OLFACTORY ORGAN 201 



Glands, situated under the olfactory mucous membrane, are now 

 also met with ; these are either diffused, or united to form definite 

 masses. They either open directly into the nasal cavity, their 

 secretion serving for the necessary moistening of the mucous mem- 

 brane (effected in Fishes by the external medium), or they pour 

 their secretion into the pharynx or posterior nostrils. The latter 

 always lie tolerably far forwards on the palate, and are for tl)e most 

 part enclosed by the vomer, as well as the palatine. 



Finally, the naso-lachrymal duct must be mentioned : it passes 

 out from the anterior angle of the orbit, through the lateral wall 



Fig. 160. — Tk.ajs-svebse Section through the Olfactoey Cavities of 

 Plethedon ghitinosus (Myctodera). 



i', S, olfactorj' mucous membrane ; N, main nasal cavity ; K, maxillary cavity ; 

 C, cartilaginous, and iS^, fibrous portion of the tiirbinal, which causes the 

 olfactory epithelium (E) to project far into the nasal cavity ; ID, inter- 

 maxillary gland, shut off from the cavity of the mouth by the oral mucous 

 membrane (J/>S) ; F, frontal ; Pf, prefrontal ; Jl/, maxilla ; Vop, vomero- 

 palatine ; Sp, nasal septum. 



of the nose, and opens into the nasal cavity on the side of the 

 upper jaw. It conducts the lachrymal secretion from the conjunc- 

 tival sac of the eye iato the nasal cavity, and arises in all Verte- 

 brates, from the Myctodera onwards, as an epithelial cord which is 

 separated off from the epidermis, and, growing down into the 

 dermis, becomes hollow secondarily. 



Reptilia. — Owing to the growth of the brain and facial region 

 and to the formation of a secondary palate (p. 92), the olfactory 

 organs, from Reptiles onwards, gradually come to be situated more 

 ventrally, beneath the cranium. 



The Lacertilia, Ophidia, and many Chelonia possess the sim- 

 plest olfactory organs amongst Reptiles. The nasal cavity of 

 Lizards 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 former being lined by 

 ordinary stratified epithelium continuous with the epidermis and 

 containing no glands. 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 nasal chamber is wanting ; it may be 

 derived from that of the Amphibia. 



