THE SENSES. 175 



Thus, after frequently tasting first one and then the other of two kinds 

 of wine, it becomes impossible to discriminate between them. 



The simple contact of a sapid substance with the surface of the gustatory 

 organ seldom gives rise to a distinct sensation of taste; it needs to be dif- 

 fused over the surface, and brought into intimate contact with the sensi- 

 tive parts by compression, friction, and motion between the tongue and 

 palate. 



Subjective Sensations of Taste. The sense of taste seems capa- 

 ble of being excited only by external causes, such as changes in the con- 

 ditions of the nerves or nerve-centres, produced by congestion or other 

 causes, which excite subjective sensations in the other organs of sense. 

 But little is known of the subjective sensations of taste; for it is difficult 

 to distinguish the phenomena from the effects of external causes, such 

 as changes in the nature of the secretions of the mouth. 



SMELL. 



Conditions Necessary. (1.) The first conditions essential to the 

 sense of smell are a special nerve and nerve-centre, the changes in whose 

 condition are perceived in sensations of odor; for no other nervous struc- 

 ture is capable of these sensations, even though acted on by the same 

 causes. The same substance which excites the sensation of smell in the 

 olfactory centre may cause another peculiar sensation through the nerves 

 of taste, and may produce an irritating and burning sensation on the nerves 

 of touch; but the sensation of odor is yet separate and distinct from these, 

 though it may be simultaneously perceived. (2.) The second condition 

 of smell is a peculiar change produced in the olfactory nerve and 

 its centre by the stimulus or odorous substance. (3.) The material 

 causes of odors are, usually, in the case of animals living in the air, 

 either solids suspended in a state of extremely fine division in the 

 atmosphere; or gaseous exhalations often of so subtile a nature that they 

 can be detected by no other re-agent than the sense of smell itself. The 

 matters of odor must, in all cases, be dissolved in the mucus of the mucous 

 membrane before they can be immediately applied to, or affect the olfac- 

 tory nerves; therefore a further condition necessary for the perception of 

 odors is, that the mucous membrane of the nasal cavity be moist. When 

 the Schneiderian membrane is dry, the sense of smell is impaired or lost; in 

 the first stage of catarrh, when the -secretion of mucus within the nostrils 

 is lessened, the faculty of perceiving odor is either lost or rendered very 

 imperfect. (4.) In animals living in the air, it is also requisite that the 

 odorous matter should be transmitted in a current through the nostrils. 

 This is effected by an inspiratory movement, the mouth being closed; 

 hence we have voluntary influence over the sense of smell; for by inter- 

 rupting respiration we prevent the perception of odors, and by repeated 



