THE SENSES. 563 



Subjective Sensations of Taste. The sense of taste seems capable of 

 being excited only by external causes, such as changes in the conditions 

 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. 



III. Smell. 



Conditions necessary. (1.) The first conditions essential to the sense 

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

 dition are perceived in sensations of odor, for no other nervous structure 

 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 reagent 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 olfactory 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 

 quick inspiration, assisted, as in the act of sniffing, by the action of the 

 nostrils, we render the impression more intense. An odorous substance 

 in a liquid form injected into the nostrils appears incapable of giving rise 

 to the sensation of smell; thus Weber could not smell the slightest odor 

 when his nostrils were completely filled with water containing a large 

 quantity of eau-de-Cologne. 



Seat. The human organ of smell is formed by the filaments of the 

 olfactory nerves, distributed in the mucous membrane covering the 



