402 



CHORDATE ANATOMY 



A lacrimal duct from each eye opens into the nasal passage, and serves to 

 moisten the olfactory epithelium. 



Some reptiles, lizards for example, have added to the nasal passage a 

 more expanded and glandular vestibule, which is apparently a mechanism 

 for eliminating dust from the air taken into the lungs. Novel also in this 

 group are the paired turbinal bones or conchae, which project into the 

 nasal passages, and serve to increase and support the olfactory membrane. 

 A nasopharyngeal cavity distinct from the mouth cavity also first appears 

 in this group, as the result of the ingrowth and extension of the palatine 

 bones. Thus a bony palate is formed, and the nariai passages open by 

 secondary choanae into the pharynx. The vomer bone, therefore, no 



ECTOTURBINALS: 



ENDOTURBINALS. 

 SEPTUM. 



Fig. 354. — Diagrams of cross sections of the nasal passage in A, ruminant and B, 

 man, showing the contrast in the number of turbinal bones. Since olfactory epithelium 

 covers these bones, the keenness of the sense of smell is proportional to the number 

 and size of turbinals. Compared with many mammals the sense of smell in man is 

 degenerate. 



longer lies in the roof of the mouth, but in the nasopharyngeal passage. 

 By the formation of the palate, the nasopharyngeal cavity is both enlarged 

 and elongated. 



In birds there are three pairs of conchae, and Jacobson's organ 



disappears. 



In mammals there is an enormous enlargement of the nasal passages, 

 and a corresponding multiplication of conchae, the single pair of reptilian 

 conchae persisting as the maxillo-turbinals, while ethmo-turbinals, upper 

 and lower, are added. Jacobson's organ nearly disappears in the higher 

 mammals. The contrast between the upper olfactory and the lower 

 respiratory region persists. 



That the olfactory organ of man is degenerate, is evidenced by the 

 reduction of. the conchae in size and number, and in the ontogenetic loss 

 of three pairs of turbinal bones present in the embryo. What is left of 

 Jacobson's organ in man enters into the formation of the incisive canal. 



