THE SENSE OF TOUCH. 707 



fundamental principles of harmony have been employed 

 empirically while the theoretical laws were unknown. 



Frequent and continued repetitions of the same taste 

 render the perception of it less and less distinct, in the 

 same way that a colour becomes more and more dull and 

 indistinct the longer the eye is fixed upon it. Thus, after 

 frequently tasting first one and then the other of two kinds 

 of wine, it becomes impossible to discriminate between 

 them. 



The simple contact of a sapid substance with the sur- 

 face of the gustatory organ seldom gives rise to a distinct 

 sensation of taste ; it needs to be diffused over the surface, 

 and brought into intimate contact with the sensitive parts 

 by compression, friction, and motion between the tongue 

 and palate. 



The sense of taste seems capable of being excited also 

 by internal causes, such as changes in the conditions of 

 the nerves or nerve-centres, produced by congestion or 

 other causes, which excite subjective sensations in the 

 other organs of sense. But little is known of the sub- 

 jective sensations of taste; for it is difficult to distin- 

 guish the phenomena from the effects of external causes, 

 such as changes in the nature of the secretions of the 

 mouth. 



SENSE OF TOUCH. 



The sense of touch is not confined to particular parts of 

 the body of small extent, like the other senses ; on the 

 contrary, all parts capable of perceiving the presence of 

 a stimulus by ordinary sensation are, in certain degrees, 

 the seat of this sense ; for touch is simply a modification 

 or exaltation of common sensation or sensibility. The 

 nerves on which the sense of touch depends are, therefore, 

 the same as those which confer ordinary sensation on the 

 different parts of the body, viz., those derived from the 

 posterior roots of the nerves of the spinal cord, and the 

 sensitive cerebral nerves. 



