OLFACTORY ORGAN 



265 



tolerably far forwards on the roof of the mouth, and are for 

 the most part enclosed by the vomer, or vomero-palatine. 



A naso-lacrymal duct passes out from the anterior angle of 

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

 the nasal cavity on the inner 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 

 epiderm, and, growing down into the derm, becomes hollow 

 secondarily. A naso-lacrymal duct is wanting in Proteus and 

 Siren. 



Reptiles. Owing to the growth of the brain and facial region 

 and to the formation of a secondary palate (p. 1 1 2), the olfactory 

 organs, from Reptiles onwards, gradually come to extend more 

 ventrally beneath the cranium. As in 

 Amphibians, a lateral or ventral respira- 

 tory, and a median olfactory portion can 

 be recognised. 



The simplest olfactory organs amongst 

 Reptiles are seen in Lizards, Snakes, and 

 many Chelonians. The nasal cavity of 

 Lizards, for example, is divided into two 

 portions, a smaller outer (anterior), and a 

 larger inner (posterior) or olfactory cham- 

 ber proper (Fig. 194). The latter alone 

 is provided with sensory cells, the former 

 being lined by ordinary stratified epi- 

 thelium continuous with the epiderm : it 

 may contain goblet-cells, but encloses no 

 aggregated glands, and externally to the 

 epithelial layer are muscular elements and 

 cavernous tissue. A large fold or tur- 

 linal, 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 nasal chamber 

 is wanting, but is very simple in the Amphisbsenida?. The 

 skeletal supports of the turbinals in Reptiles as in all higher forms, 

 are developed secondarily. 



A large gland which opens at the boundary between the inner 

 and outer nasal cavities lies within the turbinal (except in 

 Hatteria), and corresponds to the superior nasal gland of Urodeles. 

 Below the turbinal is the aperture of the lacrymal duct : in some 

 Reptiles this opens on the roof of the pharynx (Ascalabota), 

 and in others into the internal nostrils (Ophidia), which, as in 

 Amphibians, are usually situated on the anterior part of the roof 

 of the mouth. 



FIG. 194. 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 nos- 

 trils ; MS, oral mucous 

 membrane ; P, papilla of 

 Jacobson's organ (q.v.\ 

 and Ca, its aperture of 

 communication with the 

 mouth. 



