SEA'SE ORGANS. 77 



Connected with the nose in all vertebrates above the fishes 

 is a pair of accessory sensory organs, the organs of Jacobson. 

 They are outpushings of the wall of the olfactory surface, sup- 

 plied by branches of the first and fifth nerves. In the lower 

 amphibia these organs are placed on the medial side of the nasal 

 cavities ; a little higher they are ventral in position ; in the high- 

 est amphibia they have rotated to the lateral side of the olfactory 

 organ. In the amniotes they are either medial or ventral in 

 position. In the lower forms these sacs are connected only 



FIG. 83. Section through the nasal region of the Surinam toad, Pipa. c, car- 

 tilage ; en, cavum nasale ; </, Jacobson's gland and duct; <?, ethmoid cartilage; 

 /, frontal bone ; _/', organ of Jacobson ; /, lateral portion of nasal passage ; n, nasal 

 bone ; /, branch of olfactory nerve to organ of Jacobson ; nn, branches of nasal 

 nerve of trigeminal. 



with the nasal cavities ; -but in the mammals a duct (Stenson's 

 duct) sometimes leads from them into the mouth through the 

 foramina incisiva, between the premaxillary and the palatine 

 processes of the maxillary bones. In many mammals, however, 

 these foramina are closed by membrane, and are vestigial in 

 character. 



In the mammals for the first time appears an external nose 

 supported by cartilage. In some, like the tapirs and elephants, 

 this organ becomes enormously developed, and forms in the 

 latter the well-known trunk. 



