Chemical Sense Organs

The chemical stimulation of taste and smell is brought about by contact with solutions of various substances. Liquids are tasted and gases are smelled. In order to taste a solid object it must first be reduced to liquid form. Even gases must be dissolved in a liquid before they can be smelled, since the delicate terminal hairs of olfactory receptors are effective only when surrounded by a lipoid film and by mucus or water in which odoriferous gases may be received.

The two sensations which we call smell and taste are served by at least ten or a dozen different kinds of receptors, each adequate for a particular distinct kind of stimulus.

Both of the groups of chemical receptors for tasting and for smelling are primarily located, like John Bunyan’s lions before the gates of the Palace Beautiful, near the entrance of the digestive tract, where they pass upon the character of the entering food. While the nasal and mouth cavities in which these chemical receptors are for the most part located are not actually on the outside of the body, they are nevertheless integumentary invaginations and so, fundamentally, a part of the cutaneous system.

Tasting is necessarily always a matter of direct contact, but smelling, which makes an animal aware of odorous substances even at some distance away, has the wider range, since it serves not only as a food censor, but also directs locomotion in the quest for food, as well as localizing the presence of other animals. It plays a particularly important role in sexual allurement, especially in mammals whose glandular skin sends forth many characteristic odors.

Response to chemical stimuli in some of the simplest animals is not associated with definite receptors. For example, the chemical receptivity of Paramecium is confined to the anterior end of the body, while in Stentor it is diffused over the entire outside of the body. The sensitivity of sea-anemones to chemical stimuli is limited to the tentacles, while the outside of the columnar body is unresponsive to chemical stimuli but responsive to mechanical contact.

Olfactoreceptors

Moth collectors know that when a female moth is confined to a wire cage at night, males come from considerable distances and can easily be captured while fluttering around the cage, attracted by an odor scarcely perceptible to man, that emanates from the body of the imprisoned female. The antennae of the males in this case bear the receptors of smell.

There are antennal receptors for at least three different kinds of chemical stimuli also that have been demonstrated in certain ants. The presence of enemies, or ants from foreign colonies, is detected by receptors located on the 6th and 7th joints of the antennae. Nest mates having the same odoriferous password are recognized by receptors on the 8th and 9th antennal joints, while the odor of the nest itself is registered by receptors on the 12th joint.

Olfactoreceptors of vertebrates are definite neurons which, although morphologically simple and undifferentiated, have become physiologically diversified so that they are adequate for a considerable variety of different chemical stimuli.

The evolution of the olfactory apparatus in vertebrates shows it to have been originally a primary sense of first importance that has gradually been replaced in the course of phylogeny by other receptors. In the lower vertebrates the olfactory centers overshadow all the rest of the brain. Such animals live in an olfactory world. It is apparent that the earliest crawling land animals, snuffling around close to the ground among ferns and club mosses and other rank vegetation, had much less use for eyes than for an olfactory apparatus. Later on, with increasing emancipation from lowly ground life, and with the rise and dominance of the major senses of sight and hearing, the olfactory sense has taken more and more to the background, until, in the case of mankind, the sense of smell may be sacrificed with less inconvenience and regret than any other. It still plays a role in appetite and sentiment, however, although human experience and memory dwell upon sights and sounds rather than smells.

Odors

In the open air odors, or volatile substances, though rarely emanating in all directions equally as sound and light tend to do, are particularly influenced by air currents that may be present. The farther away odorous gases are from the point of release, the more diluted they become and the more slowly they disperse (Stephan’s Law). Nevertheless extremely minute quantities of certain odors are perceptible at considerable distances from their origin, as every bloodhound attests.

Allison and Katz, in the Journal for Industrial Engineering for 1919, give a table in milligrams of minimum concentrations of various chemical substances that are perceptible to man in a liter of air. The quantity of chloroform is given as 3.3; iodoform, 0.018; propyl-mercaptan, 0.006; and artificial musk, which is by far the “smelliest” known substance that was tested, 0.00004 milligrams. No one of the above tested substances is encountered freely in nature.

The diffusion of odorous substances through water is very much slower than through air, yet in spite of this fact sharks are known to be able to discover a dead body at considerable distance in a remarkably short time, although when cotton is stuffed into the nasal pits of a shark, it is unable to locate odorous food readily even in the immediate neighborhood.

