518 THE NERVOUS SYSTEM. 



So far as we can judge from the results of experiment, they are not 

 capable of receiving or transmitting any other sensations than those 

 excited by odoriferous substances. 



II. The second set of nerves distributed to the nasal passages con- 

 sists of the nasal branch of the fifth pair, and its ramifications (Fig. 

 129, 2 ). This nerve, after entering the cavity of the nose a little in 

 advance of the cribriform plate of the ethmoid bone, is distributed 

 mainly to the mucous membrane covering the inferior turbinated bone 

 and lining the inferior meatus, which it supplies with general sensi- 

 bility. Some of its filaments are also continued into the mucous mem- 

 brane of the olfactory region, in proximity with those of the olfactory 

 nerves ; and this region, according to the observations of Babuchin,* 

 possesses consequently a certain amount of general sensibility, though 

 much less than the remainder of the nasal passages. 



III. The third set of nerves are derived from the spheno-palatine 

 ganglion of the sympathetic (Fig. 129, 3 ), which supply the mucous 

 membrane of the posterior part of the nasal passages and the muscles 

 of the posterior nares. Finally, the muscles of the anterior nares are 

 supplied by filaments of the facial nerve. 



Necessary Conditions of the Sense of Smell. In order to produce 

 an olfactory impression, the emanations of the odoriferous body must 

 be drawn freely through the nasal passages. As the olfactory mem- 

 brane is situated only in the upper part of these passages, whenever a 

 faint or delicate odor is to be perceived, the air is forcibly directed 

 toward the superior turbinated bones, by a peculiar inspiratory move- 

 ment of the nostrils, very marked in many of the lower animals. As 

 the odoriferous vapors arrive in the upper part of the nasal passages, 

 they are probably dissolved in the secretions of the olfactory mem- 

 brane, and thus brought into relation with its nerves. Inflammatory 

 disorders interfere with the sense of smell, both by altering the secre- 

 tions of the part, and by causing tumefaction of the mucous membrane 

 and obstruction of the nasal passages. 



A distinction is to be made between true odors and the excitement 

 of the general sensibility of the riasal membrane by irritating sub- 

 stances. Some of the odors are similar in their nature to impressions 

 of taste. Thus there are sweet and sour smells, though none corre- 

 sponding to the alkaline or the bitter tastes. Most odors, however, are 

 of a peculiar nature and difficult to describe ; but they are always dis- 

 tinct from the simply irritating properties which belong to certain 

 vapors and gases. Thus, pure alcohol is principally a stimulant to the 

 mucous membrane ; but wines have in addition odoriferous qualities, 

 due to ingredients of vegetable origin. 



The vapor of pure acetic acid is simply irritating ; while vinegar has 

 also a peculiar odor, derived from its vegetable constituents. Ammonia 

 is an irritating gas, but contains no proper odoriferous principle. 



* Strieker's Manual of Histology, Buck's Edition. New York, 1872, p. 799. 



