SLEEP is a sensation excited by the contact with the olfactory region of certain substances, usually in a gaseous condition and necessarily in a state of fine subdivision. The sense is widely distributed throughout the animal kingdom. The lower animals, especially those breathing in water, become cognizant of the presence of odoriferous matter near them without touch, vision, or hearing, and we suppose that they do so by some sense of taste or smell, or a combination of both. In such cases smell has been appropriately termed "taste at a distance," by which is meant that particles of matter may be diffused through the water so as to come into contact with the terminal organ and give rise to a sensation such as would have been excited had the matter from which the particles emanated come directly into contact with the nerve-endings. It is therefore of no great importance whether such sensations in humble aquatic organisms are termed taste or smell. In the higher air-breathing animals, however, the senses are differentiated: that of taste is found at the entrance of the alimentary canal, whilst that of smell guards the opening of the respiratory tract. This view assists in the interpretation of various structures met with in the lower forms which have been fairly regarded by naturalists as olfactory organs.
Comparative View of Olfactory Organs.In various Medusas pit-like depressions, lined with ciliated epithelium, on the dorsal side of the excavation in which the "marginal" bodies are found, have been called olfactory regions. In many Arthropoda the sense of smell is located in delicate tubular structures, or conical projections, found on the antennae and connected with nerves. Similar organs are met with in Crustacea. In Cyclops (Copepoda), Isopoda, and Thoracostraca olfactory hairs are present as delicate appendages of the anterior antennae, chiefly in the male sex. In Schizopoda the anterior antennae have a comb-like prominence bearing a great number of olfactory hairs. Insecta have olfactory organs largely developed, usually in the form of hairs, coues, or knobs on the antennae, and connected with gangliated nerve-endings. Olfactory organs are also met with in Mollusca: in Lamelli-branchiata they appear as hairs on the margin of the mantle; in aquatic Gasteropoda as tufts of hairs scattered over the surface of the body and specially aggregated in those parts where tactile sensibility is highly developed; in terrestrial Gasteropoda the antennae have on their end plates a number of club-shaped cells with rods, which are held to be olfactory, and recently in the same class Sprengel has shown that an organ "which was supposed to be a rudimentary gill, and is innervated from the supra-intestinal ganglion," has an olfactory function. In Ascidians the olfactory region is believed to be a depression on the wall of the pharynx, situated in front of the ganglion, and lined with ciliated epithelium.
In Fishes the olfactory organs consist of a membrane (the pituitary membrane) lining one or two pits, to which the olfactory or first pair of cranial nerves are distributed. This highly vascular membrane is usually thrown into numerous folds, so as to admit of an extensive surface being packed into small space, and it is covered by ciliated epithelium. In the lowest vertebrate, Amphioxus, the olfactory organ is a simple unsymmetrical pit at the anterior end of the nervous system. In the hag fishes (Myxinidae) the olfactory pit has a posterior opening which pierces the palate and can be closed by a valvular apparatus. In the lampreys (Petromyzon) the flask-shaped nasal sac opens on the top of the head, and from this a tube descends which expands into a blind sac towards the base of the skull. In all other iishes the olfactory organs are double and have no communication with the mouth. In osseous fishes the olfactory capsules or sacs are covered with skin which is usually pierced by two openings for each sac. Some, such as the wrasses, have a single nasal opening; and where there are two the anterior can be closed by a valve. The olfactory region may be extensive owing to the pituitary membrane being thrown into plaits or folds, and it may be divided into two portions, one quite smooth and the other plicated. The smooth portion, prob-ably acting as a reservoir, may be large, extending down to the palate, as in the mackerel, or to the back part of the palate, as in the wolf-fish (Owen). The nasal cavities exist below the snout in sharks, near the angles of the mouth in the rays, and beneath the fore part of the head, behind the base of the rostrum, in the saw-fish. In such fishes the olfactory organ is guarded by valves, con-taining cartilaginous plates moved by muscles, and we may there-fore conclude with Owen "that these fishes scent as well as smell, i.e., actively search for odoriferous impressions by rapidly changing the current of water through the olfactory sac."
