1078 THE SENSES 



its acid taste in 91 of the remaining 98, sugar its sweet taste in 79, 

 and quinine its bitter taste in 71 ; 12 reacted only to tartaric acid, 

 and 3 only to sugar (Ohrwall) . Such facts indicate, although they 

 do not definitely prove, the existence of specific receptors for each 

 of the fundamental taste sensations i.e., gustatory end-organs, 

 which are easily excited by an adequate stimulus (acid, e.g., in the 

 case of an ' acid' taste-bud), with difficulty or not at all by an in- 

 adequate stimulus. 



The form of inadequate stimulation most investigated is that pro- 

 duced when a constant current is passed through the tongue. An acid 

 taste is experienced at the positive, and an alkaline or bitter taste at the 

 negative pole ; and this is the case even when the current is conducted 

 to and from the tongue by unpolarizable combinations, which prevent the 

 deposition of electrolytic products on the mucous membrane (p. 731). 

 The sensations are due to stimulation of the gustatory end-organs and 

 not of the nerve-trunks. 



Normal lymph, which bathes these end-organs, does not excite any 

 sensation of taste, but when the composition of the blood is altered in 

 disease or by the introduction of foreign substances, tastes of various 

 kinds may be perceived. Sometimes this may be due to the stimula- 

 tion of substances excreted in the saliva; but in other cases it seems 

 that, without passing beyond the blood and lymph, foreign substances 

 may excite the gustatory nerves. 



Flavour embraces a group of mixed sensations in which smell and 

 taste are both concerned, as is shown by the common observation that 

 a person suffering from a cold in the head, which blunts his sense of 

 smell, loses the proper flavour of his food, and that some nauseous 

 medicines do not taste so badly when the nostrils are held. 



In common speech, the two sensations are frequently confounded 

 with each other and with tactile sensations. Thus the ' bouquet ' of 

 wines, which most people imagine to be a sensation of taste, is in 

 reality a sensation of smell; the astringent ' taste ' of tannic acid is not 

 a taste at all, but a tactile sensation ; the ' hot ' taste of mustard is no 

 more a true sensation of taste than the sensation produced by the same 

 substance when applied in the form of a mustard poultice to the skin. 



As already remarked, the substances which affect the olfactory end- 

 organs in air-breathing animals, like those which affect the gustatory 

 end-organs, must eventually go into solution before causing stimulation. 

 The most striking distinction between the two senses is the astonishingly 

 small concentration in which substances can elicit sensations of smell, 

 as compared with sensations of taste. Thus ethyl alcohol is a stimulus 

 for both smell and taste, but it can be recognized by smell in a dilution 

 24,000 times greater than the dilution necessary for taste (Parker). 



SECTION IV. CUTANEOUS AND INTERNAL SENSATIONS. 

 Under the sense of touch it was at one time usual to include a 

 group of sensations which differ in quality and that in some in- 

 stances to as great an extent as any of the sensations which are 

 universally considered as separate and distinct but agree in this, 

 that the end-organs by which they are perceived are all situated in 

 the skin, the mucous membranes, or the subcutaneous tissue. 

 They are more correctly designated ' cutaneous sensations.' Such 

 are the common tactile sensations including pressure, tickling, 

 and itching and the sensations of temperature, or, more correctly, 



