UNIVERSITY OF TORONTO DEPARTMENT OF PSYCHOLOGY •B.I SMELL, TASTE, AND ALLIED SENSES IN THE VERTEBRATES MONOGRAPHS ON EXPERIMENTAL BIOLOGY PUBLISHED FORCED MOVEMENTS, TROPISMS, AND ANIMAL CONDUCT By JACQUES LOEB, Rockefeller Institute THE ELEMENTARY NERVOUS SYSTEM By G.H. PARKER. Harvard University THE PHYSICAL BASIS OF HEREDITY By T. H. MORGAN. Columbia University INBREEDING AND OUTBREEDING: THEIR GENETIC AND SOCIOLOGICAL SIGNIFICANCE By E. M. EAST and D. F. JONES, Bussey Institution. Harvard University . THE NATURE OF ANIMAL LIGHT By E. N. HARVEY, Princeton University SMELL, TASTE AND ALLIED SENSES IN THE VERTEBRATES By G. H. PARKER, Harvard University BIOLOGY OF DEATH By R. PEARL. Johns Hopkins University I N PREPARATION PURE LINE INHERITANCE By H. S. JENNINGS. Johns Hopkins University LOCALIZATION OF MORPHOGENETIC SUBSTANCES IN THE EGG By E. G. CONKLIN, Princeton University TISSUE CULTURE By R. G. HARRISON, Yale University INJURY, RECOVERY AND DEATH IN RELATION TO CONDUCTIVITY AND PERMEABILITY By W. J. V. OSTERHOUT. Harvard University THE EQUILIBRIUM BETWEEN ACIDS AND BASES IN ORGANISM AND ENVIRONMENT By L. J. HENDERSON, Harvard University CHEMICAL BASIS OF GROWTH By T. B. ROBERTSON. University of Toronto COORDINATION IN LOCOMOTION By A. R. MOORE. Rutgers College OTHERS WILL FOLLOW MONOGRAPHS ON EXPERIMENTAL BIOLOGY SMELL, TASTE, AND ALLIED SENSES IN THE VERTEBRATES BY G. H. PARKER, Sc.D. PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY 57 ILLUSTRATIONS PHILADELPHIA AND LONDON J. B. LIPPINCOTT COMPANY Vftky 589567 V 8. COPYRIGHT. 1922. BY J. B. LIPPINCOTT COMPANY Electrotyped and Printed by J. B. Lippincott Company The Washington Square Press, Philadelphia, U. S. A. EDITORS' ANNOUNCEMENT THE rapid increase of specialization makes it im- possible for one author to cover satisfactorily the whole field of modern Biology. This situation, which exists in all the sciences, has induced English authors to issue series of monographs in Biochemistry, Physiology, and Physics. A number of American biologists have decided to provide the same opportunity for the study of Experimental Biology. Biology, which not long ago was purely descriptive and speculative, has begun to adopt the methods of the exact sciences, recognizing that for permanent progress not only experiments are required but quantitative experi- ments. It will be the purpose of this series of monographs to emphasize and further as much as possible this develop- ment of Biology. Experimental Biology and General Physiology are one and the same science, in method as well as content, since both aim at explaining life from the physico-chemical constitution of living matter. The series of monographs on^Experimental Biology will therefore include the field of traditional General Physiology. JACQUES LOEB, T. H. MORGAN, W. J. V. OSTEBHOUT. AUTHOR'S PREFACE SENSE organs have always excited general interest, for they are the means of approach to the human mind. Without them our intellectual life would be a blank. The deaf and the blind show how serious is the loss of even a single set of these organs. Although the ear and the eye have commonly received most attention, the other sense organs, such as those of smell and of taste, are in reality equally worthy of con- sideration. These organs are of first significance in warning us of untoward conditions that may exist about us particularly in relation to our food. But they not only serve us in this protective way, they are also of the utmost importance in initiating that chain of events which cul- minates in successful nutrition. Through their action the secretion of the digestive juices and other like operations, so essential to the proper treatment of the food, are started and furthered in the alimentary canal. Thus their activities, though less associated with our mental states than are those of the ear and of the eye, are never- theless so essential to our organic well-being that they are in reality quite as necessary to us as the so-called higher senses. Smell and taste, together with certain other senses not so well known, form a more or less natural group in which there is a certain amount of functional interrelation and genetic connection, and it is from this standpoint that these senses will be considered in the following pages. They will thus illustrate in a way principles common to 7 8 AUTHOR'S PREFACE other groups of sense organs, and these principles will be found to be of an essentially dynamic character as con- trasted with the older conceptions in which function has been brought into relation less intimately with structure. The author is greatly