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Olfactory System

OLFACTORY SYSTEM, in anatomy. The olfactory system consists of the outer nose, which projects from the face, and the nasal cavities, contained in the skull, which support the olfactory mucous membrane for the perception of smell in their upper parts, and act as respiratory passages below. The bony framework of the nose is part of the skull iq.v.), but the outer nose is only supported by bone above; lower down its shape is kept by an " upper " and " lower lateral cartilage " and two or three smaller plates known as " cartilagines minores."

The expanded lower part of the side of the outer nose is known as the " ala " and is only formed of skin, both externally and internally, with fibro-fatty tissue between the layers. The inner nose or nasal cavities are separated by a septum, which is seldom quite median and is covered in its lower two-thirds by thick, highly vascular mucous membrane composed of columnar ciliated epithelium with masses of acinous glands (see Epithelial Tissues) embedded in it, while in its upper part it is covered by the less vascular but more speciahzed olfactory membrane. Near the front of the lower part of the septum a slight opening into a short blind tube, which runs upward and backward, may sometimes be found; this is the vestigial remnant of " Jacobson's organ," which will be noticed later. The supporting framework of the septum is made up of ethmoid above, vomer below, and the " septal cartilage " in front. The outer wall of each nasal cavity is divided into three meatus by the overhanging turbinated W^T OLFACTORY SYSTEM IHJO bones (see fig. 2). Above the superior turbinated is a space between it and the roof known as the " recessus spheno- ethmoidalis," into the back of which the " sphenoidal air sinus " opens. Between the superior and middle turbinated bones is the " superior meatus," containing the openings of the " posterior ethmoidal air cells," while between the middle and inferior turbinateds is the "middle meatus," which is the largest of the three and contains a rounded elevation known as the " bulla ethmoidahs." Above and behind this is often an opening for the " middle ethmoidal cells," while below and in front a deep sickle-shaped gutter runs, the " hiatus semilunaris," which communicates above with the " frontal air sinus " and below with the opening into the " antrum of Highmore " or " maxillary antrum." So deep is this hiatus semilunaris that if, in the dead subject, water is poured into the frontal sinus it all passes into[the Frontal air-sinus.

Bristle passed fr<.m it into infundibulum Opening of middle ethmoidal cells Openings of posterior ethmoidal cells Recessus spheno-ethmoidalis Sphenoidal air -sinus From R. Howden, in Cunningham's Text-Book of Anatomy. Fig. 2. - View of the Outer Wall of the Nose 1. Vestibule.

2. Opening of antrum of Highmore.

3. Hiatus semilunaris.

4. Bulla ethmoidalis.

5. Agger nasi.

antrum and none escapes through the nostrils until that cavity is fuU. The passage from the frontal sinus to the hiatus semilunaris is known as the " infundibulum," and into this open the " anterior ethmoidal cells," so that the antrum acts as a sink for the secretion of these cells and of the frontal sinus. Running downward and forward from the front of the middle turbinated bone is a curved ridge known as the " agger nasi," which forms the anterior boundary of a slightly depressed area called the " atrium."

The " inferior meatus" is below the inferior turbinated bone, and, when that is Ufted up, the valvular opening of the nasal duct (see Eye) is seen. In front of the inferior meatus there is a depression just above the nostril which is Uned with skin instead of mucous membrane and from which short hairs grow; this is called the " vestibule." The roof of the nose is very narrow, and here the olfactory nerves pass in through the cribriform plate. The floor is a good deal wider so that a coronal section through each nasal cavity has roughly the appearance of a rightangled triangle. The anterior wall is formed by the nasal bones and the upper and lower lateral cartilages, while posteriorly the sphenoidal turbinated bone separates the nasal cavity from the sphenoidal sinus above, and below there is an opening into the naso-pharynx known as the " posterior nasal aperture " or " choana." The mucous membrane of the outer wall is characteristic of the respiratory tract as high as the superior turbinated bone; it is ciliated all over and very vascular where it covers the inferior turbinated ; superficial to and above the superior turbinated the olfactory tract is reached and the speciahzed olfactory epithelium begins.


