444 THE SPECIAL, SEN^SES 



which supply the pain spots have no distinctive end organs so far 

 as known. (d) They may be stimulated by any of the usual 

 forms of cutaneous stimuli, such as temperature or pressure, pro- 

 vided these st muli are of sufficient intensity, (e) The location 

 of a pain stimulus by the brain is aided by the pressure and tem- 

 perature stimuli, parts which have lost these centers still retain- 

 ing the sense of pain but the subject has difficulty in locating 

 the seat of stimulation. 



Internal Sensations. 



The membranes of the internal organs, such as the 

 mucous membranes that line the alimentary tract and the 

 lungs and the membranes that invest the muscles or 

 cavities of the body, have no pressure, warmth, or cold 

 fibers. Increase in internal temperature or pressure can 

 be detected, therefore, onlj' by its effect upon the cutan- 

 eous fibers. These membranes are, however, supplied with 

 pain fibers and other fibers whose function it is to supply 

 the brain with information as to the health and needs of 

 these parts. Very little is known of these latter fibers or 

 their end organs, their method of distribution or stimula- 

 tion. The sensations of hunger, thirst, general health- 

 fulness or weakness are all produced b}'' impulses passing 

 over these fibers, and their fimction as indicators of the 

 condition of om- internal organs is extremely important. 

 At present, we know only of the presence of these fibers 

 and admit our ignorance of their end organs or properties. 



Gustatory Sensations. 



Distribution of fibers and end organs. — The sensation of 

 taste is produced by impulses sent over special nerve 

 fibers whose ends are distributed over various parts of 

 the mouth cavity and particularly in certain regions of 



