ODOURS. 161 



beyond the cranium, in order that it may there receive, as at first hand, 

 the impressions of which the mind is to become cognizant. 



Besides the first pair of nerves, the pituitary membrane receives 

 several branches from the fifth encephalic pair ; for example, the nasal 

 twig of the ophthalmic branch of the fifth, and filaments from the frontal 

 branch of the same ; from the spheno-palatine ganglion ; the palatine 

 nerve ; the vidian nerve ; and from the anterior dental branch of the 

 superior maxillary. One of these twigs enters the anterior naso-pala- 

 tine canal ; and, in its course to the roof of the mouth, passes through 

 a small ganglion, which has been described by M. H. Cloquet under the 

 name naso-palatine, and w r hich he conceives to be the organ of sympathy 

 between the senses of smell and taste. 



The pituitary membrane is kept moist by nasal mucus, as well as by 

 the exhalation that constantly takes place from it. It receives the 

 superfluous tears by means of the ductus ad nasum, a duct passing 

 from the inner canthus of the eye, and opening into the nasal fossae 

 below the lower spongy bone. The constant evaporation which must 

 take place from the membrane, owing to the passage of the air during 

 respiration, requires that the secretion should be continuous and copious, 

 otherwise the membrane would become dry. 



The nasal fossae communicate externally by means of the nostrils, 

 the shape, size, and direction of which vary, so as to give rise to the 

 aquiline, Roman, pug, and other varieties of nose. At the extremity 

 of the nostrils long hairs are situate technically called vibrissde whose 

 function, it is conceived, may be to sift, as it were, the air passing 

 through during respiration, and thus prevent extraneous bodies from 

 entering the fossae. The nostrils are also capable of being expanded 

 or contracted by appropriate muscles. 



In this sense, there is a more clear separation between the physical 

 and nervous part of the apparatus than in either of those already con- 

 sidered; the nose proper forming the physical portion; and the nerves 

 of smell the organic or nervous. 



2. ODOURS. 



The comprehension of the physiology of olfaction will not be complete 

 without an inquiry into odours, or those emanations from odorous bodies, 

 that give them their character, and impress the organ of smell. 



It was long maintained, as in the case of savours, that odours are 

 dependent upon a peculiar principle, which, according to its particular 

 combination with the constituents of bodies, gives rise to various odours. 

 To this principle the terms aroma and spiritus rector have been assigned; 

 but the notion has been long abandoned, because no general or common 

 characters are observable amongst odorous bodies, which should be 

 expected were they indebted for their odour to the same principle. 

 Walther, a German physiologist, expresses the opinion, that an odorous 

 body is such by virtue of a vibratory motion, analogous to that made 

 by a sonorous body. We have, however, the most satisfactory evidence, 

 that there are special odours, as there are special savoury molecules. 

 We can prevent an odorous body from impressing our olfactory nerves 

 by covering it with a glass receiver. Odours can be separated by in- 



VOL. I. 11 