In the third week of intra-uterinc life two pits make their appearance on the under side of the front of the head, and are known as the olfactory or nasal pits; they are the first appearance of the true olfactory region of the nose, and some of their epithelial lining cells send off a.xons (see Nervous System) which arborize with the dendrites of the cells of the olfactory lobe of the brain and so form the olfactory nerves (see J. Disse, Anat. Hefte, 1897; also P. Anat. Soc, J. Anat. and Pliys., 1897, p. 12). Between the olfactory pits the broad median fronto-nasal process grows down from the forehead region to form the dorsum of the nose (see fig. 3), and the anterior part of the nasal septum, while outside them the lateral nasal processes grow down, and later on meet the maxillary processes from the first visceral arch. In this way the nasal cavities are formed, but for some time they are separated from the mouth by a thin bucconasal membrane which eventually is broken through; after this the mouth and nose are one cavity until the formation of the palate in the third month (see Mouth and Salivary Glands). In the third month Jacobson's organ may be seen as a wellmarked tube lined with respiratory mucous membrane and running upward and backward, close to the septum, from its orifice, which is just above the foramen of Stensen in the anterior palatine canal. In man it never has any connexion with the olfactory membrane or olfactory nerves. Internally and below it is surrounded by a delicate sheet of cartilage, which is distinct from that of the nasal septum. No explanation of the function of Jacobson's organ in man is known, and it is probably entirely atavistic. At birth the nasal cavities are very shallow from above downward, but they rapidly deepen till the age of puberty. The external nose at birth projects very little from the plane of the face except at the tip, the button-like shape of which in babies is well known. In the second and third year the bridge becomes more prominent, but after puberty the nasal bones tend to tilt upward at their lower ends to form the eminence which is seen at its best in the Roman nose. (For further details see Quain's Atialomy, vol. i., London, 1908.)

Comparative A natomy. In Amphioxus among the Acrania there is a ciliated pit above the anterior end of the central nervous system, which is probably a rudiment of an unpaired olfactory organ. In the Cyclostomata (lampreys and hags) the pit is at first ventral, but later becomes dorsal and shares a common opening with the pituitary invagination. It furthermore becomes divided internally into two lateral halves. In fishes there are also two lateral pits, the nostrils of which open sometimes, as in the elasmobranchs (sharks and rays), on to the ventral surface of the snout, and sometimes, as in the higher fishes, on to the dorsal surface. Up to this stage the olfactory organs are mere pits, but in the Dipnoi (mud-fish) an opening is established from them into the front of the roof of the mouth, and so they serve as respiratory passages as well as organs for the sense of smell. In the higher Amphibia the nasal organ becomes included in the skull and respiratory and olfactory parts are distinguished. In this class, too, turbinal ingrowths are found, aiid the naso-lachrymal duct appears. In the lizards, among the Reptilia, the olfactory and respiratory parts are very distinct, the latter being lined only by stratified epithelium unconnected with the olfacton,- nerves. There is one true turbinal bone growing from the outer wall, and close to this is a large nasal gland. In crocodiles the hard palate is formed, and there is henceforward a considerable distance between the openings of the external and internal nares. In this order, too (Crocodilia)

-the Turbinated Bones having been removed.