The olfactory organs of Amphibia are always paired cavities, opening internally either anteriorly within the lips or further back, as in the batrachians and salamandrines. In the Perennibranchiates (Siren, Proteus, Axolotl) there are no outward signs of olfactory organs, and the thick upper lip must be raised to bring the plicated sac with its two remote orifices into view (Owen). In the Tritonidae (newts) and Salamandrinae (salamanders) the olfactory membrane is smooth and lines an oval bag having an external nostril, guarded by a valvular fold of skin, and a palatal opening. Frogs and toads (Batrachia) have also an external nostril with a flap of skin, and the palatal opening is wide and near the fore part of the mouth. The skulls of extinct saurians of marine habits (Ichthyosaurus and Plesiosaurus) show that the external nostrils opened near the orbits at a distance from the muzzle. In snakes (Ophidia) the external nostrils are double, and the internal nostril is single and in the median line. In water snakes the external orifices can bo closed by valves.
In Chelonia (turtles, tortoises) and in Crocodilia the external nasal opening is single and near the end of the snout; but in Chelonia the nostrils are really distinct, although their external apertures coincide. In the turtle the nasal cavity is large and contains a twisted shell-like cartilage, so as to give extent of surface to the darkly pigmented and highly vascular pituitary membrane. In the crocodiles and alligators the nostrils can be closed by a valvular lobe, and in the gaviáis (Rhamphostoma gangeticum and Rhynchosuchus schlegelii) the integument can be raised round the nostril in the form of a tube so as to bring the orifice to the surface of the water without exposing the other parts of the head (Owen). In all Crocodilia the nasal cavity is of great length, commencing at the fore part of the muzzle and ending beneath the occiput by a single aperture, and the surface of this long olfactory meatus is increased by the meatus communicating with large cells or sinuses. In snakes and lizards a second olfactory organ is found embedded between the turbinals and the vomer and is known as "Jacobson's organ." It has the form of a cup or depression round a cartilaginous papilla and is supplied by a nerve which arises from the end of the olfactory lobe.
The olfactory organs of Birds are somewhat similar to those of the cold-blooded reptiles and amphibians in that "the external nostrils are simple perforations, having no movable cartilages or muscles provided for dilating or contracting their apertures, as in mammalia" (Owen). The extent of the olfactory surface is increased by projections and folds of turbinated bones and not by large accessory cavities. With the exception of the apteryx and dinornis, the olfactory nerve passes out of the skull by a single foramen. The external nostrils are in the majority of birds placed at the sides of the upper mandible ; but in some cases, as in the toucans, they are found at the base of the bill, and in the apteryx they open at the extremity of the long upper mandible. In herons the apertures are so small as scarcely to admit the point of a pin ; and in the pelicans they are wanting, and odours get access to the olfactory organ from the palate. The Rasores (scratching birds) have the nostrils defended by a scale, and the crows (Corvidae) have a bunch of stiff feathers for the same purpose. The septum or partition between the nostrils is usually complete and is formed of bone and cartilage. The outer wall of each nasal passage is furnished with three turbinal or twisted shell-like bones, of which the middle is the largest, thus affording a considerable extent of olfactory surface. In most birds there are two posterior nasal apertures communicating with the palate ; but in some, as in the cormorant and gannet, the passages unite and there is only one opening. In birds the upper part of the nasal passage is more especially devoted to the sense of smell, whilst the lower part may be regarded as the beginning of the respiratory tract. This is in-dicated by the arrangement of the nerves, the olfactory nerve being distributed to the membrane covering* the septum and the superior and middle turbinated bones, whilst the lower portion and lower turbinals are supplied by the fifth nerve,a nerve of general sensi-bility. The upper turbinals reach their greatest development in the apterj'x, where they are attached, according to Owen, to the whole outer part of the prefrontals. This bird has amongst birds the largest olfactory nerves in proportion to its size, and it would appear to be guided by the sense of smell to the worms that form its food. A contrast as regards the anatomical arrangements for the olfactory sense is well seen on comparing the turkey with the vulture. In the turkey the olfactory nerve is small, about one-fifth the size of that in the vulture, and is distributed over a small middle turbinal, there being no extension over a superior turbinal. The vulture, on the other hand, has a large nerve and the olfactory region is extensive, owing to the largely developed superior turbinal bone. There can be no doubt that the carrion - eating vulture is guided from great distances to its food by the sense of smell, although it will be assisted by its powerful sense of vision.