indebted to the editors of this series of monographs for many suggestions that have led to improvements in the text. He is also under obligations to his wife for a careful revision of the manuscript. He wishes to extend his thanks to numerous persons who have permitted him to copy and use figures contained in their publications. In all such instances the sources of such figures are acknowledged in the text. Where a figure is given without reference, it is an original. The drawings for all figures were made by Mr. E. N. Fisher. G. H. P. Harvard University, Cambridge, Mass. January, 1922. CONTENTS CHAPTER PAGE I. NATURE OF SENSE ORGANS 13 II. ANATOMY OF THE OLFACTORY ORGAN 23 III. PHYSIOLOGY OF OLFACTION 42 IV. VOMERO- NASAL ORGAN OR ORGAN OF JACOBSON 92 V. THE COMMON CHEMICAL SENSE 102 VI. ANATOMY OF THE GUSTATORY ORGAN 110 VII. PHYSIOLOGY OF GUSTATION 132 VIII. INTERRELATION OF THE CHEMICAL SENSES 167 INDEX . . .187 ILLUSTRATIONS FIG. PAGE 1. Diagram of the Lateral Wall of the Right Nasal Cavity of Man . . 24 2. Diagram of a Transverse Section of the Right Nasal Cavity of Man 25 3. Respiratory Epithelium from the Nasal Cavity of a Young Pig .... 27 4. Olfactory Cleft of Man 28 5. Olfactory Epithelium from a Pig Embryo 29 6. Olfactory Epithelium from a Young Mouse 30 7. Isolated Olfactory Cells and Sustentacular Cells from Man 31 8. Isolated Olfactory Cell and Sustentacular Cell from a Frog 32 9. Olfactory Cell of a Pike Showing Flagellum 33 10. Olfactory Epithelium from a Chick Embryo 36 11. Ventral View of the Head of a Shark (Scyllium) 38 12. Diagram of the Right Nasal Cavity of Man Showing the Direction of the Inspired Air Currents 46 13. Simple Rubber Olfactometer 50 14. Double Olfactometer 51 15. Ventral View of the Head of a Hammer-head Shark 66 16. Curves of Olfactory Exhaustion 71 17. Olfactory Prism 75 18. Generalized Diagrams of the Molecular Structure of Classes of Aromatic Bodies (Olfactory Stimuli) 80 19. Head of Human Embryo showing Vomero-nasal Pore 93 20. Diagram of the Median Face of the Left Nasal Cavity of Man 94 21. Transverse Section of the Snout of a Young Frog 95 22. Transverse Section of the Head of a Snake Embryo 96 23. Transverse Section of the Nasal Septum of a Young Cat 97 24. Epithelium from the Vomero-nasal Organ of the Sheep 98 11 12 ILLUSTRATIONS 25. Dorsal View of the Human Tongue 112 26. Vertical Section of a Fungiform Papilla 113 27. Vertical Section of a Vallate Papilla 114 28. Lateral View of a Catfish Showing Gustatory Branches of the Facial Nerve 116 29. A Simple Taste-bud 117 30. A Compound Taste-bud 118 31. Taste-buds of the Rabbit 121 32. Taste-buds of the Cat 122 33. Taste-buds of the European Barbel 124 34. Diagram of the Human Tongue Showing Innervation 125 35. Diagram of the Possible Paths of the Gustatory Nerves in Man. ... 126 36. Diagrams of the Human Tongue Showing the Distribution of the Four Tastes 149 37. Diagrams of the Receptor Systems of the Vertebrate Chemoreceptors 181 SMELL, TASTE, AND ALLIED SENSES IN THE VERTEBRATES CHAPTER I. NATUEE OF SENSE ORGANS. Contents. — 1. Older Conception of Sense Organs. 2. Modified View due to Theory of Reflex Action. 3. The Genesis of Receptors. 4. Bibliography. 1. OLDER Conception of Sense Organs. In the con- ventional text-book, sense organs are commonly looked upon as structures that supply the brain with those nerv- ous impressions from which the mental life of the indi- vidual is built. During normal activity these organs are incessantly in operation and flood the central apparatus with a stream of impulses by which are carried to us evi- dences of the multitudinous alterations of the environ- ment. Through the ear and the eye pass continuous streams of change by which we adjust ourselves not only to the immediate material world about us but to the world of ideas whose elements are spoken and writ- ten words. Sense organs from a structural standpoint are organs whose cells are so specialized that they are subject to stim- ulation by only a particular category of external changes. As Keith Lucas has expressed it, sense cells approximate a unif unctional state. The changes by which they are brought into action form rather homogeneous groups of 13 14 SMELL, TASTE, ALLIED SENSES environmental alterations. Thus the chemical changes of the surroundings affect the organs of smell and of taste, the pressure changes those of touch and hearing, and al- terations in the radiant energy those of sight. These natural groups of environmental changes have been des- ignated as homologous, or, better, adequate stimuli for the sense organ that they activate. Such organs are