i8o THE FROG chap. 



are transmitted to the tactile-cells and thence through the 

 sensory nerves to the brain. Notice that the stimulus i.s 

 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 in the tongue and in the neighbour- 

 hood of the vomerine teeth, but extending also as far back 

 as the gullet. Certain of 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 tempera- 

 ture, 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 mem- 

 brane of the mouth is injured, or if the glossopharyngeal 

 and palatine nerves are cut, or if the cerebral hemispheres 

 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 sacs are lined by a 