Land amphibians, reptiles, and carnivorous birds such as the vulture depend more upon the sight of moving prey than upon the sense of smell in finding food. There is a great variation among mammals. Rodents, ruminants, and carnivores, all of which have a highly developed olfactory sense, are said to macrosmatic. Many mammals, man for example, are microsmatic, that is, poor “smellers.” Others are anosmatic, without any sense of smell at all, like cetaceans, whose nostrils have been readjusted into a periscopic position and are entirely devoted to respiratory uses.

Most evil smelling things are bad or poisonous, but not invariably so. Carbon monoxide gives out no olfactory warning although very poisonous, while certain famous cheeses, which are decidedly repulsive in odor, are not only non-poisonous but highly nutritious and beneficial.

The attempt to classify different odors objectively has not been very successful, although the existence of several specifically different kinds of olfactory receptors, adequate only for certain odors, is quite likely. Henning in 1916 proposed six different arbitrary categories of odors based upon their resemblance to well-known olfactory standards, namely, foul, flowery, fruity, burnt, spicy, and resinous.

Structure

Since smell, as contrasted with taste, has a much wider functional range, there is a greater elaboration of accessory structures in the olfactory apparatus than is necessary with organs of taste. There are not only the olfactory neurons, but also skeletal capsules and cavities in which they are housed; various supplementary chambers or sinuses connecting with the olfactory cavities; devices for securing the passage of odorous substances across the receptors; and glands for maintaining a constant film of moisture over the exposed ends of the olfactoreceptors.

The olfactory apparatus is the most anterior of all the sense organs. In man it appears first as a pair of ectodermal thickenings, which lie just in front of the developing medullary plate. By the third week of fetal existence they have become depressed into two nasal pits which are in close approximation to the olfactory lobes of the brain, and are brought into communication with the mouth cavity, both ontogenetically and phvlogenetically, by a channel, the naso-labial groove (Fig. 675), extending from the edge of each pit to the mouth.

Ventral view of the head of a dogfish, Scyllium, showing the nasolabial grooves

In some fishes and amphibians the nasal passages are formed by the fusion of the edges of the naso-labial grooves. In man, however, the grooves fill in completely, the nasal pits push in considerably deeper, and from the posterior ends of the pits secondary openings break through into the mouth cavity. This process is completed by the end of the fifth fetal month. Failure of the proper closure of the naso-labial groove results in the deformity known as “hare-lip” (Fig. 676), although shark-lip would be a more appropriate term, since the cleft in the upper lip is not a single one in the middle as in hares but on either side as in sharks and other elasmobranchs.

Head of a chick embryo of 5 days, showing the naso-labial sulcus, which would form a hare-lip if it remained open

The enlarged nasal cavity finally becomes differentiated into three regions, namely, vestibular, respiratory, and olfactory, distinguishable from each other by the character of the epithelium lining them.

The vestibular region is transitional between the skin on the outside and the mucous membrane within. It is characterized in mammals by the presence of sebaceous glands and by stiff outward-projecting, dust-arresting hairs, and forms that part of the nasal cavity which is principally under the protection of the projecting roof-like cartilaginous elements of the external nose, when such a structure is present, as in man.

Snouts have little to do with the olfactory sense but are rather associated with the tactile sense in nocturnal and burrowing forms, while in animals like pigs which “root for a living,” they are mechanical organs of the first order, strengthened sometimes by calcified cartilage. The trunk of an elephant, which is a combination of a long drawn-out nose and upper lip grown together, continues to perform the primary olfactory function, even though transformed into a grasping organ.

The respiratory region of the nasal chamber, the most extensive part of the olfactory mechanism, provides a place for the passage of air to and from the lungs. It has been described in Chapter XIII.

Diagram showing the extent and position of the olfactory membrane in man

The so-called olfactory region is the innermost recess on either side of the nasal cavity (Fig. 677). In man it is comparatively small, the olfactory membrane being distinguishable by its yellowish brown pigmentation. Although the use of the pigment here is not definitely known, it has been demonstrated that albinos which lack it have a defective sense of smell.

Evolution of sensory neurons

The cellular units of the olfactory epithelium are of three kinds, namely, sustentacular, basal, and sensory. The numerous sustentacular cells are long slender supporting elements extending through the entire thickness of the membrane. Between their slender inner ends are the basal cells. The sensory, or olfactory, cells are evenly distributed between the supporting cells. Each olfactory cell consists of a nuclear region from which a cylindrical process, comparable to a dendrite, extends to the surface where it terminates in several extremely delicate hair-like processes (Fig. 678). From the opposite end of the cell a slender neurite, covered by neurolemma but no myelin sheath, runs to the olfactory bulb of the brain. Many of these neurites together constitute the olfactory nerve, as described in the preceding chapter.

