238 Life and Health 



Such information is supplied by the general sensations. 

 Thus we have a feeling of hunger or thirst, indicating the 

 need of food, and a feeling of discomfort when imperfectly 

 clad, warning us of the need of more clothing. To these 

 may be added the sensations of pain, tickling, and itching. 



The great majority of sensations result from some outward 

 stimulus or agency ; and yet some sensations, such as those 

 of faintness, restlessness, and fatigue, seem to spring up 

 within us in some mysterious way, without any obvious 

 cause. 



366. Essentials of a Sense Organ. Certain conditions are 

 necessary for a sensation. First, there is a special structure 

 adapted to a particular kind of influence. Thus the ear is 

 formed specially for being stimulated by the waves of sound, 

 while the eye is not influenced by sound, but responds to 

 the action of light. These special structures are called 

 terminal organs. 



Again, a nerve proceeds from the special structure, and provides 

 direct communication with nerve cells in the brain in the region of 

 consciousness. This last point is important to remember, 

 for if for any reason the impression is arrested in the 

 connecting nerve, no sensation will result. Thus a man 

 whose spine has been injured may not feel a severe pinch 

 on either leg. The impression may be sufficient to produce 

 as marked a reflex act as in a person with a healthy cord, 

 but the man thus hurt does not feel the pinch, because 

 the injury has prevented the impression from being carried 

 up the cord to the higher centers in the brain. 

 J 367. The Condition of Sensation. It is thus evident that 

 while an impression may be made upon a terminal organ, it 

 cannot strictly be called a sensation until the person becomes 

 conscious of it. The consciousness of an impression is, therefore, 

 the essential element of a sensation. 



