SENSE OF TOUCH. 897 



The acuteness of the sense of taste admits of education, but is by no 

 means as delicate as the sense of smell ; thus, a solution of one part of 

 sulphuric acid to one thousand of water gives its characteristic taste 

 when only one drop, which may be said to contain about ^Vs part of a 

 gramme, is placed upon the tongue. 



E. THE SENSE OF TOUCH. 



When any portion of the external integument is brought in contact 

 with a foreign body we appreciate what is termed the sensation of touch, 

 and we are, therefore, warranted in regarding the skin as a sensory organ, 

 inclosing our entire body and adapted to render every part of the body 

 sensible to external impressions. These may be of the most manifold 

 kind, and excite peculiar sensations dependent upon the nature of the 

 contact. To a limited extent this power N of tactile sensibility possessed 

 by the integument belongs likewise to the internal mucous surfaces of 

 the body, but only for a short distance from their respective orifices. 



Tactile sensations, as we shall find directly, vary among themselves, 

 and give us means of determining the physical characters of the body 

 producing the contact and the locality at which the contact takes 

 place. When such a tactile sensation is increased in intensity it may be 

 converted into a painful sensation. If, for example, as in the illustra- 

 tion given by Weber, the edge of a knife is placed on the skin we feel 

 the edge by means of the sense of touch, a sensation is perceived which 

 is referred to the object which has caused it. If, however, the skin is 

 cut by the knife pain is felt, a feeling which is no longer referred to the 

 cutting knife, but which we feel within ourselves, and which indicates to 

 us the fact of a change of condition in our own body. By the sensation 

 of pain we are neither able to recognize the object which causes it nor its 

 nature. Thus, if a body is placed within the mouth we are able to 

 recognize its general characters and to a certain extent its shape, 

 whether it be solid or liquid, hot or cold ; but if the body be swallowed 

 tactile sensation disappears as soon as the body reaches the (Esophagus, 

 and then, if any sensation be excited, it can only be a painful sensation. 

 The sensation caused by a too hot liquid cannot be distinguished from a 

 corroding acid liquid, or from the passage of an irregular hard body 

 through the oesophagus. 



The sense of touch is the simplest and is the only universal sense, 

 and exists in all members of the animal kingdom. In the articulata, 

 whether covered by horny (insects) or by calcareous (crustaceans) cover- 

 ings, the sensation of touch is possessed by all parts of the body m 

 common, while it is especially developed in the antenna? which project 

 from the sides of the head. 



In mollusks and zoophytes sensibility is much more obtuse, but is 



57 