Accessory Parts

The circulation of water, containing dissolved gases, across the olfactory surface in the nasal pit of fishes, is effected largely by the presence of cilia. In elasmobranchs it is facilitated by a curving partition which partially subdivides each pit into intake and outgo regions. This division is carried further in many teleost fishes, as, for example, in the conger eel, Muraena (Fig. 679), in that each external naris has become double, thus presenting, instead of the two usual nasal openings, four chimnev-like nostrils on the head, none of which penetrates to the mouth cavity.

With the establishment of the choanae and the transfer of air through the nasal cavities, a system of valves and muscles becomes elaborated in connection with nasal ventilation. (See Chapter XIII, Respiration.)

Lateral outline of the head of a fish, Muraena, with projecting tubular nostrils that open into a common chamber

The remote situation of the olfactory membrane above the respiratory passage-way makes it possible ordinarily to breathe air without stimulating the olfactoreceptors to any great extent, even when odorous substances are about, unless air is drawn in vigorously over the upper olfactory route, as in “sniffing.”

Animals primarily aquatic, such as fishes and amphibians, have the olfactory membrane spreading over the entire nasal cavity, but a differentiation into vestibulo-respiratory and olfactory regions is found in air-breathing amniotes, beginning with the reptiles. In animals secondarily aquatic, like seals and possibly whales, which have largely lost the olfactory epithelium, the nasal cavities become entirely respiratory in character.

Supplementing glands in the nasal passages that serve to provide an adequate degree of moisture are the lacrimal glands, the ducts of which at the inner angles of the eyes constantly drain down into the nasal chambers. This nasal flow of excess tears is particularly well demonstrated by a sniffling child emerging from an emotional crisis that results in weeping.

Evolution

The development of the nasal cavities in man finds a parallel in the evolution of these structures in the vertebrate series.

In general fishes have olfactory pits that are culs-de-sac, not extending down into the mouth cavity. The nasal pits of elasmobranchs, however, which are on the ventral side of the snout not far from the mouth, may connect with it by means of open naso-labial grooves (Fig. 675).

In dipnoans and more completely in amphibians the grooves of the pits become closed, thus establishing, in addition to external nares, or nasal openings, a pair of internal nares, or choanae.

With the development in reptiles of a “hard palate,” or secondary roof of the mouth, the passage-way between the external and internal nares becomes much elongated. With a choanal opening into the mouth it is possible to receive olfactory stimuli from the outside world, not only through the front door of the external nares but also by way of the back door of the choanae, by means of food substances taken into the oral cavity.

Cross section through the head of a lizard, Lacerta agilis, with the lower jaw removed

Animals that hold food in the mouth have an accessory structure, Jacobson’s organ, which was first described in 1811. It is a cavity derived embryonically from the olfactory cavity and is lined with olfactory epithelium that is supplied by twigs from the olfactory (1st) nerve and the trigeminal (Vth) nerve. In position it is ventro-medial to the nasal cavity on either side. Reaching its highest development in the snakes and lizards (Fig. 680), it is reduced in turtles and crocodiles and appears only as an embryonic structure in birds. It is present in amphibians although not opening into the mouth cavity. Mammals show traccs of Jacobson’s organ embryonically, but in most instances it is a degenerate structure, its best manifestation being in monotremes, marsupials, insectivores, and rodents. It is entirely absent in whales, bats, and Old World monkeys. The history of Jacobson’s organ in man is wholly intrauterine. Arising at the beginning of the third month as small outpocketings of epithelium in the lower part of the nasal septum, it forms a slender blind sac on either side, reaching its greatest elaboration at about the fifth month. Before birth it becomes entirely reduced. This structure, which Kingsley refers to as “a kind of an olfactory organ,” probably serves as an accessory olfactory apparatus for testing odorous substances held in the mouth.

The enlargement of surfaces for the exposure of epithelium within the nasal chambers is brought about in three ways: by the folding of the mucous membrane; by sinuses in communication with the main nasal cavities; and by conchae, or skeletal shelf-like extensions of the nasal walls.

