In the cartilaginous septum of the lower animals we find a small cavity lined with mucous membrane, called after its discoverer Jacobson's organ, the minute anatomy of which has lately been described by Klein.[18] This {35} organ in man is, however, only rudimentary. The nasal cavities are lined with mucous membrane, which varies greatly in thickness in different localities, and which materially decreases the size of the cavities in the living subject from that seen in the denuded skull. This mucous membrane is covered by ciliated epithelium in man, with the exception of that portion which lines the vestibule--_i.e._ that portion of the cavity of the nose surrounded by cartilage only--which is covered by pavement epithelium. [Footnote 18: _Quarterly Journal of Mic. Science_, January, 1881.] In the lower animals we find that in the olfactory region the ciliated epithelium is either absent, or that ciliated and non-ciliated epithelium alternates in patches.[19] The author has not been able to find a statement in the literature on the subject as to the kind of epithelium found in the accessory cavities in man, but it is very probable that the mucous membrane of the frontal sinuses and the antra of Highmore is covered with ciliated epithelium; otherwise it would be difficult, if not impossible, for the secretions of that mucous membrane to pass through the narrow channels into the nasal cavities. The color of the normal nasal mucous membrane is of a light pink shade in what is termed the respiratory portion, while it is of a yellowish hue in the olfactory region, that portion of the mucous membrane which covers the roof and the outer walls of the nasal cavities down to the upper margin of the middle turbinated bone and the septum down to about the same level. It is in this region that the nerve-ends of the olfactory nerve are distributed. Immediately beneath the mucous membrane, and between it and the periosteum of the bony walls and the perichondrium of the cartilaginous portion of the septum, we find a tissue which bears a striking resemblance to the erectile tissue of the genital organs.[20] It is composed of a network of fibrous tissue, the trabeculæ of which contain a few organic muscular fibres. Its meshes of various sizes and shapes are occupied by venous sinuses lined with endothelium. These are supplied with blood by small arterioles and capillaries, which are quite numerous in the fibrous tissue and can readily be demonstrated under the microscope. In this arrangement of elements of the nasal mucous membrane we find a ready explanation of the fact that liquids of greater or less density than the serum of the blood when introduced into the nasal cavities produce pain, for we have here the most favorable conditions for osmosis, which will cause either a contraction or a distension of the sinuses. In the larger masses of fibrous tissue between the sinuses or caverns we find imbedded the