370 THE SENSORIAL FUNCTIONS. 



are insensible to many causes of mechanical 

 irritation, such as cutting, pricking, and even 

 burning : but the moment they are violently 

 stretched, that being the mode in which they 

 are most liable to be injured, they instantly 

 communicate a feeling of acute pain. The 

 bones, in like manner, scarcely ever communi- 

 cate pain in the healthy state, except from the 

 application of a mechanical force which tends 

 to fracture them. 



The system of nerves, comprising those which 

 are designed to convey the impressions of touch, 

 is universally present in all classes of animals ; 

 and among the lowest orders, they appear to con- 

 stitute the sole medium of communication with 

 the external world. As we rise in the scale of 

 animals we find the faculties of perception ex- 

 tending to a wider range, and many qualities, 

 depending on the chemical action of bodies, are 

 rendered sensible, more especially those which 

 belong to the substances employed as food. 

 Hence arises the sense of taste, which may be 

 regarded as a new and more refined species of 

 touch. This difference in the nature of the im- 

 pressions to be conveyed, renders it necessary 

 that the structure of the nerves, or at least of 

 those parts of the nerves which are to receive 

 the impression, should be modified and adapted 

 to this particular mode of action. 



I 



