116 PHYSIOLOGY. 



relation to its ordinary stimuli ; the odorous particles are brought 

 into contact with it in the act of inspiration. These particles are so 

 small as to elude detection by the most delicate experiments. The 

 whole mucous membrane of the nose is not endowed with the sense 

 of smell ; it seems to be limited to that portion expanded over the 

 superior and 'part oftlie middle spongy bones. This region is there- 

 fore called the olfactory region. It is to this portion only that the 

 olfactory nerve is distributed. The other portions of the mucous 

 membrane are supplied with branches of the fifth pair, {"rom which 

 they obtain their general sensibility. These branches are derived 

 from the ophthalmic and superior maxillary divisions. When the 

 fifth nerve is diseased, irritating substances may be introduced into 

 the nose without discomfort to the patient. Section of the fifth pair 

 also arrests the secretion from the mucous membrane, and in that 

 way diminishes the acuteness of smell. This nerve is the afferent 

 or excitor nerve in the reflex act of sneezing. 



The organ of smell is seated high up in the nose, not only to 

 protect it from mechanical injury, but that it may be screened from 

 the contact of air either too cold or too dry. The convoluted ar- 

 rangement of the turbinated bones, with their expansion of mucous 

 membrane, effects this. These parts break the force of the current 

 of air, warm it, and impart that degree of moisture which is best 

 calculated to aid the solution of the odorous particles on the sentient 

 surface to which they are to be applied. 



The olfactory nerve passes down from the olfactory bulb or gan- 

 glion through the cribriform plate of the ethmoid bone, and is dis- 

 tributed in minute threads having a plexiform arrangement, (Fig. 

 32.) Their ultimate distribution is probably in loops. 



The conditions requisite for the exercise of the sense of smell are, 

 in addition to the integrity of the nervous apparatus, a healthy con- 

 dition of the mucous membrane. If it be dry, or in a raw irritable 

 state, with a watery discharge, the sense is lost or impaired. This 

 is familiarly seen in a cold in the Jiead. The substance to be smelt 

 must also be soluble ; insoluble substances cannot be perceived by 

 this sense. When we wish to examine any substance closely, it is 

 drawn up into the nose with some force, in order that the odorous 

 particles may reach the olfactory region. 



The sense of smell, although not an intellectual sense, is suscep- 

 tible of cultivation ; by it individuals are often capable of recognising 

 others, and even, as in one instance, to discover their own clothes 

 among many others. 



Subjective plienomena sometimes occur in this sense, arising, as 

 in the others, from irritation by an internal cause. Disease of the 

 olfactory bulb, or of the anterior lobe of the brain, has been known 

 to produce them. Muller relates the case of a man, who ever after 

 a fall from his horse, believed that he smelt a bad odour. Whether 



