CHAPTER V. 



THE COMMON" CHEMICAL, SENSE. 



Contents. 1. Common Chemical Sense in Man. 2. In 

 Lower Vertebrates. 3. Nerve Terminals. 4. Relation 

 to Other Senses. 5. Bibliography. 



1. COMMON Chemical Sense in Man. It was long 

 ago made clear by Frohlich that on the nasal surfaces 

 of man there were two systems of receptors that could 

 be stimulated by gaseous or vaporous materials : olfac- 

 tory cells representing the olfactory nerve, whose stim- 

 uli, delicate perfumes and odors, call forth few observable 

 responses, and free-nerve terminals probably represent- 

 ing the trigeminal nerve, whose stimuli, irritants for the 

 most part, are usually followed by vigorous reactions 

 such as sneezing. This distinction has been generally 

 accepted among physiologists, but it has not been so 

 clearly seen that the receptors for irritants are found in 

 other parts of the body than the nose and that they rep- 

 resent a fairly well defined category of sense organs 

 which, if not so sharply marked off as those of taste and 

 of smell, are fairly comparable in distinctness with the 

 receptors for heat, cold, or pain. The extent of their 

 occurrence is easily recognized. Thus the vapor of 

 ammonia not only irritates the nose, but also the eye, 

 causing watering, as well as the mouth and the upper 

 respiratory region whence arise impulses that lead to 

 coughing and choking. Irritants of this kind also stim- 

 ulate the anus and the genital apertures and in fact any 

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