4. _Smell._--On a fine, filmy membrane, which lines the labyrinth of the nasal cavities, are distributed the delicate ramifications of the olfactory nerves, or the nerves of smell. These nerves are constructed for transmitting to the mind impressions arising from the floating exhalations of odoriferous substances. It is through these nasal cavities, principally, that man and quadrupeds breathe; hence the respiration of the air informs us of our proximity to objects disgusting and noxious, or, on the contrary, attractive from their fragrance. Surely we need not adduce instances in proof of our position. The scent of putrescent animal or vegetable matters is in itself a warning to us to retire from the locality of mortific corruption, and escape from the region of disease. How loathsome to our sense are these putrescent effluvia! They nauseate us even to sickness--nay, sometimes almost to fainting; indeed, the writer has seen a violent fit of epilepsy produced by the overpowering and horribly disgusting odour of the matter contained in the scent pouch of a small quadruped, the grison, (_Galictis vittata_,) which he was dissecting, the sufferer being a practical zoologist, accustomed to the dissecting-room. But when the voice of the nightingale is heard, and the air is loaded with the perfume of the hawthorn blossom, and of thousands of spring flowers, are we not invited, as it were by a secret call, to walk abroad through woodlands and meadows, contemplating the works of God? We are thus lured to beneficial exercise--to inhale "The breezy fragrance of the morn"-- to forget the trials of life, and muse upon a still brighter world, where the flowers perish not, where storms come not, and where winter is unknown. Thus, to our bodily health do our senses administer; and thus the sense of smell calls us from the close room of the populous city, into the fresh air of the country--from the crowded town, "Where houses thick and sewers annoy the air," to issue forth on a summer's morn, and breathe the pure air redolent of sweetest odours. When overpoweringly offensive effluvia disgust us, we endeavour to stop our breath--we breathe at protracted intervals, take in as little air as possible, and endeavour to escape from the infected locality; but do we flee away from the gale bearing the scent of the honey-suckle, the bean-field, or the new-mown hay? It is, indeed, rather to our pleasures, to our mental enjoyment, to our delight, or to our disgust and abhorrence, that our sense of smell administers. We depend little upon it for our discrimination between things acting as poison, and things nutritious or wholesome. How pleasant is the smell of laurel-water or prussic acid--how dull and insipid the smell of wheaten flour! We must not, then, always be guided by our sense of smell, although in general it serves us as a monitor.