6. Opening of anterior ethmoidal cells.

7. Cut edge of superior turbinated bone.

8. Cut edge of middle turbinated bone.

9. Pharyngeal orifice of Eustachian tube.

air sinuses are first found extending from the olfactory cavities into the skull-ljones. The birds' arrangement is very like that of the reptiles; olfactory and respiratory chambers are present, and into the latter projects the true turbinal, though there is a pseudo-turbinal in the upper or olfactory chamber. In mammals the olfactory chamber of the nose is variously developed; most of them are " macrosmatic," and have a large area of olfactory mucous membrane; some, like the seals, whalebone whales, monkeys and man are " microsmatic," while the toothed whales have the olfactory region practically suppressed in the adult, and are said to be " anosmatic." There are generally five turbinal bones in macrosmatic marnmals, so that man has a reduced number. The lowest of the series or " maxillo-turbinal " is the equivalent of the single true turbinal bone of birds and reptiles, and in most mammals is a double scroll, one leaf turning upward and the other down. Jacobson's organ first appears in amphibians, where it is found as an anteroposterior gutter in the floor of the nasal cavity, sometimes being close to the septum, at other times far away, though the former position is the more primitive. In reptiles the roof of the gutter closes in on each side, and a tube is formed lying below and internal to the nasal cavity, opening anteriorly into the mouth and ending by a blind extremity, posteriorly to which branches of the olfactory and trigeminal nerves are distributed. In the higher reptiles (crocodiles and chelonians) the organ is suppressed in the adult, and the same applies to birds; but in the lower mammals, especially the monotremes, it is very well developed, and is enclosed in a cartilaginous sheath, from which a turbinal process projects into its interior. In other mammals, with the exception of the Primates and perhaps the Chiroptera, the organ is quite distinct, though even in man, as has been shown, its presence can be demonstrated in the embryo. The special opening through which it communicates with the mouth is the foramen of Stensen in the anterior palatine canal.

See J. Symington on the organ of Jacobson in the Ornithorynchus, P. Zool. Soc. (1891), and in the kangaroo, J. Anal, and Phys., vol. 26 (1891); also G. Eliot Smith on Jacobson's organ, Anatom. Anseiger, xi. Band No. 6 (1895). For general literature on the comparative anatomy of the olfactory system up to 1906, see R. Wiedersheim's Comparative Anatomy of Vertebrates, translated and adapted by W. N. Parker (London, 1907;. (F. G. P.)

Fig. 3. - From A. H. Young and A. Robinson, in Cunningham's Text-Book of Anatomy.

I. Side view of the head of human embryo about 27 days old, showing the olfactory pit and the visceral arches and clefts (from His).
II. Transverse section through the head of an embryo, showing the relation of the olfactory pits to the forebrain and to the roof of the stomatodaeal space.
III. Head of human embryo about 29 days old, showing the division of the lower part of the mesial frontal process into the two globular processes, the intervention of the olfactory pits between the mesial and lateral nasal processes, and the approximation of the maxillary and lateral nasal processes, which, however, are separated by the oculo-nasal sulcus (from His).
IV. Transverse section of head of embryo, showing the deepening of the olfactory pits and their relation to the hemisphere vesicles of the fore-brain.

Diseases of Olfactory System

External Affections and Injuries of the Nose. - Acne rosacea is one of the most frequent nasal skin affections. In an early stage it consists of dilatation or congestion of the capillaries, and later of a hypertrophy of the sebaceous follicles. This may be accompanied by the formation of pustules. In an exaggerated stage the sefmceous glands become overgrown, forming l.irge protuberant nodular masses over which the dilated capillaries are plainly visible. This condition is termed lipoma nasi (rhinophynia or hammer nosej, though there is no increase in fatty tissue. Nasal acne occurs mainly in dyspeptics and tea drinkers, and the more advanced condition, lipoma nasi, chielly in elderly men addicted to alcoholism. The treatment of acne is the removal of the dyspepsia with the local application of sulphur ointment or of a lotion of perchloride of mercury. Unsightly capillaries may be destroyed by an application of the galvano-cautery or by electrolysis. Free dissection of the redundant tissue from around the nasal cartilages is necessary in lipoma nasi, skin being grafted on to the raw surface. The nasal bones are frequently fractured as the result of direct violence, as by a blow from a cricket ball or stick. The fracture is usually transverse, and may be communicated, leading to much deformity if left untreated. The treatment is the immediate reposition of the bony fragments. The old-standing cases where tliere is considerable depression Cerebral wiring the fragments may be resorted to. hemi- In numerous cases the subcutaneous Sphere injection of paraffin may improve the shape of the organ. Deflection of the septum may also result from similar injuries, and lateral displacement may cause subsequent nasal obstruction and -,, require the straightening of the septum, pit^*^ ° Lesions involving considerable loss of substance due to injury or to syphilitic or tuberculous disease have led to many methods being devised to supply the missing part. In the Indian method of rhinoplasty a flap is cut from the forehead, to which it is left attached by a pedicle; the flap is then turned downwards to cover the missing portion of the nose; when the parts have united, the pedicle is cut through. In the Italian operation devised by Tagliacotius (Tagliacozzi), a flap was taken from the patient's arm, the arm being kept fixed to the head until the flap has united.