ordi- narily arranged under five heads each with an adequate stimulus and productive of a special sensation ; they are the organs of smell, taste, touch, hearing, and sight. Experience has also shown that when in a given per- son a sense organ exhibits complete congenital incapacity, such an individual lacks certain mental elements that can never in reality be made good to him by the activity of the remaining parts. A state of this kind implies a certain mental deficiency in the given individual. If a person has been blind from birth, no amount of description can supply to him the sensations of the wealth of color that the external world holds for the normal man. Where blind- ness is an acquired defect, the rememberance of the former color sensations as compared with the present deprivation, makes the state of deficiency still more pro- nounced. And in those rare cases where therms a unilateral defect in color vision with sight otherwise unim- paired, the subject can contrast most vividly tne state of deficiency with that of normal completeness. Such con- ditions, which are known to occur not only in sight but in the other senses as well, have had a most profound influ- ence on the interpretations that naturalists have placed upon the states presented by the lower animals. It has been commonly assumed, and with no small show of reason, that where an animal is found to possess NATURE OF SENSE ORGANS 15 an eye or an ear, for instance, it should be accredited with all the central nervous activities, sensations and the like, that accompany such an organ in man, qualified only by the degree of development to which the particular organ in the given animal has arrived. Conclusions based upon such a course of reasoning were commonly ad- mitted as valid by the workers of a few decades ago (Lubbock, 1882; Graber, 1884) and the text-books of that period in dealing with the sense organs of the lower animals discuss these parts ordinarily under the conven- tional five heads of the older human physiology (Jour dan, 1889). From this standpoint one of the lower animals is like a defective human being in that its full sensory ac- tivity falls short of that of the normal man. Or it may be compared to a person whose sensory development is un- symmetrical and whose relations with the surroundings have come to be predominant through a limited number of sensory channels rather than through all. It is likewise perfectly clear that a given animal, whose organization in general may be simpler than that of man, may nevertheless excede him in a particular sensory capacity and in this respect at least stand above him. It is commonly admitted that the dog far outruns man in the keeness of his sense of smell and it has long been known that cats hear tones of a pitch much too high for the human ear. These and other like examples show that though the senses of the lower animals are in general less efficient than those of man, the reverse is occasionally true. Moreover among some of the lower forms, sense or- gans have been discovered that are not represented in man. Thus fishes possess, in addition to the five classes of human sense organs, the so-called lateral-line organs. 16 SMELL, TASTE, ALLIED SENSES Here then must be a wholly novel set of sensory relations. As to the sensations arising from these organs man can form no direct conception, for they are entirely outside the range of his experience. Hence Leydig, the discoverer of the sensory nature of these parts, wrote of them as organs of a sixth sense. Thus to the older workers the senses of the lower animals were like those of a human being that had suffered either curtailment or expansion even to the extent of excluding or including whole categories of stimuli. But quite aside from the question of the number and variety of these parts, is the opinion held by most of the early workers that the sense organs of the lower ani- mals are primarily concerned with providing the brain or corresponding structure of the given creature with that body of sensation which was supposed to represent all the significant changes in the effective environment. 