XI 



NOSE AND EYE 



181 



sacs are lined by a delicate mucous membrane, some of 

 the epithelial cells of which are of the ordinary columnar 

 type, while others are extremely slender and produced 

 into delicate processes at their free ends. With these 

 latter the fibres of the olfactory nerves are connected, 

 and they are distinguished as olfactory cells (Fig. 56). 

 As the epithelial cells of the skin are 

 affected by direct contact or by heat, so 

 the olfactory cells are affected by the 

 minute particles given off from odorous 

 bodies : the contact of these particles 

 acts as a stimulus, which, being trans- 

 mitted by the olfactory nerves to the 

 brain, gives rise to the sense of smell. 

 This sense can be destroyed as in the 

 case of feeling and taste- either by 

 destruction of the olfactory mucous 

 membrane or by cutting the olfactory 

 nerves, or by destroying the brain. 



The organ of sight or eye of the frog 

 is so similar in structure to that of man 

 that the reader may be referred for 

 details both of structure and of function 

 to the text-books of Physiology, and it 

 will only be necessary to give a brief 

 outline here. 



Each eye (Fig. 57) is a nearly glo- 

 bular organ, and when removed from 

 the orbit and cleaned by dissecting away the attached 

 muscles, etc., two regions can be distinguished in it an 

 opaque portion of a dark bluish colour which forms some 

 two-thirds of the entire globe and is hidden within the 

 orbit in the entire animal ; and a clear, transparent por- 

 tion which is directed outwards and freely exposed 

 between the eyelids in the living frog. The outer coat 



FIG. 56. Epithelial 

 cells of the olfac- 

 tory mucous mem- 

 brane of an Am- 

 phibian, highly 

 magnified. 



E. interstitial 

 cells ; R. olfactory 

 cells. (From \Vie- 

 dersheim's Comp. 

 Anatomy.) 



