260 NUTRITIONAL PHYSIOLOGY 



by the worn places on the floors and thresholds and by the 

 grimy edges of the doors just where the tenants came and 

 went most often. The lifeless brain must bear a more 

 subtle registry of the same order. 



Afferent fibers reach the brain from all parts of the body. 

 Many of these have had their origin within its tissues, 

 where they are normally stimulated by conditions that 

 belong to the organism itself rather than to its environ- 

 ment. The impulses that enter the brain from such 

 sources are most of the time serving their purpose in pro- 

 moting subconscious adaptive reflexes. When they affect 

 consciousness it is to bring to the attention the so-called 

 "general sensations" those feelings which we refer to 

 states of the organs. Such are hunger and thirst, many 

 kinds of pain, satiety, nausea, faintness, fatigue, and the 

 like. The majority of these general sensations seem to 

 signify conditions that need to be rectified and they are 

 mostly unpleasant. 



Contrasted with these are the "special sensations," 

 which are referred to causes acting upon the afferent ap- 

 paratus from outside. Various terminal structures are 

 developed at or near the surface of the body which serve 

 to transmute different forms of environmental energy 

 into nerve-impulses. Such structures are called sense 

 organs or "receptors." A very suggestive distinction has 

 been made between the "proprio-receptors," which are 

 affected only by the literal contact of the stimulating sub- 

 stance, and the "distance-receptors," which respond to 

 forms of energy radiating from places more or less remote. 

 The nerve-endings in the skin which are acted upon by 

 pressure and by temperature changes are proprio-recep- 

 tors. So are those in the tongue on which various dis- 

 solved substances take effect, giving the conscious sub- 

 ject sensations of taste. The olfactory endings high up in 

 the nasal cavities are reckoned by most writers to be in the 

 same class. 



The ear is a distance receptor. The energy which sets 

 its intricate mechanism into vibration may have originated 

 as far away as the thunder-cloud hanging near the horizon. 



