs are not present in Crocodiles, Chelonians, and 

 Birds, but rudiments have been observed in embryos of Crocodilus 

 biporcatus, and certain cartilages on the nasal floor in Birds 

 appear to correspond with the Jacobson's cartilages of other forms. 



Amongst Mammals, Jacobson's organ is most marked in 

 Monotremes (Fig. 165, H), in which it is much more highly developed 

 than in Lizards. It contains a well-marked, turbinal-like ridge, 

 supported by cartilage continuous with that enveloping the organ 

 arid covered with ciliated epithelium, and numerous glands are 

 present in the mucous membrane. In other Mammals (G, l) it 

 becomes more or less reduced, though often well-marked, consisting 

 of two tubes lying at the base of the septum nasi, usually enclosed 

 by separate cartilages (Marsupials, Edentates, Insectivores, Rodents, 

 Carnivores, Ungulates). A branch of the olfactory nerve enters the 

 tube posteriorly, and anteriorly the cavity of the organ communi- 

 cates with the mouth through the incisive or naso-palatine canals. 

 Rudiments of the organ exist even in Man (Fig. 163, c). 



The function of Jacobson's organ may consist in bringing the 

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

 nerve. 



EYE. 



As already mentioned (p. 154, Fig. 167, A and B), the optic 

 nerve is developed from the stalk of an outgrowth of the primary 



1 A curious apparatus exists in Ccecilians in connection with the nasal cavity 

 and orbit. It consists of a fibrous capsule with muscles and a large gland, 

 opening near the snout. Its function is not certainly known. 



