SENSE OF SMELL. 539 



properties, as soon as they are converted into vapour, by the aid of 

 heat for example. There are some solid substances, which possess very 

 strong odorous properties, without losing weight in any appreciable 

 degree by the diffusion of their particles through the air. This is the 

 case, for example, with Musk ; a grain of which has been kept freely 

 exposed to the air of a room, whose door and windows were constantly 

 open, for a period of ten years ; during which time the air, thus con- 

 tinually changed, was completely impregnated with the odour of musk ; 

 and yet, at the end of that time, the particle was not found to have per- 

 ceptibly diminished in weight. We can only attribute this result to the 

 extreme minuteness of the division of the odorous particles of this sub- 

 stance. There are other odorous solids, such as Camphor, which rapidly 

 lose weight by the loss of particles from their surface, when freely 

 exposed to the air. 



947. The conditions of the sense of Smell are very simple. The Ol- 

 factory nerve is minutely distributed over the Schneiderian membrane, 

 which is itself highly vascular. The arrangement of the ultimate fibres 

 of this nerve has not been ascertained. The Schneiderian membrane 

 is kept constantly but moderately moist, by a mucous secretion from its 

 surface ; and this condition is essential to the acute perception of odours. 

 If the mucous surface be too dry, as happens when the fifth pair is 



Fig. 160. 



The Olfactory nerve, with its distribution on the septum nasi. The nares have been divided by a longitu- 

 dinal section made immediately to the left of the septum, the right nares being preserved entire. 1. The 

 frontal sinus. 2. The nasal bone. 3. The crista gall i process of the ethmoid bone. 4. The sphenoidal sinus 

 of the left side. 5. The sella turcica. 6. The basilar process of the sphenoid and occipital bones. 7. The 

 posterior opening of the right nares. 8. The opening of the Eustachian tube in the upper part of the 

 pharynx. 9. The soft palate, divided through its middle. 10. Cut surface of the hard palate, a. The olfac- 

 tory peduncle. &. Its three roots of origin, c. Olfactory ganglion, from which the filaments proceed that 

 spread out in the substance of the pituitary membrane, d. The nasal nerve, a branch of the ophthalmic 

 nerve descending into the left nares from the anterior foramen of the cribriform plate, and dividing into 

 its external and internal branch, e. The naso-palatine nerve, a branch of the spheuo-palatine ganglion 

 distributing twigs to the mucous membrane of the septum nasi in its course to (/) the anterior palatine 

 foramen, where it forms a small gangliform swelling (Cloquet's ganglion) by its union with its fellow of the 

 opposite side. g. Branches of the naso palatine nerve to the palate. 7i. Posterior palatine nerves, t, i.The 

 septum nasi. 



paralysed, the sensation is blunted or even destroyed ; and the same 

 effect is produced by the presence of too copious a secretion, as when 



