490 THE SPECIAL SENSES. 



nature and endowments. The nasal passages, however, are supplied 

 with other nerves beside the olfactory. The nasal branch of the 

 ophthalmic division of the fifth pair, after entering the anterior part 

 of the cavity of the nares, just in advance of the cribriform plate of 

 the ethmoid bone, is distributed to the mucous membrane of the in- 

 ferior turbinated bone and the inferior meatus. Thus the organ 

 of smell is provided with sensitive nerves from two different sources, 

 viz., at its upper part, with the olfactory nerves proper, derived 

 from the olfactory ganglion (Fig. 154, i), which are nerves of special 

 sensation ; and secondly, at its lower part, with the nasal branch of 

 the fifth pair (2), a nerve of general sensation. Beside which, the 

 spheno-palatine ganglion of the great sympathetic (a) sends fila- 

 ments to the mucous membrane of the whole posterior part of the 

 nasal passages, and to the levator palati and azygos uvula3 muscles. 

 Finally, the muscles of the anterior nares are supplied by filaments 

 of the facial nerve. 



The conditions of the sense of smell are much more special in 

 their nature than those of taste. For, in the first place, this sense is 

 excited, not by actual contact with the foreign body, but only with 

 its vaporous emanations; and the quantity of these emanations, 

 sufficient to excite the smell, is often so minute as to be altogether 

 inappreciable by other means. We cannot measure the loss of 

 weight in an odorous body, though it may affect the atmosphere 

 of an entire house, and the senses of all its inhabitants, for days 

 and weeks together. Secondly, in the olfactory organ, the special 

 sensibility of smell and the general sensibility of the mucous mem- 

 brane are separated from each other and provided for by different 

 nerves, not mingled together and exercised by the same nerves, as 

 is the case in the tongue. 



In order to produce an olfactory impression, the emanations of 

 the odorous body must be drawn freely through the nasal passages. 

 As the sense of smell, also, is situated only in the upper part of these 

 passages, whenever an unusually faint or delicate odor is to be per- 

 ceived, the air is forcibly directed upward, toward the superior 

 turbinated bones, by a peculiar inspiratory movement of the nos- 

 trils. This movement is very marked in many of the lower animals. 

 As the odoriferous vapors arrive in the upper part of the nasal 

 passages, they are undoubtedly dissolved in the secretions of the 

 olfactory membrane, and thus brought into relation with its nerves. 

 Inflammatory disorders, therefore, interfere with the sense of smell, 

 both by checking or altering the secretions of the part, and by 



