This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
Smell, the sense by which we distinguish odours, is located within the nasal cavities, and depends on a simpler mechanism than any other of the special senses.
The nasal cavities or fossæ extend from the nostrils back to the pharynx, into which they open behind; the communication being termed the posterior nares (fig. 49). Superiorly, they are separated from the cranial cavity by a thin plate of bone, the cribriform plate of the ethmoid, which is perforated by the filaments of the olfactory nerve; and inferiorly their floor is made by the hard and soft palate; while between them is placed a vertical septum dividing one fossa or cavity from the other.
The outer wall, in the human subject, presents three ledges of bone, one above another, projecting inwards with a downward curve, and termed turbinated bones. The inferior is a distinct bone; while the others are portions of the ethmoid, and project little lower than the floor of the orbit.
The ethmoid bone consists of a central plate, the cribrifrom plate, and two lateral masses. The central plate descends in the middle line from the cribriform plate, and forms the upper part of the nasal septum; while the lower part of the septum is formed behind by another bone, the vomer, and in front is cartilaginous. The lateral masses of the ethmoid form, by two smooth surfaces, part of the inner walls of the orbit, and are hollowed out into air cells, with thin papery walls lined with mucous membrane, and communicating with the nasal fossæ. The ethmoidal turbinated bones form part of these lateral masses. The superior one exists only in the hinder half of each mass, and overhangs a space between it and the middle turbinated bone called the superior meatus of the nose, which communicates in front with ethmoidal cells, and has the opening of a space hollowed out of the sphenoid bone, the sphenoidal sinus, opposite it behind (fig. 17). The other turbinated bone of the ethmoid, called the middle turbinated bone, is at the lower part of the lateral mass, and overhangs a gallery between it and the inferior turbinated bone, the middle meatus of the nose. This communicates with a hollow in the upper jaw, the maxillary sinus or antrum, and also, by a passage through the fore part of the lateral mass of the ethmoid, with the frontal sinus, a large hollow in the frontal bone, in the lower part of the forehead. A third gallery or passage, the inferior meatus, lies between the inferior turbinated bone and the floor of the nose; and into it there opens near the front the nasal duct, a canal by which the tears are conveyed from the eye; while, opposite its extremity behind, is the orifice of a communication leading from the ear, the Eustachian tube.
163. I have thought it better to give the student at once a connected account of the whole interior of the nose, but it must not be supposed that all the structures now described are connected with smell. What is called the nose, in the more extended sense of the word, has a number of different functions. The nose proper, or feature so called, is simply an organ of expression. The nostrils are of use as the openings into the nasal cavities, but the prominence above them is merely ornamental. The nasal cavities are connected with three functions—breathing, voice, and smell. Only the ethmoidal part has the filaments of the olfactory nerve distributed in its mucous membrane; the lower part is furnished with ciliated epithelium like the rest of the respiratory tract, as is also the part of the pharynx into which it opens; and in quiet breathing the upper edge of the epiglottis comes almost in contact with the edge of the soft palate which forms its floor behind. When the mouth is shut, the air passes to and from the larynx through the nasal fossæ; and, even when the mouth is open, a considerable quantity of air passes through them; and the current is broken by the various turbinated projections, so that part of it is directed upwards to the olfactory region, and the whole of it is assimilated to the temperature of the body before entering the larynx. In " sniffing," or drawing air into the nose to assist smell, the inspirations are short, abrupt, and repeated, so as to put small quantities of air into rapid motion; and in each inspiration there is a slight but quick contraction of the nostrils, both increasing the rapidity of the air and directing it upwards.
Both the nasal fossæ and the various sinuses opening off from them act as reverberating cavities to improve the timbre of the voice; and it is in connection with the voice, not with smell, that the nasal fossæ in the human subject are extensive. They differ from those of quadrupeds in their greater vertical height and diminished extent from before backwards, and in communicating with larger and more numerous sinuses. But, in addition, it will be seen on comparison with any common mammal, such as the dog or the sheep, that in the human subject there is a very small amount of surface provided for the distribution of the olfactory nerves. In those animals the turbinated bones are far more complex, and finer secondary turbinations come off from the main laminæ, while all are directed with their ends towards the nostrils, so that the inhaled air is exposed to a most extensive sensitive surface. This corresponds with what one would expect, both from the very small comparative size in man of the olfactory bulbs of the brain, and from the obviously acute sense of smell in the lower animals, exceeding in some of them, such as the dog, anything which our own senses enable us to conceive.