THE SENSES. (375 



ceru the presence of bodies in quantities so minute as to be undiscover- 

 able even by spectrum analysis; y^VW.^ of a grain of musk can be dis- 

 tinctly smelt (Valentin). Opposed to the sensation of an agreeable odor is 

 that of a disagreeable or disgusting odor, which corresponds to the serra- 

 tions of pain, dazzling and disharmony of colors, and dissonance in the 

 other senses. The cause of this difference in the effect of different odors is 

 unknown; but this much is certain, that odors are pleasant or offensive 

 in a relative sense only, for many animals pass their existence in the 

 midst of odors which to us are highly disagreeable. A great difference 

 in this respect is, indeed, observed amongst men: many odors, generally 

 thought agreeable, are to some persons intolerable; and different per- 

 sons describe differently the sensations that they severally derive from 

 the same odorous substances. There seems also to be in some persons 

 an insensibility to certain odors, comparable with that of the eye to cer- 

 tain colors; and among different persons, as great a difference in the 

 acuteness of the sense of smell as among others in the acuteness of sight. 

 We have no exact proof that a relation of harmony and disharmony exists 

 between odors as between colors and sounds; though it is probable that 

 such is the case, since it certainly is so with regard to the sense of taste ; 

 and since such a relation would account in some measure for the differ- 

 ent degrees of perceptive power in different persons; for as some have 

 no ear for music (as it is said), so others have no clear appreciation of 

 the relation of odors, and therefore little pleasure in them. 



Subjective sensations. The sensations of the olfactory nerves, inde- 

 pendent of the external application of odorous substances, have hitherto 

 been little studied. The friction of the electric machine produces a 

 smell like that of phosphorus. Ritter, too, has observed, that when a 

 galvanic current is applied to the organ of smell, besides the impulse 

 to sneeze, and the tickling sensation excited in the filaments of the fifth, 

 nerve, a smell like that of ammonia was excited by the negative pole, and 

 an acid odor by the positive pole; which ever of these sensations were pro- 

 duced, it remained constant as long as the circle was closed, and changed 

 to the other at the moment of the circle being opened. Subjective sen 

 sations occur frequently in connection with the sense of smell. Fre- 

 quently a person smells something which is not present, and which othev 

 persons cannot smell ; this is very frequent with nervous people, but it oc- 

 casionally happens to every one. In a man who was constantly conscious 

 of a bad odor, the arachnoid was found after death to be beset with 

 deposits of bone, and a lesion in the middle of the cerebral hemispheres 

 was also discovered. Dubois was acquainted with a man who, ever after 

 a fall from his horse, which occurred several years before his death, 

 believed that he smelt a bad odor, 



