NOSE AND EVE 



i8i 



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 

 nerve are connected, and they are distinguished as olfactory 

 cells (Fig. 56). As the epithelial cells of the skin are af- 

 fected 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 nerve 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 de- 

 stroying 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 (compare Fig. 57) is a nearly 

 globular organ, and when removed from 

 the orbit and cleaned by dissecting away 

 the attached muscles, &c., 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, some- 

 what flattened portion which is directed outwards and freely 

 exposed between the eyelids in the living frog. The outer 



Fig. 56.— Epithelial 

 ceils of the olfac- 

 tory mucous mem- 

 brane of an Am- 

 phibian. 



E. interstitial 

 cells ; R. olfactory 

 cells. (FromWie- 

 dersheim's Verte- 

 brata.) 