The sense of smell reaches its highest development in Mammalia. The anatomical surface is enormously extended in many cases, not only by the complication of the ethmoidal labyrinth, but also by the nasal passages communicating with spaces in the neighbouring cranial and facial bones. The olfactory nerves also are very numer-ous and arise from a special encephalic centre. They pass out of the skull by numerous holes in the cribriform or sieve-like plate of the prefrontal bone, which, on account of this peculiarity, is called the ethmoid bone. These nerves ramify on the olfactory membrane, covering the upper or ethmo-turbinal bones. The cavity containing the organ of smell is bounded by the prefrontal, vomerine, nasal, sphenoid, pterygoid, palatine, maxillary, and premaxillary bones, and it is usually in connexion with air-cavities or sinuses in many or all of the bones of the skull. The median partition by which the two nostrils are formed consists of bone and cartilage and is built up by processes of the prefrontals, the vomer, and by the ridges of the nasals, palatines, maxillaries, and premaxillaries with which the vomer articulates. Each passage thus formed is the beginning of the respiratory tract, and is continued forwards into a more or less mobile part called a nose, snout, or proboscis, whilst posteriorly it communicates with the upper part of the pharynx, into which opens the windpipe. On the outer wall there are three turbinal bonessuperior, middle, and inferiordividing partially the nasal cavity into three meatuses or passages. The superior meatus is between the superior and middle turbinated bones, the middle meatus between the middle and inferior turbinated bones, and the inferior meatus between the inferior turbinated bones and the floor of the nose (see ANATOMY, vol. i. p. 823, fig. 7 ; also vol. i. pi. XIX. fig. 2). Many of the lower mammals have in addition a process from the frontal and nasal bones, sometimes called the superior spongy bone, which is not the same as the superior turbinated, as described in the anatomy of the human being. The extent of olfactory surface is enormously increased by numerous plicae or processes of bone which to a great extent mask the comparatively simple arrangement above described. In Ornithorhynchus there is a single olfactory nerve escaping through an aperture in the prefrontal bone; in Echidna, the other member of the Monotremata, there are numerous olfactory nerves and a largo development of ethmo-turbinals. In many Marsupials the sense of smell is largely developed, and in some (Osphranter) the turbinated bones are so large as to cause a lateral bulging of the nasal cavity, forming a marked feature of the skull. In Rodents the ethmo-turbinals may be subdivided into lamellae so as to increase the olfactory surface ; such is the case in the common hare. In the porcupine the sinuses developed from the olfactory cavity are of large size, forming a spongy mass surrounding the cavity of the skull in which the anterior portion of the brain lies. In Inscctivora the olfactory surface is very large. Thus in the mole the ethmo-tur-binal has not fewer than eight lamellae or plates and the external nose is developed into a snout capable of considerable movement. Such a snout is very large and mobile in the elephant shrews. Armadillos and ant-eaters (Edentata) have a strong sense of smell. Thus in Dasypus the nasal portion of the skull is about equal in volume to all the rest, and in Chlamydophorus (dwarf armadillo) the frontals are raised "into a pair of domes" by sinuses in them com-municating with the large olfactory cavity. In most armadillos the external nose is strengthened by small bones. The air sinuses in the sloth extend upwards into the frontals and downwards into the sphenoid bone. No Cetaceans have olfactory organs, except the baleen or whalebone whales, and thus are devoid of the sense of smell. In the manatee (Sirenia) the nasal openings