242 THE SPECIAL SENSES 



of the body. Taste depends upon the concentration of the 

 solution. There is evidence that the sensation may be divided 

 into four primary ones bitter, sweet, sour, and salt, with 

 special nerves and end organs for each. Thus, the tip of the 

 tongue perceives acids acutely, sweets less, and bitter sub- 

 stances hardly at all. Saccharin appears sweet at the tip of the 

 tongue and bitter at the base. The fungiform papillae scattered 

 over the surface of the tongue were tested with succinic acid, 

 quinine, and sugar. Out of 125, 27 did not respond at all, 

 showing that they were devoid of taste endings; 12 reacted to 

 succinic acid alone; 3 to sugar alone; while quinine affected them 

 all. An extract of a tropical . plant has been found to paralyze 

 the sense endings for sweet and bitter substances. Cocaine 

 abolishes the sensibility of the tongue in the following order: 

 (1) general feeling; (2) bitter taste; (3) sweet taste; (4) salt 

 taste; (5) acid taste; (6) tactile perception. 



The tongue has a highly developed sense of touch, tempera- 

 ture, pressure, and pain, which aid the accuracy of speech, 

 mastication, and deglutition. Some aromatic substances leave 

 an impression of their taste, called an after-taste, and when 

 tasted in rapid succession a number of times the appreciation 

 of their flavor is lost. 



Cutaneous Sensations. A specialized peripheral organ 

 for the reception of an external impression, an afferent nerve, 

 and the brain for the perception of the sensation, constitute 

 an organ for sensation. It is by means of impressions so 

 received and conducted to it that the mind is able to control 

 the body, and to take cognizance of the external world. Cuta- 

 neous sensations include the sense of touch, temperature, and 

 pain. Touch is due to sensory nerve endings in the skin and 

 mucous membrane. The nails and teeth are peculiarly in- 

 volved in the sense of touch, and also the hair in certain regions, 

 c. g., the eyelashes. The relation between the strength of the 

 stimulus and the resulting sensation is expressed by Weber's 

 law: " The amount of stimulus necessary to provoke a perceptible 

 increase of sensation always bears the same ratio to the amount 

 of stimulus already applied." This law is only approximately 

 correct for small and for large weights. Fechner's " psycho- 

 physical" law attempts to express the relation more exactly: 

 " The intensity of sensation varies with the logarithm of the 