2. Modified View due to Theory of Reflex Action. The belief that sense organs were chiefly concerned with providing the brain with the elements of which the mental life is composed suffered an important limitation from the work of the physiologist. This limitation arose from the development of the idea of reflex action. Originating about the time of Descartes in the seventeenth century, the conception of the reflex action grew in time into a most important principle for the interpretation of nervous operations. It was at first applied to that form of nervous activity whose outcome is fairly constant and in a way mechanical in that it is unassociated with conscious- ness, but it was gradually extended to include those per- formances in which consciousness is involved and at present it commonly refers to any chain of nervous activity in which a sensory stimulation produces an im- NATURE OF SENSE ORGANS 17 pulse that, after passage through the central nervous organs, results in action. From the beginning many reflexes were believed to be unassociated with consciousness and though this view was subsequently combated and the idea of the reflex extended to nervous operations that included an obvious sensa- tional element, it nevertheless remained true that a host of reflex operations could be pointed out that were with- out representation in consciousness. Thus the impulses that flow from the vestibular portion of the human ear and that are of the utmost importance in maintaining equilibrium provoke no obvious sensations and the vast flux of afferent nerve action that moves from the mus- cle to the spinal cord and that is so essential to the coordination of bodily movements, runs its course without exciting sensation. These and many like instances have made it clear that the reflex, even in the most special ap- plication of the term may as often be unassociated with sensation as associated with it. As the first step in every reflex is the excitation of a sense organ and as many reflexes are unassociated with consciousness, it must be admitted that sense organs, not- withstanding the name, are not always necessarily con- cerned with sensations. Many certainly have nothing whatever to do with such central nervous states. Thus it is doubtful if the normal activity of the sensory endings in our muscles and tendons is ever productive of sensation. In consequence of this condition a reasonable objection was raised to the term sense organ and it was proposed by Bethe (1897) to use in place of it the word receptor. Although the theoretic force of this objection has not always carried conviction, the term receptor has come into 2 18 SMELL, TASTE, ALLIED SENSES common use and the emphasis that it places on the organs to which it is applied as receivers of environmental change rather than as originators of impulses to sensation is certainly a step in the right direction. Human receptors belong to one or other of two classes. Either they are concerned purely and simply with the excitation of reflex acts and take no part in the pro- duction of sensations, in which case they may be called activators, or they are at the same time effective in arousing sensations, the elements of the intellectual life and hence may be appropriately termed sense organs. All receptors belong to either one or the other of these classes though in some instances a certain degree of temporary vacillation occurs. Hence it may be that these classes exemplify in a way two receptive functions, one of which predominates in one class and the other in the other. How these functions are related can best be gathered from the genetic history of receptors. 3. The Genesis of Receptors., Eeceptors such as the eye and the ear, the organs of smell and taste, and the more diffuse sensory equipment of the skin, are found in all the more complex animals. They abound in the verte- brates, the mollusksjthe arthropods, and to a less extent in the worms. They may be said to occur even in the cce- lenterates, as, for instance, among the jelly fishes, though in the majority of these animals the receptors present a diffuse condition more like that seen in the vertebrate skin than in the vertebrate eye or ear. This diffuse state seems to be characteristic of the receptors in the simpler sessile invertebrates. The more complex animals such as are capable of active locomotion exhibit almost invari- ably specialized types of organs. NATURE OF SENSE ORGANS 19 So far as the neuromuscular system of the inverte- brates is concerned, forms as low in the scale as the annelid worms appear to possess all the elements of the corre- sponding system in the vertebrates. Such worms may have specialized receptors, eyes and the like, often of a highly complex structure. They possess a well-differ- entiated central nervous system as represented in their so-called brain and ventral ganglionic chain. Finally, they have an abundant variety of specialized effectors in their various muscles, glands, and luminous organs. Their receptors, central nervous organs, and muscles are so related that reflexes can be demonstrated on them as readily as on