Fig. 147—*Sense organ of smell.* _A._ Distribution of nerves in outer wall of nasal cavity. 1. Turbinated bones. 2. Branch of fifth pair of nerves. 3. Branches of olfactory nerve. 4. Olfactory bulb. _B._ Diagram showing connection of neurons concerned in smell. *The Sensation of Smell.*—The sense organs of smell are found in the mucous membrane lining the upper divisions of the nasal cavities. Here are found two kinds of cells in great abundance—column-shaped epithelial cells and the cells which are recognized as the sense organs of smell. These olfactory cells are spindle-shaped, having at one end a slender, thread-like projection which reaches the surface, and at the other end a fiber which joins an olfactory nerve (B, Fig. 147). In fact, the olfactory cells resemble closely the cell-bodies of neurons, and are thought to be such. The divisions of the olfactory nerve pass through many small openings in the ethmoid bone to connect with the olfactory bulbs, which in turn connect with the cerebrum (A, Fig. 147). *The Olfactory Stimulus.*—Only substances in the gaseous state can be smelled. From this it is inferred that the stimulus is supplied by gas particles. Solids and liquids are smelled because of the gas particles which separate from them. The substance which is smelled must be kept moving through the nostrils and made to come in direct contact with the olfactory cells. There is practically no limit to the number of distinct odors that may be recognized. *Value of Smell.*—Although the sense of smell is not so acute in man as in some of the lower animals, it is, nevertheless, a most important and useful gift. It is the only sense that responds to matter in the gaseous state, and is, for this reason, the only natural means of detecting harmful constituents of the atmosphere. In this connection it has been likened to a sentinel standing guard over the air passages. Many gases are, however, without odor, and for this reason cannot be detected by the nostrils. It is of especial importance that gases which are likely to become mixed with the air supply to the body have odor, even though the odor be disagreeable. The bad odors of illuminating gas and of various compounds of the chemical laboratory, since they serve as danger signals to put one exposed to them on his guard, are of great protective value. *Sight and Hearing.*—The sense organs of sight and hearing are highly complicated structures, and will be considered in the chapters following.