The method of membranous duplication is common among fishes. The mucous lining of the nasal pits of the dogfish, for example, bears a distant resemblance to the leaves of a book because of its numerous folds. Such a device would not be effective out of water, for the folds would tend to adhere together when not kept separate by immersion in an aqueous medium.

Reptiles have a single unrolled concha, or projection from the ectethmoidal wall of the nasal chamber on either side, which is slight in turtles, but of considerable size in crocodiles and alligators.

Diagrammatic cross section through the nasal region of a mammalian head

The nasal chamber of birds is compressed, in accordance with the general policy of compactness that characterizes avian anatomv. Its surfaces are compensatingly increased, however, by the presence of three conchae on either side, the most anterior of which is located in the vestibulo-respiratory region, while the smaller middle and posterior ones, that serve as supporting foundation for the olfactory membrane, are no doubt chiefly of value in supplying a moisture-producing surface for the respiratory mechanism.

Conchae reach their greatest elaboration in mammals, particularly ungulates, rodents, and carnivores, often becoming rolled like a scroll, thus presenting a maximum surface within a minimum space (Fig. 681). The nasal space in a sheep is larger than the brain cavity. In man, whose sense of smell is much inferior to that of a truffle-hunting pig, the conchae are smaller in size and reduced in number to three (Fig. 677).

Gustoreceptors

Just as the chemical sense of smell is located with special reference to the entrance of the respiratory passages, so the sense of taste acts as a sentinel at the portal of the digestive tube, inspecting all substances that enter there.

Taste Buds

Gustoreceptors are composed of clusters of cells called taste buds, which follow the same plan throughout the vertebrate series, differing in the various groups principally in their arrangement with relation to different kinds of papillae, as well as in their distribution.

The parent tissue that gives rise to taste buds is ectodermal in derivation. In many fishes, such as carp, sticklebacks, suckers, and catfishes, the ability to taste is not confined to the mouth cavity, but extends over the entire ectodermal covering of the body, even as far as the tail (Fig. 682). These external taste buds are supplied by the facial nerve. With emergence from water to land, gustoreceptors are withdrawn to the moist environment of the mouth cavity which is the logical place for encountering food solutions.

Cutaneous gustatory branches of the facial nerve of Ameiurus

A typical taste bud (Fig. 683) consists of a compact group of receptor and sustentacular cells, the former of which are in contact with free nerve endings from cranial nerves. Each sensory receptor cell, which is slender and elongate, terminates peripherally in a delicate protoplasmic ‘"hair” which extends from the bud through the taste pore.

A section through a taste bud

Taste buds are innervated by the facial (VIIth), glossopharyngeal (IXth), and vagus (Xth) nerves. The facial supplies the anterior part of the mouth cavity, including the anterior two-thirds of the tongue with its fungiform papillae in mammals. The glossopharyngeal supplies the posterior part of the mouth cavity, including the posterior third of the tongue with its vallate and foliate papillae in mammals. The vagus supplies the pharynx, including the taste buds of the epiglottis in mammals.

Although taste buds occur in various parts of the mucous membrane of the mouth and pharynx, they are especially abundant on the papillae of the tongue.

Comparative Anatomy

Amphioxus has cellular structures that resemble taste buds upon the cirri around the mouth, but their function is unknown.

The sense of taste in fishes is, as already pointed out, much more generalized than in land forms, the gustoreceptors extending over the outside of the body where they appear more as free endings than as taste buds.

Not much is known of the sense of taste in amphibians, but these animals do possess nerve endings in the skin that serve as chemical irritoreceptors. They have groups of cutaneous cells also which are probably more tactile than chemical in function. Whatever sense organs may be found in the amphibian mouth have slight opportunity to be of gustatory service, since the food is swallowed at once without being held in the mouth.

In reptiles, particularly crocodiles and snakes, taste buds are located in the posterior part of the mouth cavity, which is less cornified than the anterior part.

Taste buds are present but scarce in the bird’s mouth, and are likewise situated posteriorly instead of on lingual papillae as in mammals. The majority of birds with small cornified tongues have most of the taste buds in the floor of the mouth, while birds like parrots, with large fleshy tongues, give lodgment to the gustatory organs on the tongue itself, or along the sides of the maxillary half of the beak.

Taste buds reach their greatest development in mammals, such as ruminants, that have specialized grinding molars and retain forage within the mouth cavity for a prolonged period before consigning it to the absorptive part of the digestive tract. The taste buds are not confined solely to the tongue, but are found also on the ventral surface of the soft palate, on the epiglottis, on the pharyngeal wall, and even on the inside surfaces of the cheeks.

