THE EAR, HEARING, TASTE AND SMELL 



239 



thought to be mediated by structures of the vestibule, the utricle 



and saccule. These are hollow structures having stiff hairs pro- 



jecting into their cavities and tiny stones caught among the hairs. 



The weight of the stones will affect the hairs among which it rests 



in one way when the head is erect, in quite another way when 



the head is horizontal. Thus the 



nerves may be stimulated differently 



for different positions of the head, ful- 



filling the conditions that the sense re- 



quires. In many invertebrate animals 



structures similar to the utricle and 



saccule represent their only organs re- 



sembling our ears in any way. Experi- 



ments upon these animals have shown 



that in them these .structures are not 



hearing organs but organs of equilib- 



rium. 



Smell. The region of the nostril 

 nearest its outer end possesses the 

 sense of touch: the olfactory organ 

 proper consists of the upper portions 

 of the two nasal cavities, over which 

 the endings of the olfactory nerves 

 are spread and where the mucous 

 membrane has a brownish-yellow color. 

 This region (regio olfactoria) covers 

 the upper and lower turbinate bones, 



Which are expansions of the ethmoid On 



.. . , ,Mi i 



the outer wall of the nostril chamber, 

 the opposite part of the partition between the nares, and the part 

 of the roof of the nose separating it from the cranial cavity. The 

 epithelium covering the mucous membrane contains three varieties 

 of cells (2, Fig. 76). The cells of one set are much like ordinary 

 columnar epithelium, but with long branched processes attached to 

 their deeper ends; mixed with these are peculiar cells, each of which 

 has a large nucleus surrounded by a little protoplasm; a slender 

 external process reaching to the surface; and a very slender deep 

 one. The latter cells have been supposed to be the proper olfac- 

 tory end organs, and to be connected with the fibers of the ol- 



FIG. 76. Cells from the ol- 

 factory epithelium. 1, from the 

 frog; 2, from man; a, columnar 

 cell, with its branched deep 

 process; 6, so-called olfactory 

 cell; c, its narrow outer process; 



tory nerve, seen dividing into 

 fine peripheral branches at a. 



