The pituitary or Schneiderian membrane, which lines the cavities of the nose, is a highly vascular mucous membrane, inseparably united, like that investing the cavity of the tympanum, with the periosteum and perichondrium, over which it lies. It is continuous with the skin through the nostrils; with the mucous membrane of the pharynx through the posterior apertures of the nasal fossae; with the conjunctiva through the nasal duct and lachrymal canaliculi; and with the lining membrane of the several sinuses which communicate with the nasal fossae.
The pituitary membrane, however, varies much in thickness, vascularity, and general appearance in these different parts. It is a– thickest and most vascular over the turbinate bones (particularly the inferior), from the most dependent parts of which it forms projections in front and behind, thereby increasing the surface to some extent. On the septum nasi the pituitary membrane is also very thick and spongy; but in the intervals between the turbinate bones, and over the floor of the nasal fossae, it is considerably thinner. In the maxillary, frontal, and sphenoidal sinuses, and in the ethmoidal cells, the mucous lining membrane, being very thin and pale, contrasts strongly with that which occupies the nasal fossae.
In respect of the characters of the mucous membrane, three regions of the nasal fossae may be distinguished. Thus, the region of the external nostrils, including all the part which is roofed by the nasal cartilages, is lined with stratified squamous epithelium ; and the remainder is divisible into two parts, viz., the olfactory region in which the epithelium is non-ciliated and columnar, and the respiratory region in which, as also in the sinuses, it is ciliated and columnar. The membrane in the respiratory part, consisting of the inferior turbinated and all the lower portions of the fossae, is studded with numerous racemose mucous glands, which open by orifices apparent on the surface.
These are most numerous about the middle and hinder parts of the nasal fossae, and are largest at the back of the septum near the floor of the nasal cavity. They are much smaller and less numerous in the membrane lining the several cavities which communicate with the nasal fossae.
Olfactory mucous membrane
The olfactory region or that, in which the olfactory nerve is distributed, includes the upper and middle turbinated parts of the fossae, and the upper portion of the septum.
It is extremely vascular, a close plexus of large capillary vessels being found under the lining membrane throughout its whole extent. Its mucous membrane is thicker and more delicate in consistence than that of the ciliated region, being soft and pulpy. It has a distinct yellow color in man; brown in some animals. The glands of this region are numerous, but are of a more simple structure than those in the lower part of the fossae ; amongst them, however, some ordinary racemose glands are occasionally to be found The columnar cells on its surface are prolonged at their deep extremities into a process generally somewhat thickened towards its deeper end, which usually includes a number of fatty granules; and from this thickened part branches proceed, which are stated by Exner and Martin to communicate with those of neighboring cells, so as to form a communicating network through-out the extent of the membrane, underneath the epithelium. Between and amongst these branching central ends of the columnar cells there are a large number of peculiar spindle-shaped cells, each consisting of a large, clear nucleus -surrounded by a relatively small amount of granular protoplasm. From each cell proceed a superficial and a deep process. The superficial process is a cylindrical or slightly tapering thread passing directly to the surface, and terminating abruptly at the same level as the epithelial cells between which it lies: the deep process is more slender, and passes vertically inwards. This last frequently presents a beaded appearance similar to that observed in fine nerve-filaments, and considered to be of a similar accidental origin. These cells were termed by Max Schultze, their discoverer, olfactory cells, to distinguish them from the columnar epithelium cells above described, than which they are much more numerous, and which they entirely surround with their fine rod-like peripheral processes. It is probable that their fine varicose central processes are directly continuous with the fibrils of the olfactory nerve, but the continuity has never been actually observed. The nucleated bodies of the olfactory cells are several rows deep, and form a layer of considerable thickness beneath the columnar cells.
The superficial process of the olfactory cell was observed by Schultze to be surmounted by a short, stiff projection and has been so described by others; but it is now agreed that this appearance results from the coagulation of albuminous matter escaped from the interior of the process. Long and fine hair-like processes do, however, exist on the olfactory membranes of amphibia, reptiles, and birds, but they have not been observed in mammals.
A doubt has been thrown by Exner upon the definiteness of the distinction between the epithelial and the olfactory cells of this region: he states that every transition may and that the central processes of both end in moreover, the nerve fibrils are distributed.
The subject has since been again carefully investigated by Martin, who upholds the correctness of Max Schultze's views.*
Olfactory Nerve
The filaments of this nerve, lodged at first in grooves on the surface of the bone, enter obliquely the substance of the Schneiderian membrane, and pass to their distribution between its raucous and fibrous layers. The nerves of the septum are rather larger than those of the outer wall of the nasal fossae; they extend over the upper third of the septum, and as they descend become very indistinct. The nerves of the outer wall are divided into two groups - the posterior branches being distributed over the surface of the upper spongy bone, and the anterior branches descending over the plain surface of the ethmoid and the middle spongy bone.
The nerves as they descend, ramify and unite in a plexiform manner, and the filaments join in brush-like and flattened tufts, which, spreading out laterally and communicating freely with similar offsets on each side, form a' fine network, with elongated and narrow intervals between the points of junction.
In their nature the olfactory filaments differ much from the ordinary dark-bordered fibers of the cerebral and spinal nerves: they possess no medullary sheath, are pale, and finely granular in aspect, firmly adherent one to another, and have oval corpuscles on their surface.
The greater part of the mucous membrane of the nasal fossae is provided also with nerves of common sensibility, derived from branches of the fifth pair: the distribution of these has already been described.
From a branch of the olfactory nerve of the sheep ; at a, a, two dark bordered or medullated fibers, from the fifth pair, associated with the pale olfactory fibers.
From Quain's anatomy.
