742 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



attenuated, and can be traced inwards for a considerable distance, when 

 they expand into a broad flat sheet or plate, which, whilst it frequently pre- 

 sents a granular appearance, is never colored. The processes which pass off 

 from this sheet appear to be continuous with the fibres of the submucous 

 connective tissue. Towards the margin of the true olfactory region, cells 

 (c, Fig. 258) in every respect analogous to those just described are found, 



excepting only that they present a well-defined 

 baud or seam at their free extremity, which is 

 surmounted by a circle of cilia. The cells of the 

 second set (b, Fig. 258) have been described by 

 Schultze and Babuchin as continuations of the 

 nerves, and have hence been termed by them 

 " Olfactory cells." They are thin, fibrous- or rod- 

 like bodies, terminating at the same level as the 

 proper epithelial cells in a leash of long fine cilia 

 (Babuchiu), and presenting when traced inwards 

 a large oval nucleus and a series of moniliform or 

 varicose swellings which are directly continuous 

 with outrunners of more deeply-seated nerve-cells. 

 Closely analogous appearances have been seen and 

 described by Lockhart Clarke, who states that the 

 Olfactory Nerve-fibres on reaching the base of the 

 epithelial layer divide into finer and still finer 

 branches, to form a network with numerous in- 

 terspersed nuclei, through which they are proba- 

 bly connected with the "Olfactory cells" (/, 

 Fig. 258), although he has never been able satis- 

 factorily to convince himself of such connection. 

 The Proper epithelial cylinders (d, e) are con- 

 uected at their bases with the septa formed ot 

 connective tissue belonging to the subepithelial 

 glandular layer. The remainder of the nasal surface is supplied by the 

 Fifth pair only, and is not endowed with sensibility to odors, although it is 

 susceptible of irritation from such as are of a pungent nature ; and hence it 

 is that we cannot distinguish faint odors, unless, by a peculiar inspiratory 

 effort, we draw the air charged with them to the upper part of the nose. In 

 animals living in the air, it is a necessary condition of the exercise of the 

 sense of Smell, that the odorous matter should be transmitted by a respi- 

 ratory current through the nostrils, and that the membrane lining these 

 should be in a moist state. Hence, by breathing through the mouth, we 

 may avoid being affected by odors even of the strongest and most disagree- 

 able kind ; and in the first state of a catarrh, when the ordinary mucous secre- 

 tion is suspended, the sense of Smell is blunted from this cause, as it after- 

 wards is from the excess in the quantity of the fluid, which prevents the 

 odoriferous effluvia from coming into immediate relation with the sensory 

 extremities of the nerves. Hence we may easily comprehend how section 

 of the Fifth pair, which exerts a considerable influence over the secretions, 

 will greatly diminish the acuteness of this sense, and will have the further 

 effect of preventing the reception of any impressions of irritation from acrid 

 vapors, which are entirely different in their character from true odorous im- 

 pressions, and are not transmitted through the Olfactory nerve ( 515). 



GOT. The importance of the sense of Smell among many of the lower Ani- 

 mals, in guiding them to their food, or in giving them warning of danger, 

 and also in exciting the sexual feelings, is well known. To Man its utility 

 is comparatively small under ordinary circumstances; but it may be greatly 



d, e, f, after Lockhart Clarke. 



