Many drugs, such as musk and ethereal oils, have a marked and characteristic smell, due to their effect upon the terminal branches of the olfactory nerve. This nerve is soon exhausted, so that in a very short time the smell is no longer perceived with anything like the intensity it was at first. Such smells as these just mentioned cannot be perceived by persons suffering from anosmia, but certain drugs, such as ammonia or acetic acid, can be recognised by them. The reason of this is that although such persons are incapable of perceiving any true smell, the nasal branches of the fifth nerve are irritated by pungent vapours, and thus produce a certain kind of sensation. The power of distinguishing smells seems to be increased by strychnine; which appears at the same time to render such disagreeable odours as those of asafoetida, garlic, and valerian agreeable. This effect may be due to the action of strychnine on the olfactory apparatus, but it is very probably due rather to the action of the drug on the cerebral centre for smell, which, according to Ferrier, is situated at the tip of the temporo-sphenoidal lobe. The power to distinguish smells is diminished by such drugs as lessen the sensibility of the brain, or by those which cause alterations in the nasal mucous membrane, as, for example, iodide of potassium given in such doses as to produce coryza.