vertebrate preparations. In other words, they possess in completeness, though in simple form, a working neuromuscular mechanism essentially like that of the higher animals. When, however, an examination of such forms as the coelenterates is made, it is found that the coral animals, the sea-anemones, the hydroids, and the like, possess scarcely any trace of a central nervous apparatus. In these animals fairly well specialized sensory surfaces occur, whose nervous prolongations connect either imme- diately with the subjacent musculature or give rise to a nerve-net which in turn connects with the contractile ele- ments. Thus the receptor is applied to the muscle very directly and without the intervention of a central organ. Such an arrangement allows of simple reflexes, for, when the receptive surface is stimulated, the animal responds at once by an appropriate muscular movement. Thus if meat juice is discharged on the tentacles of a sea-anemone, these organs carry out vermiculate movements and the gullet opens ; or if the pedal edge of the column is touched, 20 SMELL, TASTE, ALLIED SENSES the whole animal contracts. The fact that meat juice will not excite the pedal edge of the column and that a touch applied to the tentacles is seldom followed by more than a slight local activity shows that the external surface of the sea-anemone, though generally receptive, is locally specialized. As a matter of fact this surface in degree of differentiation stands between a diffuse receptive surface, such as the vertebrate skin, and a specialized organ like the eye or the ear. In the literal sense of the word the outer surface of a sea-anemone is not sensory though abundantly receptive. There is no reason to suppose that the receptive areas of these animals are concerned with initiating impulses to sensation. They connect very directly with muscles and serve quite obviously as trigger-like organs by which the muscle is set in action. A careful examination of the activities of sea-anemones has failed to reveal any evi- dence, such as can be produced from the more complex animals, to show that these simple creatures possess central nervous functions. Such functions apparently have no part in their organized performances. Hence their receptors have nothing whatever to do with initia- ting impulses to sensation, but are limited in their action to the excitation of the muscles after the type of the most mechanical reflex. The presence in ooelentrates of eye spots, olfactory pits, statocysts and other such special receptors is, therefore, no indication that these animals are endowed with corresponding sensations, as many of the older workers believed, but this condition merely shows that their possessors are especially open to a particular stimulus. An eye spot does not mean that the animal pos- sesses sight, but that it is readily excited to action by light. NATURE OF SENSE ORGANS 21 Thus of the two functions that have been attributed to receptors, the capacity to excite action and the ability to initiate impulses for sensation, the former is much the more widely distributed of the two and is without question the more primitive. Since sponges are known to possess muscles but are devoid of nervous tissue, it is probable that they represent a type of organization which in point of time preceded that in which the nervous elements arose. So far as can be judged these elements originated in connection with the previously differentiated muscle and as a special means of exciting it to contraction. This earliest nervous mate- rial must have been, therefore, essentially receptive in character and must have served as the source of the more obvious receptors of specialized types. Thus receptors must be regarded as the original form of nervous struc- ture, concerned in the beginning with the simple excita- tion of muscle (activators) and subsequently involved, after the development of the central organs, with that supply of impulses which yields the elements of the intel- lectual life (sense organs). The extent to which a natural group of receptors may undergo differentiation and yet maintain a striking degree of mutual interdependence can nowhere be better illus- trated than with the chemical receptors, the organs of smell and of taste. It is from this standpoint that the structure and function of these receptors will be con- sidered in the following chapters. 4. BlBLIOGBAPHY. BEEB, T., A. BETHE, und J. VON UEXKUIX. 1899. Vorschlage zu einer objektivierenden Nomenklatur in der Physiologie des Nervensystems. Biol. CentralU., Bd. 19, pp. 517-521. 