174 COMPARATIVE ANATOMY. 



Finally, the naso-lacrymal duct of Amphibia must be 

 mentioned : it passes out from the anterior angle of the orbit, goes 

 through the lateral wall of the nose, and opens into the nasal 

 cavity on the side of the upper jaw. It conducts the lacrymal 

 secretion from the conjunctival sac of the eye into the nasal 

 cavity, and arises in all Vertebrates, from the Myctodera onwards, 

 as an epithelial cord which is separated off from the epidermis, and, 

 growing down into the derma, becomes secondarily hollowed. 



Reptilia. The Lacertilia and Ophidia possess the sim- 

 plest olfactory organs amongst Reptiles. The nasal cavity of the 

 former group is divided into two portions, a smaller outer 

 (anterior), and a larger inner (posterior), or olfactory chamber 

 proper. The latter only is provided with sensory cells, the for- 

 mer being lined by ordinary stratified epithelium continuous with 

 the epidermis, and glands being entirely absent in it. 



A large turbinal, slightly rolled on itself, arises from the outer 

 wall of the inner nasal chamber, and extends far into its lumen ; 

 this is also well developed in Ophidia, in which a distinct outer 



FIG. 145. DIAGRAM OF THE OLFACTORY ORGAN OF A LIZARD. (Longitudinal 



vertical section. ) 



AN, IN, outer and inner nasal chambers ; t, tube-like connection between them ; 

 Ch, internal nostrils ; P, papilla of Jacobson's organ ; Ca, aperture of com- 

 munication of the latter with the mouth ; MS, oral mucous membrane. 



nasal chamber is wanting ; it may be derived from that of the 

 Amphibia. 



A large gland which opens in the boundary between the 

 inner and outer nasal cavities lies within the turbinal. Below the 

 latter is the aperture of the lacrymal duct, though this in some 

 cases opens on the roof of the pharynx (Ascalabota), and in others 

 into the internal nostrils (Ophidia). 



The structure of the nose in Cheloniaus is very complicated and varied. 

 In marine Chelonians it is divided into two passages, one of which lies 

 above the other, but they are connected by means of a perforation of the 

 septum. The comparative paucity of glands in the olfactory organ of Lizards 

 and Snakes forms a marked contrast to the condition seen in Chelonians, the 

 nasal organ of which is characterised by a great abundance of them. 



From the Crocodilia onwards the olfactory organ, which 

 up to this point lies entirely in front of the brain, gradually 