Diseases of the Interior of the Nose. - Epistaxis or bleeding of the nose may arise from many conditions. It is particularly common in young girls at the time of puberty, being a form of vicarious menstruation. It also occurs in cerebral congestion, heart disease, scurvy, haemophylia, or as a sign of local disease. The treatment will depend upon the cause. In patients with high arterial tension epistaxis may be of direct benefit. In other cases rest on the back may be tried, %vith the local application of tanno-gallic acid or hazelin or adrenalin, either in a spray or on absorbent cotton. If these should not stop the haemorrhage the nose must be plugged. In cases which arise from specific forms of ulceration, such as tuberculosis and syphilis, the area should be rendered anaesthetic by cocaine, the bleeding points found, and the vessels obliterated by the electrocautery. Polypi in the nasal passages are also a frequent cause of epistaxis.

Rhinitis, or inflammation of the mucous membrane of the nose, occurs both in acute and chronic forms. Of the acute the simple catarrhal form termed " coryza " forms the widely known " cold in the head." The tendency of acute cor>'za to affect entire families, and to be communicable from one person to another, points to its infectious nature, though probably some predisposing condition of health is necessary for its development. It is considered proved that the symptoms are due to the presence and development of several distinct micro-organisms. Of these the most important is the micrococcus catarrhalis described by Martin Kirchner in 1890, but Friedlander's pneumo-bacillus has also been found. In ordinary cases of coryza, sneezing, congestion of the nasal mucous membrane and a profuse watery discharge usher in the attack, and the inflammation may extend to the pharynx, larynx and trachea, blocking of the Eustachian tube producing a temporary deafness. Later the discharge may become muco-purulent. One attack of coryza conveys no immunity from subsequent attacks, and some persons seem particularly susceptible. The treatment is directed towards increasing the action of the kidneys, skin and bowels. A brisk mercurial purgative is indicated, and salicin and aspirin are useful in many cases. Considerable relief may be obtained by washing out the nasal cavities several times a day with a warm lotion containing boric acid. Those who are unusually prone to catch cold should habituate themselves to an open air life by day and an open window by night, adenoids or enlarged tonsils should be removed, and the diet should be modified so as not to contain an excess of starchy foods. An acute croupous inflammation occasionally attacks the nasal mucous membrane when the Klebs-Loffler bacillus is not present, but the nasal membrane often shares in true diphtheria, or it may be the only organ to be infected thereby. The diagnosis is of course bacteriological.

As a result of frequent catarrhal attacks the nasal mucous membrane may become the seat of a chronic rhinitis in which the turbinals become swollen with oedema, and congested and finally thickened by increase in the fibrous tissue. There is an excessive muco-purulent discharge, and the patient is unable to breathe through the nose; deafness and adenoid vegetations may be the result. In the early stages the nasal cavity should be washed out night and morning with an alkaline lotion, such as bicarbonate of soda, or a caustic, such as chromic acid, should be used in swabbing over the affected part. The application of the galvano-cauter>' here is useful, but when the areas are much hypertrophied the hypertrophied portion of the inferior turbinals may have to be removed under cocaine. A special form of recurrent hypertrophic rhinitis is hay fever (q.v.).