22 SMELL, TASTE, ALLIED SENSES BETHE, A. 1897. Das Nervensystem von Carcinus msenas. Arch. mik. Anat., Bd. 50, pp. 460-546. GBABEB. V. 1884. Grundlinien zur Erforschung des Helligkeits- und Farbensinnes der Tiere. Prag & Leipzig, 322 pp. JOUBDAN, E. 1889. Les sens chez les animaux inferieurs. Paris, 314 pp. LUBBOCK, J. 1882. Ants, Bees, and Wasps. New York, 448 pp. PARKEB, G. H. 1910. The Reactions of Sponges, with a Consideration of the Origin of the Nervous System. Jour. Exp. Zool., vol., 8, pp. 1-41. PABKEB, G. R. 1917. The Sources of Nervous Activity. Science, vol. 45, pp. 619-626. PABKER, G. H. 1919. The Elementary Nervous System. Philadelphia, 229 pp. CHAPTER II. ANATOMY OF THE OLFACTORY ORGAN. Contents. — 1. Nasal Cavities in Man. 2. Nasal Mem- branes. 3. Olfactory Epithelium. 4. Intermediate Zone. 5. Polymorphic Cells. 6. Sense Buds. 7. Free-nerve Endings. 8. Development of Olfactory Nerve. 9. Com- parative Anatomy of Olfactory Organs. 10. Bibliography. 1. NASAL Cavities in Man. In man the olfactory organs are paired and are situated one in each nasal cavity. Each of these cavities possesses an external opening, the anterior naris, and an internal one, the posterior naris or choana, which communicates with the pharynx. (Fig. 1). The two nasal cavities are separated by the nasal septum, a partly bony, partly cartilaginous wall, which forms a smooth median partition between them. The lateral walls of these cavities are thrown into a series of more or less horizontal folds, the nasal conchae. These are commonly three in number for each cavity though in some instances only two are present and in others a fourth, fifth or even a sixth can be discerned. Of the three conchae usually present the most ventral one, the inferior concha, is the largest and extends through much of the length of the cavity in a direction approxi- mately parallel to its floor. Immediately above the inferior concha is the somewhat smaller middle concha which is followed by the still smaller superior concha. When only two conchas are present, they are the inferior and the 23 24 SMELL, TASTE, ALLIED SENSES middle, the superior being absent. When a fourth concha is to be seen, it is found above and behind the superior. It has been designated the first supreme concha and it FIG. 1. — Diagram of the lateral wall of the right nasal cavity of man. 1, Inferior concha; 2, middle concha; 3, superior concha; 4, first supreme concha; 5, second supreme concha; the apertures numbered 6 to 10 are covered from sight by the conehse, but their positions are indicated by vertical lining; 6, aperture of the nasolacrimal duct opening into the inferior meatus; 7, opening of the maxillary sinus (middle meatus); 8, opening of the frontal sinus (middle meatus); 9, and 10, openings of the ethmoid cells, 9, into the middle meatus, 10, into the superior meatus; 11, opening of the Eustachian tube; 12, vesibule; 13, atrium; 14, choana; 15, frontal sinus; 16, sphenoidal sinus whose opening is indicated by an arrow; 17, olfactory region whose limits are marked by the dotted line. The vertical dotted line shows the plane of section from which Fig. 2 was drawn. may be followed by a second or even a third supreme concha. According to Schaeffer(1920), the first supreme concha is to be observed in about 60 per cent of all adult human beings. The three conchae ordinarily present project from ANATOMY OF THE OLFACTORY ORGAN 25 the lateral wall of each nasal chamber into its cavity and partly divide that cavity into three approximately hori- zontal passages: the inferior meatus under the inferior concha, the middle meatus under the middle concha and the superior meatus under the superior concha. (Fig. 2). The external naris leads at once to the first chamber of the nose, the vestibule, which connects almost directly with the inferior meatus, less directly with the su- perior meatus and through the so-called atrium with the middle meatus. Between the median sep- tum of the nose and the laterally situated conchas is a considerable space known as the common meatus. Dorsally this space is continuous with a narrow slit lying between the superior concha and the septum and called the olfactory cleft. Air these pas- sages and spaces communicate more or less directly and freely through the posterior naris or choana with the pharynx. In the bones about the nose in man are large paired air- spaces or sinuses that communicate with the exterior through the nasal cavity. These spaces, which have been very fully described by Schaeffer (1916), are of consid- erable size and are lined with a mucous epithelium con- tinuous with that of the nose. They are somewhat variable in number and connections and yet they fall more or less Fio. 2. — Diagram of a trans- verse section of the right nasal cavity in man made at the plane indicated by the vertical dotted line in Fig. 1. 