The number of taste buds to each vallate papilla varies greatly in different mammals. Hesse has estimated that sheep have 480; cows, 1760; and pigs, 4760. A cow, having about 20 papillae, would consequently possess approximately 35,000 taste buds, while a giraffe, which is known to have over 30 papillae, would probably prove to be still more generously endowed.

Diagrams of the right half of the human tongue, illustrating the distribution of the taste buds

Stahr has made the interesting observation that taste buds on the tongue of the domesticated white rat are considerably reduced in number as compared with those of the wild rat that works for a living.

In man there is an oval area on the dorsal surface of the tongue that is free from taste buds, as is also the under side of the tongue (Fig. 684). That the sense of taste in man is degenerating is proved not only by the fact that taste buds are more widely distributed during the fifth to the seventh months of fetal life than in the adult, but also by the fact that the inner surfaces of the cheeks respond to taste far more keenly in children than in older persons.

Classification

The subjective classification of gustatory sensations into pleasant, unpleasant, indifferent, or absent is better replaced by an objective grouping according to the types of gustatory stimuli. Four such categories may be accepted as demonstrable, namely, sweet, salty, sour, and bitter. Intergrades probably occur, and there is no doubt that confusion often results, particularly with olfactory and tactile stimuli. Thus when one speaks of a “slimy” or a “gritty” taste, he is referring in reality to tactile stimulation, while the so-called “taste” of onions, wine, coffee, tobacco, tea, fruits, and spices is largely a matter of olfactory stimulation. Most “flavors” which are commonly associated with the sense of taste arise largely from olfactory stimuli, as is realized when with a “head cold” the swollen mucous membranes largely occlude the choanal passages and food “loses its taste.”

Although indistinguishable from each other histologically, there are specific gustoreceptors, adequate for each of the four kinds of stimuli just mentioned, since, upon application of cocaine to the tongue the sense of bitter disappears first, then those of sweet, salty, and sour in succession.

The distribution of the four kinds of receptors over the surface of the human tongue has been carefully mapped (Fig. 684), thus demonstrating that the receptors of gustation are different. It will be seen that sour receptors predominate along the edge of the tongue; salty, both at the tip and at the sides; bitter, at the base; and sweet, at the tip.

Extensive tests by Blakeslee and others with phenyl-thiocarbamide, a harmless substance which tastes bitter to most people, reveal the fact that all human beings are not equipped with the same array of gustoreceptors. Out of 2550 individuals tested in one series of experiments 65.5 per cent reported the substance as bitter; 28.0 per cent, tasteless; 2.3 per cent, sour; and 4.2 per cent, some other taste. It is obvious that all people do not interpret the same chemical world the same way, because of a difference in their gustoreceptors.

Irritoreceptors

The moist skin of amphibians and fishes is sensitive to certain chemical stimuli that act as irritants. In reptiles, birds, and mammals, whose integument is adapted to conditions in dry air, the stimulus produced by common chemical irritation aside from that interpreted as taste or smell is confined mostly to moist mucous surfaces. Here it may be very noticeable, as when a whiff of ammonia causes “watering of the eyes” or choking reflexes. These are stimuli transferred to the brain by the pathway of the fifth cranial nerve, instead of the first, and the receptors may properly be termed irritoreceptors rather than olfactoreceptors.

Irritoreceptors are closely related to those of contact and pain, but are nevertheless distinct from them, as is demonstrated by the fact that cocaine differentiates them from each other, and also because they do not become simultaneously exhausted upon rapidly repeated stimulation. Parker has shown that the tail of amphioxus, for instance, after it no longer responds to the application of weak nitric acid, is still fully responsive to the touch of a camel’s-hair brush. Cole has also demonstrated that a frog, anaesthetized by a one per cent solution of cocaine to the point at which it is entirely oblivious to the mechanical stimulation of pinching or scratching, will still respond vigorously to the application of a salt solution.

“We are, therefore, entirely justified,” says Parker, “in concluding that the common chemical sense is a true sense with an important set of receptors and a sensation quality entirely its own.”

By experiments of elimination it has been shown that irritoreceptors are free nerve endings with spinal or cranial connections, whose distribution is confined to the skin of aquatic vertebrates, or to the moist cutaneous surfaces of land forms.