are placed far forwards and have movable cartilages, and the bony walls of the nasal passages are not extensive in proportion to the size of the rest of the skull. The elephants (Proboscidea) have the part of the nasal cavity concerned in smell contracted and narrow, but the cavity is prolonged into the trunk, at the end of which are the nostrils; the nasal cavity communicates with sinuses permeating every bone of the cranium. The tapirs have a shorter but very mobile proboscis, and the development of the nasal passages is extensive. The horse has the power of dilating and contracting each nostril, and the cribriform plates transmit very numerous olfactory nerves from the olfactory bulbs, which are large in proportion to the size of the rest of the brain. The Suidse (swine) have a large and complex olfactory region; the accessory sinuses or spaces attain a great development; the nose is prolonged and truncate, the cartilages forming a complete tube, which is a continuation of the bony nostrils, and these tubes open on a naked disk. In the ox and sheep the olfactory region is large, but not so large as in the horse. The external, glandular, and moist part of the nose is a linear tract running from the mid-furrow of the upper lip to the oblique nostril in the sheep, and this portion passes through many gradations in size, as seen in the roebuck, fallow-deer, red-deer, and the ox. The Carnívora have the ethmo-turbinal and maxillo-turbinal regions even more largely developed than in Herbívora, and the latter portion reaches its maximum in the seals, where "these turbinals seem to block up the entry of the nasal respiratory passages, and must warm the air in arctic latitudes as well as arrest every indication from the effluvia of alimentary substances or prey" (Owen). In Quadrumana the nasal chamber becomes shorter and gains in depth, but not proportionally. In the platyrhine monkeys the cartilage forming the septum becomes flattened anteriorly, pushing the nostrils outwards. In the catarrhines this flattening is much less, so that the nostrils are approximated. In both groups the nostrils are not terminal. In Man the chief characteristic is the prominence of the fore part of the chambers, with the nostrils on the lower surface, and the nose is supported by eleven pieces of cartilage, of which one is medial, the others lateral, in five pairs. The size and form of the septal or medial cartilage mainly determine the shape and prominence of the nose. It is least developed but thickest in the Negro and Papuan races. (For a description of the muscles of the nose in man, see ANATOMY, vol. i. p. 837.)
The interior of the nose is divided physiologically into two portions,(1) the upper (regio olfactoria), which embraces the upper part of the septum, the upper turbinated bone, and a portion of the middle turbinated bone; and (2) the lower portion of the cavity (regio respiratoria). The olfactory region proper has a thicker mucous membrane than the respiratory; it is covered by a single layer of epithelial cells, often branched at their lower ends and containing a vellow or brownish red moment: and it contains peculiar tubular glands named "Bowman's glands."
Longitudinal section through the olfactory membrane of guinea-pig. x about 400. 1, Olfactory epithelium on free surface ; 2, plexus of olfactory nerve-fibres ; 3, pouches of serous glands containing epithelial cells. From Klein's Atlas of Histology.
The respiratory portion contains ordinary serous glands. In the olfactory region also are the terminal organs of smell. These are long narrow cells passing to the surface between the columnar epithelium covering the surface. (See ANATOMY, vol. i. p. 885, fig. 76.) The body of the cell is spindle-shaped and it sends up to the surface a delicate rod-like filament, whilst the deeper part is continuous with varicose nerve-filaments, the ends of the olfactory nerve. In the frog the free end terminates in fine hairs.
