438 



THE SPECIAL SENSES 



required by any given fiber. This relation of amount of 

 stimulus to sensation is one of the factors which determine 

 the sensitiveness of a sense center. The tips of the fin- 

 gers, which require much less increase in stimulus to pro- 

 duce sensation than the skin of the back requires, are said 

 to be more sensitive than the latter. 



Classification of sense organs. — The end of every affer- 

 ent nerve is a sense organ, but for convenience these end 

 organs are grouped under various heads according to the 

 region where they end and the quality of their impulses. 

 The following scheme presents the commonly accepted 

 classification : 



CLASSES OF SENSE ORGANS. 



Name of Group 



Cutaneous 



Internal . 



Gustatory- 

 Olfactory 

 Visual . . 

 Auditory . 



Location of End Organs 



Skin and mucous membrane J 

 of the mouth 1 



Internal membranes and I 

 muscles ' 



Tongue 



Nasal passages 



Eyes 



Ears 



Quality of Impulse 



Pressure 



Warmth 



Cold 



Pain 



Pain 



Muscle sense 



Hunger and thirst 



Taste 



Smell 



Sight 



Hearing 



Cutaneous sensations. — It was formerly customary to 

 group all the sensations which were produced by the 

 impulses from the sensory nerve endings in the skin as 

 touch sensations. Modern exj^eriments have shown that 

 touch sensations show four distinct qualities, namely, pres- 

 sure, warmth, cold, and pain, and that this difference in 



