576 



COMPARATIVE ANATOMY 



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

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

 longer lies in the roof of the mouth, but in the naso-pharyngeal passage. 

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

 and elongated. (Fig. 183) 



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

 disappears. 



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

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

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



CORPUS CALLOSUM 



CYRUS DCNTATUS 



MAMMILLARY BODY 

 TUBER CINEREUM 



Fig. 475. — A diagram of the nervous relations of the olfactory system of fibers. 

 (Redrawn after Rasmussen's "Principal Nervous Pathways," The Macmillan Co.) 



and lower, being 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, 

 which connects the anterior part of the nasal passage with the mouth 

 cavity. (Fig. 474) 



Development of Olfactory Organs 



In elasmobranchs, each olfactory organ develops from a placode-like 

 thickening at the anterior termination of the series of lateral line organs; 

 Subsequently each placode, by invagination, is converted into a pit, which 

 lies in close apposition to the telencephalon vesicle. In man a groove 

 similar to the nasobuccal groove of elasmobranchs connects each olfactory 