Physical Causes of Smell.Electrical or thermal stimuli do not usually give rise to olfactory sensations. Althaus states that electrical stimulation caused a sensation of the smell of phosphorus. To excite smell it is usually supposed that substances must be present in the atmosphere in a state of fine subdivision, or existing as vapours or gases. The fineness of the particles is remarkable, because if the air conveying an odour be filtered through a tube packed with cotton wool and inserted into the nose a smell is still discernible. This proceeding completely removes from the air organisms less than the 1/100000th of an inch in diameter which are the causes of putrefaction and fermentation. A grain or two of musk will scent an apart-ment for years and at the end of the time no appreciable loss of weight can be detected. Substances exciting smell are no doubt usually gases or vapours. Only a few ten-tative efforts have been made to connect the sense with the chemical constitution of the substance. One of the most important of these is in an Essay on Smell, by Dr. William Ramsay of University College, Bristol. The following gases have no smell:hydrogen, oxygen, nitrogen, water gas, marsh gas, defiant gas, carbon monoxide, hydro-chloric acid, formic acid vapour, nitrous oxide, and ammonia, (It is necessary, of course, to distinguish between the sensa-tion of smell and the irritant action of such a gas as ammonia.) The gases exciting smell are chlorine, bromine, iodine, the compounds of the first two with oxygen and water, nitric peroxide, vapours of phosphorus and sulphur, arsenic, antimony, sulphurous acid, carbonic acid, almost all the volatile compounds of carbon except those already mentioned, some compounds of selenium and tellurium, the compounds of chlorine, bromine, and iodine with the above-named elements, and some metals. Chlorine, bromine, iodine, sulphur, selenium, and tellurium, which are volatile and give off vapour at ordinary temperatures, have each a characteristic smell. Ramsay points out that as a general rule substances having a low molecular weight have either no smell or simply cause irritation of the nostrils. He also shows that in the carbon compounds increase of specific gravity as a gas is associated to a certain point with a sensation of smell. Take the marsh gas or methane series commonly called the paraffins. The first two have no smell; ethane (fifteen times as heavy as hydrogen) has a faint smell; and it is not till butane (thirty times heavier than hydrogen) that a distinct sensation of smell is noticed. Again, a similar relation exists among the alcohols. Methyl alcohol has no smell. Ethyl, or ordinary alcohol free from ethers and water, has a faint smell; "and the odour rapidly becomes more marked as we rise in the series, till the limit of volatility is reached, and we arrive at solids with such a low vapour tension that they give off no appreciable amount of vapour at the ordinary temperature.''' Acids gain in odour with increase in density in the form of gas. Thus formic acid is devoid of smell; acetic acid has a characteristic smell; and the higher acids of the seriespropionic, butyric, valerianicincrease in odour. It would appear also that " the character of a smell is a property of the element or group which enters into the body producing the smell, and tends to make it generic." Many compounds of chlorine, hydrogen, compounds of sulphur, selenium, and tellurium, the paraffins, the alcohols, he acids, the nitrites, the amines, the pyridine series, the benzene group, have each a characteristic odour. Ramsay has advanced the theory that the sense of smell "is excited by vibrations of a lower period than those which give rise to the sense of light or heat," and he points out a series of important facts in support of this view. He states that to produce the sen-sation of smell a substance must have a molecular weight at least fifteen times that of hydrogen. For instance, the specific gravity of marsh gas is eight (no smell), of ethane fif-teen (faint smell), of propane twenty-two (distinct smell). Again, prussic acid has a specific gravity of fifteen, and many persons fail to detect its odour. Further, Ramsay sup-poses that smell may be excited by vibrations, and suggests that the period of vibration of the lighter molecules is too rapid to affect the sense ; at last a number of vibrations is reached capable of exciting the sense organ; and beyond an upper limit the sense is again lost. Graham pointed out that odorous substances are in general readily oxidized. [168-1] Tyndall showed that many odorous vapours have a con-siderable power of absorbing heat. Taking the absorptive capacity of the air as unity, the following absorptions were observed in the respective cases:
== TABLE ==
In comparison with the air introduced in the experi-ments the weight of the odours must be almost infinitely small. " Still we find that the least energetic in the list produces thirty times the effect of the air, whilst the most energetic produces 109 times the same effect." [168-2]
Venturi, B. Prévost, and Liégeois have studied the well-known movements of odoriferous particles, such as camphor, succinic acid, &c, when placed on the surface of water, and they have suggested that all odoriferous sub-stances in a state of fine subdivision may move in a similar way on the moist surface of the olfactory membrane, and thus mechanically irritate the nerve-endings. This explanation is too coarse ; but it is well known that the odours of flowers are most distinctly perceived in the morning, or after a shower, when the atmosphere contains a considerable amount of aqueous vapour. It would appear also that the odours of animal effluvia are of a higher specific gravity than the air, and do not readily diffuse,a fact which may account for the pointer and bloodhound keeping their noses to the ground. Such smells are very persistent and are apparently difficult to remove from any surface to which they have become attached. The smell of a corpse may haunt a living person for days, notwithstanding copious ablutions and change of clothes. [168-3]
Special Physiology of Smell.It is necessary that the air containing the odour be driven forcibly against the membrane. Thus the nostrils may be filled with eau de Cologne, or with air impregnated with sulphuretted hydrogen, and still no odour is experienced if the person does not breathe. When a sniff is made the air within the nasal passages is rarefied, and, as the air rushes in to equili-brate the pressure, it is forcibly propelled against the olfactory surface. The olfactory surface must be moist ; if it is dry, or is covered with too thick a layer of mu-cus (as in catarrh), the sense is much weakened or lost.