Rhinitis Sicca is a form of chronic rhinitis in which there is but little discharge, crusts or scabs which may be difficult to remove forming in the nasal cavities; the pharynx may be also affected.

Atrophic rhinitis or ozaena usually attacks children and young adults, following on measles or scarlet fever. Crusts form, and favour the retention of the purulent discharge. The disease may e-xtend to the nasal sinuses and septic absorption take place. The treatment is to keep the nasal cavity clean by irrigation with solution of permanganate of potash or carbolic acid lotion, the nose then being wiped and smeared with lanolin or partially plugged with a tampon of cotton-wool, the process being repeated at frequent intervals, the general treatment being that for anaemia. Disease of the middle turbinated bone is also a cause of an offensive nasal discharge, and rhinitis occurring in infants gives rise to the obstructed respiration known as " the snuffles."

Three forms of nasal polypi are described, the mucous, the fibrous and the malignant. The general symptoms of nasal polypus are a feeling of stuffiness in one or both nostrils, inability to breathe down the nose and a thin watery discharge. A nasal tone of voice, together with cough and asthma, may be present, or there may be partial or complete loss of the sense of smell (anosmia). The treatment of mucous polypi is their removal by the forceps or the snare, the base of the growth being afterwards carefully examined and cauterized with the galvano-cautery.

Fibrous polypi are usually very vascular, and may be a cause of severe epistaxis as well as of obstruction of breathing, " dead voice," sleepiness and deafness. The increasing growth may lead to expansion of the bridge of the nose and deformity of the facial bones, known as " frog-face." The tendency of fibrous polypi to take on malignant sarcomatous characters is specially noticeable. Extirpation of the growth as soon as its nature is recognized is therefore urgently demanded.

The chief diseases of the nasal septum are abscesses, due to the breaking down of haematomata, syphilitic gummata (leading to deep excavation and bony destruction), tuberculous disease in which a small yellowish grey ulcer forms and what is known as perforating ulcer of the septum, which is met with just within the nostril. The latter tends to run a chronic course, and the detachment of one of its crusts may cause epistaxis. Rhinoscleroma was first described by F. Hebra in 1870, and is endemic in Russian Poland, Galicia and Hungary, but is unknown in England, except amongst alien immigrants. The infecting organism is a specific bacillus, and the disease starts as a chronic smooth painless obstruction with the formation of dense plate-like masses of tissue of stony hardness. Treatment other than that of excision of the masses has proved useless, though the recent plan of introduction of the injection of a vaccine of the bacillus may in future modify the progress of the disease.

The accessory sinuses of the nose are also prone to disease. The maxillary antrum may become filled with muco-pus, forming an empyema, pus escaping intermittently by way of the nose. The condition causes pain and swelling, and may require the irrigation and drainage of the antrum. The frontal sinuses may become filled with mucous, owing to the swelling of the nasal mucous membrane over the middle turbinated bone, or an acute inflammation may spread to the frontal sinuses, giving rise to an empyema in that locality. There is severe frontal pain, and in some cases a fulness on the forehead over the affected side, the pus often pointing in this site, or there may be a discharge of pus through the nose. The treatment is that of incision and irrigation of the sinus (in some cases scraping out of the sinus) and the re-establishment of communication with the nose, with free drainage. The ethmoidal and sphenoidal sinuses are also frequently the site of empyemata, giving rise to pain in the orbit and the back of the nose, and a discharge into the nasopharynx. In the case of the ethmoidal sinus it may give rise to ' exophthalmus and to strabismus (squint), with the formation of a tumour at the inner wall of the orbit and fever and delirium at night. In the young the condition may become rapidly fatal. Suppuration in the sphenoidal sinus may lead to blindness from involvement of the sheath of the optic nerve, and dangerous complications such as septic basal meningitis and thrombosis of the cavernous sinus may occur. Acute ethmoiditis and sphenoiditis are serious conditions demanding immediate surgical intervention. (H. L. H.)

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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