PAIN. COMMON SENSATIONS. 587 



(bone felon) under the periosteum of a finger bone is often 

 felt all over the forearm; and a single diseased tooth may 

 cause pain felt over the whole of that side of the face. This 

 is probably due to imperfection in brain and spinal cord of the 

 isolation of the paths of conduction of the nervous impulses 

 concerned. 



Common Sensations^ These agree with pain sensations in 

 calling attention to conditions of our Bodies and not of outer 

 things. Some of them, as general malaise and ' ' feeling well, ' ' 

 are probably due to modifications of the general inflow of im- 

 pulses through the apparatus of common sensibility, not suffi- 

 cient to cause a feeling of definite pain or pleasure. Others, 

 as hunger, thirst and nausea, may have similar origin, but in 

 a more localized region. 



Hunger and Thirst. These sensations, which regulate 

 the taking of food, are peripherally localized in consciousness, 

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

 local conditions no doubt play a part in their production; 

 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 stom- 

 ach has been empty some time, and it 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; provided these are 

 absorbed and reach the blood, their mode of entry is unessen- 

 tial ; the hunger may be stayed by injections of food into the 

 rectum as well as by putting it into 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. 



While both sensations depend in part on local peripheral 

 conditions, they may also be, and more powerfully, excited by 

 poverty of the blood in foods and water; such deficiency 

 probably directly stimulates hunger and thirst brain-centres. 



Smell. The region of the nostril nearest its outer end 

 possesses the sense of touch, and most of its lining mucous 

 membrane has common sensibility, which can be aroused by 

 such substances as ammonia vapor: the nerve-fibres concerned 

 in these feelings are derived from the superior maxillary branch 

 of the fifth nerve. 



