236 HUMAN PHYSIOLOGY. 



The nerves of taste in the superficial layer of the mucous membrane 

 form a fine plexus, from which branches pass to the epithelium and 

 penetrate it ; others enter the taste-beakers, and are directly con- 

 nected with the taste-cells. 



The seat of the sense of taste has been shown by experiment to 

 be the whole of the mucous membrane over the dorsum of the tongue, 

 soft palate, fauces, and upper part of the pharynx. 



The sense of taste enables us to distinguish the savor of sub- 

 stances introduced into the mouth, which faculty is different from 

 tactile sensibility. The sapid qualities of substances appreciated by 

 the tongue are designated as bitter, sweet, alkaline, sour, salt, etc. 



The essential conditions for the production of the impressions of 

 taste are : 



1. A state of solubility of the food. 



2. A free secretion of the saliva, and 



3. Active movements on the part of the tongue, exciting pressure 

 against the roof of the mouth, gums, etc., thus aiding the solution 

 of various articles and their osmosis into the lingual papillae. 

 Sapid substances, when in a state of solution, pass into the interior 



of the taste-beakers, and come into contact, through the medium of 

 the taste-cells, with the terminal filaments of the gustatory nerves. 



THE SENSE OF SMELL. 



The sense of smell is located in the mucous membrane lining the 

 upper part of the nasal cavity, in which the olfactory nerves are 

 distributed. 



The nasal fossae are two cavities, irregular in shape, separated by 

 the vomer, the perpendicular plate of the ethmoid bone, and the tri- 

 angular cartilage. They open anteriorly and posteriorly by the an- 

 terior and posterior nares, the latter communicating with the pharynx. 

 They are lined by mucous membrane, of which the only portion 

 capable of receiving odorous impressions is the part lining the upper 

 one third of the fossae. 



The olfactory nerves, the olfactory bulb and tracts, unite the ol- 

 factory epithelium with the cortical areas of smell in the cerebrum. 



In animals which possess an acute sense of smell there is a cor- 

 responding increase in the development of the olfactory bulbs. 



