The early developement of the human no has not been particularly studied, but is pro bably very similar to that just described fro' observations in the lower animals. In a wel formed embryo an inch in length, I have foun the nasal cavities of proportionally large siz On their lateral walls they present distinct trac of the rudiments of the two lower turbi bones in prominent horizontal folds of the membrane. The palate is at this time fon onlyanteriorly and at its sides ; its middle po tion is widely open, exposing to the view fron below the free inferior border of the septum. The upper edge of the septum is firmly fixed to the base of the skull, and its posterior edge gradually slopes back to the upper and back part of the wide cavity of the pharynx. There is no appearance of a vomer ; and the nose does not project upon the face. Its position is marked externally by the nostrils, which are elongated vertically, oval and narrow, situated about three-fourths of a line from the margin of the upper,lip and at the like distance from each other. They are not at this time closed, but lead straight backwards into the common nasal and oral cavity.
In the following periods the chief changes are effected by the gradual closure of the palate, the fixing of the lovver margin of the septum, the developement of the vomer, and the growth of the anterior part of the septum and of the rudiments of the nasal and superior maxillary bones. With these changes the nose gradually becomes more prominent, and the nostrils, which at first look straightforwards, are gradually turned obliquely downwards, and at last are directed as in the adult nearly straight down wards. During the third and fourth months, according to Burdach, the nostrils are closed by a fine membrane, which in the fifth month is again removed. Together with the change in their direction already spoken of, the septum becomes narrower and the distance between them is diminished. Changes perfecting these are con tinued. even long after birth in the gradual elevation and elongation of the bridge of the nose, and in the narrowing of its base ; and it is in these changes subsequent to birth that noses, which present little variety in infants, acquire the ahnost infinite diversities of form by which they characterize the faces of adults.
Physiology of the nose.—Most of the pur poses to which the nose is subservient in the economy are described in other articles. [SmELL, LARYNX, MUCUS, FAcm.] Here, however, it may be considered as the first portion of the respiratory passage, and as a feature character istic of the human race and of its several varieties.
The DOSC is the proper channel through which the air is drawn into and expelled from the lungs. It alone is habitually used in respi ration by most animals, and though in man the mouth is as often used in breathing as the nose, (and, indeed, oftener in our own climate, in which, from various causes, few persons have at all times both the nasal passages free,) yet it is not adapted to this office so well as to be used long without inconvenience. Most persons must have suffeted the discomforr of breathing through the month during a few hours' sleep : all its lining membrane, as well as that of the fauces and of the upper part of the larynx, be comes dry, and an excretion of saliva must be artificially produced before the annoyance and the danger of choking can be removed. No such inconvenience attends the breathing through the nose for any length of time. Its more extended mucous membrane supplies a fluid sufficient to keep its own epithelium moist, and to saturate with vapour the air which passes over it, so that this air does not abstract so much moisture from the surface of the epiglottis and the glottis as the drier air which has passed through the mouth alone.
Again, the nose is far better adapted than the mouth is for the arrest of the particles of foreip,m solid bodies which float in the air. If such
particles have passed through the hairs Which lie at the orifices of the nostrils, and which are sufficiently close-set to stop even very minute bodies, they are in their further course liable to be caught in the irregular surfaces of the walls of the nasal fossw and entangled in the moisture of their lining membrane. Hence, most per sons can breathe through the nose for some time without inconvenience even in a cloud of dust : and the nasal cavities of the horse and other Mammalia are, in this respect, still better adapted for the protection of the lungs. Some experiments were performed in France to de termine whether great injury of the respiratory passages of horses were produced by their ex posure to the dust of roads. Horses were made to trot for a considerable distance in the clouds of dust thrown up by the wheels of carriages driven before them ; they were killed directly afterwards, and not a particle of dust appeared, on the closest scrutiny, to have passed beyond their nasal fossw.
The nose is further adapted to be the first portion of the respiratory passage by the acute and peculiar sensibility of its mucous mem brane, and by the connection of its nerves in the nervous centres with the nerves of all the set of respiratory muscles. Through the ol factory nerve the nose detects the impurity of the air from those gases whose deleterious pro perties are indicated by odour; and its acute common sensibility affords a warning of the presence of any mechanical or other common irritant. The act of sneezing, which in this last case is excited through the already-men , tioned connection of these nerves, is an ex ample of that class of half-involuntary acts* which are consequent on acute sensations; and, in this respect, it is widely disting-uished from the other reflex acts with which it is commonly classed, but which are never, or'at least not necessarily, connected with sensation. Every one must have felt that a certain acute sensation is necessary in order that a sneeze should occur ; and if the sensation does not arise to that cer tain degree of acuteness, the disposition to the sudden forcible expiration gradually passes offi though the act had been desired and had seerned on the point of being accomplished. In this respect sneezing is exactly analog,ous to coughing,—an act which is never thoroughly effected except in consequence of a certain acuteness of sensation at the glottis. And the analogy is maintained in this also,—that the cause of irritation which produces sneezing rnay be seated either in the nose itself, where it is always felt, or in another part. In cough ing, the sensation which immediately precedes the act may be the consequence of direct irri tation of the glottis, or of irritation of another part, such as the distant bronchial tubes, from which the impression is conveyed to the brain, and there, as it is supposed, is radiated to the central extremities of the nerves of the glottis, and is jdt as if it were applied to their peri pheral extremities. In eithcr case the peculiar sensation at the glottis is the necessary pre cedent of the act of coughing; and it is the same in sneezing. A sudden vivid impression of light upon the retina, or, sometimes, the irritation of a tender point on the skin of the face, produces a sensation of pain or irritation in the mucous membrane of the nose, and sneezing follows. The sequence of events may be supposed to be,—an impression on the pen pheral filaments of the retina,—its conveyance to their central extremities in the brain,—its radiation to the central extremities of the sen sitive nerves of the nose, producing the same sensation as if their peripheral extremities had been irritated,—and, through that sensation, whether it be objective or subjective, the half involuntary act.