114 



PHYSIOLOGY. 



upon the tactile surface, we discover the roughness or smooth- 

 ness of the substance. The knowledge of form and iveight is 

 derived by the additional assistance of the muscular sense. 



The sense of touch is exceedingly educable, as is seen in the case 

 of the blind, who can be taught to read, and even distinguish colours 

 by its agency. 



Impressions made upon the organ of touch continue perceptible 

 for some time after the stimulus has been removed; for instance, the 

 stinging of a smart blow does not soon subside, and ihe simple con- 

 tact of an article of clothing often leaves the impression of its presence 

 after it has been removed. The subjective sensations pertaining to 

 the nerves of this sense are among the best known. The tingling 

 of a limb that is " asleep^'' which commonly depends on pressure 



on its trunk, may also result 

 Fig. 31.* from changes in the centre ; 



the same is true of formica- 

 tion, heat, chilliness, itching, 

 and also of pain. 



SENSE OF TASTE. 



In the sense of taste, as well 

 as in touch, the substance to 

 be examined must be brought 

 into contact with the organ of 

 the sense, which organ is the 

 mucous membrane of the 

 tongue and fauces. The mu- 

 cous membrane of the tongue 

 is largely supplied with papillae 

 of various forms, which are 

 abundantly supplied with 

 nerves and blood-vessels. The 

 papillse are of three varieties ; 

 1st. The calycifortn, or cir- 

 cumvaUate^ situated at the 

 base of the tongue in a V 

 shaped line, a (Fig. 31); the 

 fungiform, on the sides and 

 apex, b ; and the conical, or 

 filiform, the most numerous, 

 and most abundant, in the cen- 

 tral part, d. The latter are supposed to be concerned rather in the 

 sense of touch than of taste. When these papillse are called into 



* Upper surface of tongue, a. One of the circumvallate papillae ; b. one of the fungiform ; 

 d. conical papilla'.^ 



