itegrative Systems 



fiHIGHT CEREBRAL HEMISPHERE 



199 



NASAL BONE 

 SPHENOID BONE 

 NASAL CONCHAE 

 EUSTACHIAN 



MAXILLA 

 MOUTH CAVITY- 

 PALATINE BONE 

 VESTIBULE 

 SOFT PALATE 

 M GENI0GL0S5US 

 MANDIBLE 

 M. GENIOHYOID 



M. MYLOHYOID 



SEMISPINALS 

 CERVICIS 



esophagus' 



Fig. 171. A sagittal section of the human head showing the relations between 

 digestive and respiratory passages in the pharyngeal region. (After Braus. Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



groups the development of a horizontal partition (secondary palate) 

 across the oral cavity and more or less of the pharynx extends the nasal 

 passages backward and brings the choanae into a much more posterior 

 position (Fig. 171). Stimulation of the olfactory receptors by sub- 

 stances contained in the air is favored by this respiratory arrangement. 



In fishes the olfactory receptors are stimulated by substance dis- 

 solved in the external water which is drawn into the nasal chambers. 

 In lung-breathers numerous small serous glands secreting a watery 

 fluid, and other glands secreting mucus, maintain over the olfactory 

 surfaces a thin layer of moisture, in which substances present in the 

 air may become dissolved and thus brought within stimulating range 

 of the sense-cells whose "hairs" are bathed by the secreted moisture. 

 x\ lacrimal duct leads from the space beneath the eyelids into the 

 corresponding nasal passage and contributes to the moistening of the 

 nasal surfaces. 



The olfactory receptors are very much more keenly sensitive than 

 the gustatory receptors. 



