196 RECEPTORS 



the same order as the pressure spots. For example, the minimum 

 perceptible difference of temperature in degrees centigrade is given 

 in the succeeding list for various regions : 



TABLE XXXI. 



Back - 0-9 



Leg - 0-6-0-2 



Thigh 0-5 



Back of foot 0-4 



Cheek 0-4 



Temple - 0-3 



Palm of hand - 0-4 



Back of hand - - 0-3 



Arm - - 0-2 



Taste and smell are the chemical senses (partly chemical and 

 partly physical) and are closely allied to touch. To stimulate the 

 end organs of chemical sense, the substance must be in a fine state 

 of division and capable of going into solution in the fluid on the 

 superficies of the sense organ. In spite of much research, little 

 more can be added to this brief statement. Of the two senses, 

 taste is the more limited as well as the less useful. Four kinds 

 of taste may be discriminated, viz. sweet, salt, acid and bitter. 

 Flavours are odours and really give rise to an olfactory sensation. 

 Smell is the ancestral chemical sense and may be classed, especially 

 in the lower animals, as a distance receptor. In civilised man, 

 this sense, unless rendered acute by training, is merely vestigial. 



The areas of nasal mucosa associated with this perceptive 

 mechanism, are small rectangular strips in the upper part of each 

 nasal cavity, just above the superior turbinate bone. In ordinary 

 respiration, air does not pass directly over the olfactory mucous 

 membrane, but some air diffuses backways through the posterior 

 nares (Fig. 42). This is important for the preservation of the 

 sense. The receptor neurons have retained their primitive con- 

 dition of cell body in the epithelium itself (Parker). They are 

 rapidly fatigued and readily destroyed. Now, by their situation 

 in a backwater they do not come directly into contact with high 

 concentrations of odoriferous substances and, furthermore, air 

 attains body temperature and moisture and is freed from suspended 

 particles (dust, bacteria, etc.) before reaching the sensory surface. 

 The physical details of the mechanism for the perception of smell, 

 that is, for the conversion of chemical into nervous energy, have 

 not yet been brought to light. The sense is extraordinarily 

 delicate. Mercaptan, in as low a concentration as 0*0000000004 



