CHAPTER XVII 

 RECEPTOR ORGANS 



ORGANISMS of all kinds maintain their existence by adapting themselves to changes 

 in the world of things surrounding them. They depend upon it for their food 

 and must avoid, or defend themselves from, other organisms which want them 

 for food. 



As we have seen, nerve fibres themselves can be excited by various kinds 

 of stimuli, chemical, electrical, mechanical, and so on. But it will be obvious 

 that an organism is the better equipped the more delicate its sensibility to 

 the actions of the outer world. Nerve fibres themselves, in fact, require rather 

 powerful stimuli to excite them. Contrast, for example, the force required to 

 stimulate a delicate organ of touch, such as the human skin, with that necessary 

 to stimulate the trunk of the sciatic nerve of the frog. 



FIG. 159. FREE NERVE ENDINGS. 



Intra-epithelial nerve terminations in the larynx. Gplgi method. On the left, stratified epithelium. On the 

 right, ciliated columnar epithelium, w, Nerve fibres in the corium. 



(After Retzius.) 



It is natural to suppose that stimuli of an injurious nature would be amongst 

 the first to be responded to in the course of evolution. We noticed that there 

 is reason to believe that free nerve endings are sufficient for the appreciation 

 of these stimuli, since it is not desirable that a stimulus, too weak to be harmful, 

 should evoke a reaction of flight. In fact, we actually find that free nerve 

 endings are present in the skin (Fig. 159). 



THE CHEMICAL SENSE 



The earliest animals arose in the sea, and would naturally be exposed to a 

 great variety of chemical substances dissolved therein. Parker (1912) finds that 

 the skin of fish is sensitive to acid, alkali, and salts, also to quinine. It appears 

 that it is the hydrogen and hydroxyl ion concentration of the two first which are 

 the active properties. It requires a considerably higher concentration of sodium 

 chloride than of hydrochloric acid to stimulate these receptors, although, in the 

 concentrations used, both would be almost completely dissociated, and the 

 number of chlorine ions would be proportional to the concentration. Similarly, 

 sodium hydroxide is more powerful than sodium chloride. 



We may also take note that the concentration of acid or alkali required, about 0'05 molar, 

 is very much higher than that to which the differentiated receptors in the human tongue are 

 sensitive. This fact suggests that it is free nerve endings that are stimulated in the fish by 



