392 ORGANS OF THE SENSES. 



organs of touch, sight, and hearing ; and, from the initial 

 parts of the digestive and the respiratory mucous surface, we 

 have the organs of taste and smell. 



I. FEELING AND TOUCH. 



THIS is a mixed sense, for it enables us to recognize, 1, the 

 pressure exercised by different bodies upon the integuments, 

 and, 2, the temperature of these bodies. 



The organ of touch includes the whole external integu- 

 ment and a part of the mucous surface, especially of the 

 primce vice, the mucous surface of the alimentary canal (the 

 tongue and also the teeth). These organs are formed of two 

 parts essential to every integument, the epidermis or epithe- 

 lium, and the dermis / the epithelial covering, indeed, is in- 

 dispensable to touch, and if its globular elements are injured 

 or destroyed, the sense of touch ceases simultaneously. By 

 means of its growth towards the exterior, the epidermis forms 

 crests, or hollow papilla?, into which the dermis penetrates, 

 bringing with it the vessels and the nerves; we are still, 

 however, unable to explain exactly this indispensability of 

 the epidermis, since the nerves appear to terminate in the 

 dermis, and their connection with the epithelial globules are 

 still hypothetical; although it has been demonstrated in the 

 case of certain organs (as we shall see in regard to the nasal 

 chambers and the internal ear) ; it is, however, certain that 

 the more delicately formed, and highly developed is the 

 covering of the papilla?, the more exquisite is the sensibility 

 of the papilla?. Some important growths of the epidermis 

 even appear to be essentially connected with the exercise of 

 the sensation of touch : the teeth, which are extremely hard, 

 and covered with a thick layer of modified epithelium (the 

 enamel), are nevertheless the seat of the most delicate sense 

 of feeling; cats feel with the long hairs growing from their 

 mouth (see p. 431 : tactile hair) ; insects have horny feelers; 

 the sole of the foot is covered with a thick layer of corneous 

 epidermis, and yet its sensibility is exquisite. The thickness 

 of the epidermis is, therefore, no obstacle to the sensibility of 

 the skin. 



The papillas of the dermis contain the terminations of the 

 nerves; the papillse do not, however, all contain nervous 

 elements, many of them having only vascular network (Fig. 

 101, B, C, D) ; the papilla? of the dermis are more highly 

 developed in proportion to the exquisite sensibility of the 



