180 THE FROG CHAP. 



touch or by the heat of the sun, are transmitted to the 

 tactile cells and thence through the sensory nerves to 

 the brain. Notice that the stimulus is transmitted to 

 the nerve-ends through the epithelial cells of the skin ; 

 if the skin be wounded and a stimulus applied directly 

 to the tactile cells or the nerves, the sensation is one, 

 not of touch, but of pain. 



The sense of taste is lodged in the mucous membrane 

 of the mouth, especially on the tongue and in the neigh- 

 bourhood of the vomerine teeth, but extending also as 

 far back as the gullet. Certain ot the epithelial cells 

 have an elongated form and are arranged in groups 

 known as taste-buds, to which the fibres of the ninth and 

 palatine branch of the seventh cerebral nerves, or nerves 

 of taste, are distributed ; on the tongue these taste- 

 buds are situated on papilla of the mucous membrane. 

 In this case the stimulus is supplied, not by direct touch 

 or by alteration of temperature, but by the contact of 

 sapid or tasty substances. As before, the stimulus is 

 applied to epithelial cells, and by them transmitted to the 

 nerves and so to the brain, when the sensation of taste 

 becomes manifest. Thus, just as common sensation may 

 be abolished, in any part of the body, in three ways 

 by destruction of the skin, by cutting the sensory 

 nerve, or by destroying the cerebral hemispheres so 

 the sense of taste is lost if either the mucous membrane 

 of the mouth is injured or if the glossopharyngeal and 

 palatine nerves are cut, or, again, if the cerebral hemi- 

 spheres are destroyed. 



The sense of smell is lodged in the nasal or olfactory 

 sacs, which are enclosed in the olfactory capsules of the 

 skull and separated from one another by a partition, the 

 nasal septum. Each sac has two apertures, the external 

 nostril, opening on the surface of the snout, and the 

 internal nostril, opening into the mouth (p. 17). The 



