SMELL. 569= 



Hunger and Thirst. These sensations, which regulate 

 the taking of food, are peripherally localized in conscious- 

 ness, the former in the stomach and the latter in the throat, 

 and local conditions no doubt play a part in their produc- 

 tion; though general states of the Body are also concerned. 

 Hunger in its first stages is probably due to a condition 

 of the gastric mucous membrane which comes on when the 

 stomach has been empty some time, and may be temporarily 

 stilled by filling the organ with indigestible substances. 

 But soon the feeling comes back intensified and can only 

 be allayed by the ingestion of nutritive substances; pro- 

 vided these are absorbed and reach the blood, their mode of 

 entry is unessential; the hunger may be stayed by injec- 

 tions of food into the rectum as well as by putting it inta 

 the stomach. 



Similarly, thirst may be temporarily relieved by moisten- 

 ing the throat without swallowing, but then soon returns;, 

 while it may be permanently relieved by water injections 

 into the veins, without wetting the throat at all. 



While both sensations thus depend in part on local 

 peripheral conditions of afferent nerves (pneumogastrio 

 and glossopharyngeal), they may be also, and more power- 

 fully, excited by poverty of the blood in foods and water; 

 this probably directly stimulates the hunger and thirst 

 brain-centres. 



Smell, The olfactory organ consists of the upper por- 

 tions 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 olfactorid) covers the upper and lower turbinate 

 bones (o, p, Fig. 89*), which are expansions of the ethmoid 

 (p. 75) on the outer wall of the nostril chamber, the oppo- 

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

 the roof of the nose (n, Fig. 89) separating it from the 

 cranial cavity. The epithelium covering the mucous mem- 

 brane contains two varieties of cells arranged in several 

 layers (2, Fig. 154). Some cells are much like ordinary 

 columnar epithelium but with long branched processes 

 attached to their deeper ends; the others have a large 



* P. 309. 



