THE SENSE ORGANS 



575 



pits with the pharynx. As a special adaptation to aquatic life the nasal 

 pits of elasmobranchs become incompletely divided by a transverse fold 

 of skin, so that as the fish swims, water flows into the anterior and out of 

 the posterior opening. 



The dipnoi take an important step forward in the evolution of the nose 

 by accjuiring a connexion between the olfactory pits and the mouth. 

 True choanae opening into the pharynx first appear in this group, and 

 thus the functions of smelling and breathing become associated. In the 

 embryo, the narial passages are formed by the closure of the edges of 

 paired nasobuccal grooves, such as are seen in adult elasmobranchs. 



ECTOTURBINALS: 



ENDOTURBINALS. 

 SEPTUM. 



Fig. 474. — Diagrams of cross sections of the narial 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. 



The connexion between olfactory organs and respiratory passages 

 which was invented by dipnoi, is also found in amphibia; but the lamellated 

 olfactory epithelium disappears as unsuited to life in the air. Narial 

 passages enlarge in amphibia, and become divided into a more dorsal 

 olfactory region and a more ventral respiratory passage. Olfactory hair- 

 cells are limited to the upper region. A supplementary olfactory organ, 

 the organ of Jacobson, arises in connexion with each narial passage and 

 thus with the mouth. It may serve to test the food taken into the mouth. 

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

 moisten the olfactory epithelium. 



Some reptiles, lizards for example, have added to the narial 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 



