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cells ; and it is provided with numerous mucous-secreting glands. 

 Nerve filaments from the terminating bulb of the olfactories are 

 distributed through it. The other parts of the interior of the nose 

 are covered with a mucous membrane also, but the nerves supplied 

 to these parts are only the nerves of general sensibility — i.e., they 

 are only sensitive to irritants, and not to odours. In birds and 

 reptiles the structure of this organ is similar. In fishes there is no 

 posterior opening, the organ being situated in a species of cul de 

 sac. In the invertebrates, as far down as the mollusca, a sense of 

 smell has been detected. It is sometime* very acute, as in bees, 

 moths, ants, &c. The precise locality of the organ has created 

 considerable discussion. In the articulata, a German naturalist 

 has recently shown this sense to be situated on certain parts of the 

 antennae. Among mammals, the carnivora seem endowed with 

 the most acute sense of smell. It is confined here, however, 

 chiefly to animal emanations. An exception to this we find in the 

 cat, which goes wild with delight over valerian, or cat-mint. Man, 

 if not so acute in his scent, is sensitive to a far greater range of 

 odours than the lower animals. The conditions of smell in the 

 higher animals are : — i. Healthy condition of the olfactory fila- 

 ments. 2. The mucous secreted must have its normal constituents. 

 When this is altered in quality, as in a cold in the head, the sense 

 of smell is either lost, or is morbid. There have been various 

 theories about the nature of odours. Some have made their action 

 dynamical or electrical, affecting the nose through the air as a 

 sounding body, or as a source of light through the ether. It is 

 now generally admitted that odours are minute particles of volatile 

 bodies floating in the air ; that they are borne by inspiration to the 

 olfactory region, where they are held, and brought into contact 

 with the nerves, by means of the mucous secreted there. In order 

 that a body should be odorous, it must be volatile. Chemical 

 instability, or a tendency to change, particularly by combining with 

 oxygen, is another condition. Hence, all imperfectly oxydized 

 bodies, such as the various essential oils, are odorous. When they 

 are oxydized they become either resins or acids, and are then com- 

 paratively inodorous. It is curious to note that hydrogen, the 