The first moment of contact is the most acute and the sense quickly becomes blunted. The first scent of a flower is the strongest and sweetest ; and after a few minutes' ex-posure the intensity of even a ftid odour may not be perceived. This fact may be accounted for on the sup-position that the olfactory membrane becomes quickly coated with a thin layer of matter, and that the most intense effect is produced when the odoriferous substances are applied to a clean surface. The intensity of smell depends on (1) the area of olfactory surface affected, and (2) the degree of concentration of the odoriferous matter. It is said that musk to the amount of the two-millionth of a milligramme, and one part of sulphuretted hydrogen in 1,000,000 parts of air, may be perceived. If the two nostrils are filled with different odorous substances, there is no mixture of the odours, but we smell sometimes the one and sometimes the other (Valentin). Morphia, mixed with sugar and taken as snuff, paralyses the olfactory apparatus, while strychnine makes it more sensitive (Lichtenfels and Frôhlich).
The delicacy of the sense is much greater in many of the lower animals than in man, and it is highly probable that the dog or cat obtain information by means of this sense which a human being cannot get. Odours may excite in the minds of many animals vivid impressions, and they have probably a memory of smells which the human being does not possess. Even in man the sense may be greatly improved by exercising it. A boy, James Mitchell, was born blind, deaf, and dumb, and chiefly depended on smell for keeping up a connexion with the outer world. He readily observed the presence of a stranger in the room and he formed his opinions of persons apparently from their characteristic smells. In some rare cases, the sense of smell is congenitally absent in human beings, and it may be much injured by the practice of snuffing or by diseases of the nose affecting the olfactory membrane. Subjective im-pressions of smells, like spectral illusions or sounds in the ears, are occasionally, but rarely observed in the insane. Finally, it may be observed that the sense of odour gives information as to the characters of food and drink and as to the purity of the air. In the lower animals, also, the sense is associated with the sexual functions.
[Further Reading] See art. "Olfaction" by François Franck, in Dictionnaire Encyclopédique des Sciences Médicales, 2d séries, where a full historical bibliography is given; Hermann's Handbuch der Physiologie: d. Sinnesorgane: Zweiter Theil, Geruchsinne, by Prof. V. Vintschgau, p. 226; Owen's Comp. Anatomy and Physiol. of Vertebrates; Bain, op. cit., p. 147 ; Grant Allen's Physiological Aesthctics, p. 77 ; Ramsay, Nature, vol. xxvi. p. 187 ; and for James Mitchell's case, see Dugald Stewart's Works, vol. iv. p. 300. (J. G. M.)
168-1 Bain, Senses and Intellect, 3d ed., p. 152.
168-2 Tyndall, Contributions to Molecular Physics in Domain of Radiant Meat, p. 99.
168-3 Liégeois, Archiv de Physiol., 1868.