THE ORGAN OF SMELL.
The Nostrils contain in their mucous mem brane the structures devoted to the sense of smell. Reference to Fig. 12, p. 59, shows the cavity of the nostrils so far as formed by bone. The roof of the cavity is formed by the ethmoid bone, the upper surface of which forms part of the floor of the brain cavity, so that this hori zontal plate above separates the brain cavity from the cavity of the nostrils. Part of the side walls of the nostrils, as low as the floor of the cavities for the eye (see p. 59), are formed by light scroll-like prolongations of the same ethmoid bone, the remainder of the side walls being formed of part of the upper jaw-bone. A central perpendicular plate of the ethmoid divides the upper part of the cavity into a right and left portion, and this division is continued down wards by the ploughshare bone (p. 60), and completed by gristle. The bony palate forms the floor of the nostrils. There is an opening to each nostril behind, into the back part of the throat, as well as in front. The walls of the
cavities are lined by mucous membrane, richly supplied by vessels and nerves. The nervousdis tribution is shown in Fig. 172, where branches of the fifth cranial nerve are teen traversing the cavity and passing over its walls. The nerves of smell proper are, however, shown spreading in a thick brush over the upper and middle scroll - like bones of the outer wall, and over the upper part of the inner wall. They are found in the mucous membrane. These nerve fibres are derived from the first pair of cranial nerves—the olfactory (p. 151), which rest on the upper surface of the ethmoid bone, and send branches through openings in its horizontal plate, which, because of the many perforations in it, is called the cribriform plate. The ter minations of these nerve-fibres seem to be con