872 



OF SENSATION, AND THE ORGANS OF THE SENSES, 



Fig. 207. 



Olfactory filaments of the Dog: a. In water. 

 b. In acetic acid. Magnified 250 diameters. 



their want of the characteristic white substance, and the absence of distinction 



between the nuclei of the minuter fibres 

 and those of the nucleated tissues through 

 which they pass. It would appear that 

 every part of the Schneiderian membrane 

 is not equally endowed with the faculty of 

 distinguishing odors, which is a very dif- 

 ferent power from that of becoming sensi- 

 ble of irritation from them. The distribu- 

 tion of the Olfactory nerves seems limited 

 to the membrane covering the superior 

 three-fourths of the septum of the nose, 

 the superior turbinated bone and the upper 

 half of the middle turbinated bone, and the 

 upper wall of the nasal cavities beneath 

 the cribriform plate of the ethmoid bone ; 

 all which surface is covered (as Messrs. 

 Todd and Bowman have pointed out) with 

 a tessellated epithelium of a rich sepia- 

 brdwn hue. The remainder of the nasal 

 surface is supplied by the 5th Pair only; and hence it is that we cannot dis- 

 tinguish 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 respiratory 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 disagreeable kind ; and in the first stage 

 of a catarrh, when the ordinary mucous secretion is suspended, the sense of 

 Smell is blunted from this cause, as it afterwards 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 5th Pair, which exerts a con- 

 siderable 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 impressions, and are not transmitted through the 

 Olfactory nerve ( 739). The nasal passages may indeed be considered as 

 having, in the air-breathing Vertebra ta, two distinct offices ; they constitute the 

 organ of Smell, through the distribution of the olfactory nerve upon a part of 

 their surface ; but they also constitute the portals of the respiratory organs, 

 having for their office to take cognizance of the aeriform matter which enters 

 them, and to give warning of that which would be injurious; this latter function 

 is performed by the 5th Pair, as by the Pneumogastric in the glottis. It is 

 through this nerve that the act of Sneezing is excitable; the evident purpose of 

 which is the ejection of a strong blast of air through the nasal passages, in such 

 a manner as to drive out any offending matter they may contain. 



873. 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 com- 

 paratively small under ordinary circumstances ; but it may be greatly increased 

 when other senses are deficient. Thus, in the well-known case of James Mit- 

 chell, who was deaf, blind, and dumb from his birth, it was the principal means 

 of distinguishing persons, and enable him at once to perceive the entrance of a 

 stranger. It is recorded that a blind gentleman, who had an antipathy to cats, 



