454 



Coinparatwe Animal Physiology 



in the superior part of the nasal cavity. In deer the area is much greater. 

 During normal respiration of man most of the air does not pass directly over 

 the olfactory organ, but this is supplied with air (and odors) primarily by 

 eddy currents (Fig. 146). The secluded position of the olfactory epithelium 

 seems to be important in preventing desiccation of the protoplasmic processes 

 of the olfactory cells. Hou'cver, in sniffing, the volume of air passing the 

 olfactory organ is greatly increased. In dogs more of the air passes directly 

 over the olfactory epithelium. Jacobson's organ or the naso-vomerine organ, 

 located in the nasal septum, is well developed in some fishes and reptiles and 

 is believed to function as an accessory olfactory organ. 



Histologically, the olfactory epithelium (Fig. 147) consists of the olfac- 

 tory receptors (primary neurones) embedded in columnar epithelial cells, 

 some of which contain a yellowish-brown pigment and give the organ a 



Fig. 146. Diagram of the right nasal cavity of man, showing the direction of air 

 flow. Eddy currents around the superior concha are created by sniffing and bring the 

 inspired air into contact with the olfactory epithelium. After Evans"' from Fulton." 



yellowish appearance macroscopically. The primary neurones are bijx)lar, 

 with the dendrite forming the receptor proper and the cell body performing 

 the function of "ganglion cells" in supplying the fibers (unmyelinated) of 

 the olfactory nerve. 1 he olfactory epithelium also contains free nerve end- 

 ings of the trigeminal ner\'e which function as part of the common chemical 

 sense receptor. The olfactory organ is the simplest sense organ of the body 

 and is also very sensitive. Stimulation probably occurs through the hairlike 

 processes, and the sense cell, serving as both receptor and conductor, trans- 

 mits this to the brain. Compared with the eye and ear, the olfactory organ 

 is extremely simple. 



Tbe sense of taste, or gustation, is mediated by sense organs located in the 

 moutb, ahhough in Hshes they are frequently found outside of the mouth 



