180 PHYSIOLOGY. 



chest, and intended to stop every foreign body that may be 

 mixed with the air, particularly the odours. 



5. What is called odour, or smell, resides in nearly all 

 bodies, and is given off by moisture, heat, motion, or friction. 

 Those which do not possess this quality are called inodorous. 

 Those little particles which convey the odour, are scattered 

 through the air, and in breathing drawn into the upper part 

 of the nostrils, where the sense of smell principally resides. 

 If we breathe through the mouth, odours in general will not 

 be perceived, unless very pungent. The fluid which moistens 

 the lining membrane of the nose, is supposed not only to 

 render it more sensitive and delicate, but also to entangle 

 the odorous particles, and in this way detain them longer in 

 contact with the olfactory nerve. The constant evaporation 

 which takes place from the membrane, owing to the passage 

 of the air in respiration, requires that the secretion should 

 be constant as well as copious ; otherwise the membrane 

 would soon become dry and parched. 



6. It is very difficult to describe an odour, except to those 

 who have smelled it, or something with which it may be 

 compared. We can say that odours are pleasant or dis- 

 agreeable ; that they are aromatic, or sweet, rancid, or fetid, 

 &c. ; but we can give no correct idea of the peculiar smell 

 of bodies, such as camphor, musk, garlic, the rose, &c., with- 

 out experiencing the sensation which their smell produces. 



7. The odorous particles of bodies must be very small to 

 excite any sensation on the animal organs. A grain of 

 musk will, it is said, scent a room for years, and not lose any 

 of its weight ; Mr. Boyle asserts that the smell of cinnamon, 

 from Ceylon, is perceived at sea at the distance of twenty- 

 five miles from the island. Scales in which a few grains of 

 musk have been weighed, have been found to retain the smell 

 for twenty years, though during all this time, they must have 

 been constantly giving off odorous particles. Haller kept 

 some papers forty years, which had been perfumed by a single 

 grain of amber, and at the end of that time they did not 



