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OLFACTORY SYSTEM

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Originally appearing in Volume V20, Page 79 of the 1911 Encyclopedia Britannica.
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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 (q.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." From R. Howden, in Cunningham's Text-Book of Anatomy. the Nose. 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 specialized 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 1. Vestibule. 6. Opening of anterior ethmoidal cells. 2. Opening of antrum of Highmore. 7. Cut edge of superior turbinated bone. 3. Hiatus semilunaris. 8. Cut edge of middle turbinated bone. 4. Bulla ethmoidalis. 9. Pharyngeal orifice of Eustachian tube. 5. Agger nasi. 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 " bulls ethmoidalis." 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 "7 8 OLFACTORY SYSTEM bones (see fig. 2). Above the superior turbinated is a space 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 specialized olfactory epithelium begins. Embryology. In the third week of intra-uterine life two pits make their appear- ance 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 axons (see NERVOUS SYSTEM) which arborize with the dendrites of the cells of the olfactory lobe Opening of middle ethmoidal cells of the brain and so form the olfactory Openings of posterior ethmoidal cells nerves (see J. Disse, Anat. Hefte, 1897; Recessus spheno•ethmoidalis also P. Anat. Soc., J. Anat. and Phys., 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 well-marked tube lined with respiratory mucous membrane and running upward and back-ward, 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 olfaotory 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 Anatomy, vol. i., London, 1908.) Cut edge of inferior turbinated bone Bristle passed into opening of nasal duct Sphenoidal air-sinus antrum and none escapes through the nostrils until that cavity is full. 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 lifted 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 lined 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 right-angled triangle. The anterior wall is formed by the nasal bones and the upper and lower lateral cartilages, while posteriorly Comparative Anatomy. 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, aad 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 olfactory 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) air sinuses are first found extending from the olfactory cavities into the skull-bones. 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 mammals, 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 Maxillary process Mandibular arch f / III From A. H. Young and A. Robinson, in Cunningham's Text-Book of Anatomy. FIG. 3. 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. 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. Zoo!. Soc. (1891), and in the kangaroo, J. Anat. and Phys., vol. 26 (1891); also G. Eliot Smith on Jacobson's organ, Anatom. Anzeiger, 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.)
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