1, inferior concha; 2, middle concha ; 3, superior con- cha; 4, nasal septum; 5, inferior meatus; 6, middle meatus; 7, superior meatus; 8, common meatus; 9, olfactory cleft (left side); 10, ethmoid cells; 11, maxillary sinus. 26 SMELL, TASTE, ALLIED SENSES naturally into four sets, the maxillary, frontal, and sphe- noidal sinuses and the ethmoidal cells. Each maxillary sinus is a large space in the maxillary bone above the teeth. It opens by a considerable sli t into the anterior part of the middle meatus. (Figs. 1 and 2). The frontal sinus,in the frontal bone also opens into the middle meatus at a point above and anterior to the opening of the maxillary sinus. Each sphenoidal sinus opens into the posterior end of the appropriate olfactory cleft in a region known as the spheno-ethmoidal recess. The remaining accessory nasal spaces, the ethmoid cells, are more or less variable ; some of them open into the middle meatus by several apertures well above the slit for the maxillary sinus. Others open, more commonly by a single aperture, into the superior meatus. In addition to these various openings, the naso- lacrimal duct, by which the lacrimal secretions from the eye are carried to the nasal cavity, opens between a pair of lips on the lateral wall of the inferior meatus near its anterior extremity, 2. Nasal Membranes. The nasal vestibule is lined with a delicate continuation of the outer skin. The walls of the deeper part of the nasal cavity are covered with a mucous membrane which is divisible into two regions, the restricted olfactory region in the dorsal part of the cavity and the much more extended respiratory region embrac- ing the remainder of the cavity. The mucous membrane of the respiratory region is reddish in color and consists of a pseudo-stratified epi- thelium containing ciliated cells and basal cells backed up by a well developed tunica propria. (Fig. 3.) The cilia of this region lash towards the choana. The secretion covering the surface of the epithelium comes from numer- ANATOMY OF THE OLFACTORY ORGAN ous branched alveolo-tubular glands which contain both mucous and serous cells. The conchas of the respiratory region have long been known to be extremely vascular and to be possessed of a structure like that of erectile tissue. This is especially true of their edges. They can be excited through reflex channels to considerable enlargement and the swelling thus produced may be suffi- cient to close completely the respir- atory passages. It is believed that this high vascularity of the respira- tory region is concerned with the moistening and warming of the current of respiratory air. The secretions of this portion of the nose are also believed to be inimical to pathogenic germs and thus to afford a protection to the deeper parts against the invasion of disease. The olfactory region in man is yellowish in color as it is in the calf and in the sheep. In the dog and the rabbit it is of a more brownish hue. According to the older anatomists it was supposed to extend in man over the dorsal half or even more of the nasal cavity. Von Brunn (1892) , however, claimed by a reconstruction from sections that the olfactory epithelium was much more restricted than had been originally supposed. According to this author only a small portion of the superior concha and a correspondingly small part of the nasal septum represent the unilateral area of distribution of the olfactory nerve. This area in one subject measured 257 sq. mm. and in an- Fia. 3. — Respiratory epi- thelium from the nasal cavity of a young pig; b, basal cell; c, ciliated cell. After Alcock, 1910, Fig. 2. 28 SMELL, TASTE, ALLIED SENSES other 238 sq. mm. The more recent results of Read (1908), however, show that in man the olfactory fibers spread from the dorsal portion of the olfactory cleft ven- trally over the superior concha almost to its free edge and correspondingly over the septum to about one third its ex- tent. (Fig. 4). The antero-posterior spread of the nerve, according to this author, is about twice that of its ventral distribution on either the concha or the septum; hence the whole area innervated by each olfactory nerve, if spread out flat, would be approximately square in outline and not far from 25 mm. 4 —olfactory deft to a side, somewhat over twice the lmnaUTsemy 8" extent ascribed to it by von Brunn. ward; the blackened area 0 /~\-in -rn • ji T mi. if shows the distribution of 3. Oltactory Jt