Book - The Nervous System of Vertebrates (1907) 10
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Chapter X. The Olfactory Apparatus
The olfactory apparatus is treated here because of the physiological relationship between it and the gustatory apparatus. Whether a morphological relationship exists or not has yet to be determined.
The peripheral organ of smell consists of a sensory epithelium situated at the extreme anterior end of the head. It arises in all vertebrates as a pair of thickenings of the ectoderm one at either side of the lower or anterior border of the neuropore. The thickenings are therefore formed from ectoderm which lies close at the border of the neural plate before it rolls up into the neural tube. While the brain and optic vesicles are developing and the secondary divisions of the brain are taking form, the two epithelial thickenings become depressed and sink into the angle between the cephalic surface of the optic vesicles and the brain, and eventually form deep pits or sacs each with an external opening. At the same time that these nasal pits are forming there appears below and between them a single invagination which has been described as the hypophysis (p. 66). The hypophysis is the remnant of the ancient mouth and in some forms still has a connection with the archenteron for a short time during development. It appears that the olfactory organ was originally situated in the roof of the mouth or in front of it. In this position the organ must have been useful both in finding and selecting food. When the hypophysis ceased to serve as mouth and a new mouth was formed from a pair of gill slits the olfactory organ was farther removed from the mouth but still in front of it. Finally, in air-breathing vertebrates the olfactory sac comes to have a connection with the pharynx and to serve as a passage for air in respiration. In this position the organ is favorably placed for detecting odors and so for finding food.
In the wall of the nasal sac a part of the cells develop into sense cells, the remainder act as supporting cells. The sense cells differ from those in any other sense organs of vertebrates and resemble a common type of sense cells in invertebrates. Each is a slender spindle-shaped cell from the inner end of which a nerve fiber arises and runs toward the brain (Fig. 93). The fibers grow out from the olfactory cells during development and enter the olfactory bulbs of the forebrain, where they end in a manner to be described below. These fibers constitute the olfactory nerve. They never have myelin sheaths.
FIG. 93. A section of the olfactory epithelium of a bony fish at the time of hatching, to show the origin of the fibers of the olfactory nerve.
The Olfactory Centers
In fishes the greater part of the forebrain is related exclusively to olfactory impulses. The primary center for olfactory fibers is the olfactory bulb. In cyclostomes the bulb consists of several layers of cells surrounding the ventricle and of a superficial zone of fibers (Fig. 94). Nearly all the cells are either spindle-shaped and provided with a single dendrite vertically placed in the wall of the bulb or are stellate and have two or more dendrites which run obliquely toward the surface. The final branches of the dendrites are intermingled with the end-branches of olfactory fibers near the surface of the bulb. The olfactory fibers run in bundles as they enter the bulb and as the individual fibers divide and subdivide the bundles form larger or smaller cylindrical masses of interwoven nerve fibrils. Into each of these masses the dendrites of numerous cells penetrate and so receive impulses from the olfactory fibers. The combined mass of fibers and dendrites is a glomerulus. There is no observable difference in the relations of the various nerve cells of different sizes and shapes to these glomeruli. There is to be seen, however, the beginning of a special type of cells which come to predominate in higher vertebrates. In the outer portion of the cell layer are seen a few cells of relatively large size whose dendrites are short and thick and more profusely branched than are those of other cells, and enter only one or two glomeruli. Although these cells are not highly specialized in cyclostomes they can be recognized as the forerunners of the so-called mitral cells.
FIG. 94. An oblique section of the forebrain of Lampelra, passing through the optic chiasma and the olfactory commissure. a. o., olfactory lobe; c. h., optic chiasma; c. o., olfactory commissure; /. b. ven., fore brain ventricle; /. o. olfactory bulb; m, mitral cells; r. po., recessus postopticus; s, striatum; II, optic tract.
In true fishes and amphibia the structure of the bulb is much the same as in cyclostomes but there is a growing specialization among the cells. The structure of the bulb in the sturgeon, which is shown in Fig. 95, may be taken as typical for fishes and amphibia. There are a great number of stellate and spindleshaped cells which enter into functional relations with the olfactory fibers as in cyclostomes. The mitral cells also are well developed and show essentially the same character as in higher vertebrates. They are very large cells lying in the fiber zone which have thick dendrites with a comparatively small number of branches. Each of these branches enters a glomerulus where it divides into a very complex bush of end twigs. There also end in the glomerulus the branches of a number of olfactory fibers. These may subdivide repeatedly and bend back and forth, so that the glomerulus becomes an exceedingly complex interlacing of dendrites and olfactory fibers. The glomeruli also receive the dendrites of stellate and spindle cells (Figs. 96, 97) and some small glomeruli are formed by the dendrites of these cells alone. The transition from this to the condition found in mammals could probably be found in reptiles but has not yet been studied.
FIG. 95. A horizontal section through the olfactory bulb of the sturgeon.
FIG. 96. A spindle cell and two granules from the olfactory bulb of the sturgeon. g, granules; gl., glomeruli; sp., spindle cell.
In mammals the mitral cells are very highly developed and there are in addition to them numerous cells with short neurites. (Fig. 98) In the deeper parts of the bulb are a great number of small cells which seem to have no neurites, and whose dendrites rise toward the glomerular zone without entering the glomeruli. These are known as granule cells and probably represent the greater number of spindle and stellate cells of lower forms. It is supposed that hi the course of evolution the mitral cells became predominant while the smaller and less efficient cells lost their function and now exist only as vestigeal structures, the granule cells. By some authors, however, the granule cells are described as of two kinds; one possessed of short neurites ending within the bulb, the other giving rise to excessively fine neurites which run hi the olfactory tract to the forebrain nuclei. Mention must be made of centrifugal fibers which come forward from the forebrain in the olfactory tract and end in the olfactory bulb. It is probable that these are commissural fibers coming from the bulb of the other side.
FIG. 97. An olfactory glomerulus from the brain of the sturgeon to show the part taken by the dendrites of deep 'cells in forming the glomeruli.
In all vertebrates the cells whose dendrites help to form the glomeruli send their neurites inward toward the ventricle and backward toward the forebrain, and there is no evidence that the neurites from the different types of cells have any different destinations or behavior. The whole number of fibers arising in the olfactory bulb (or ending in it from behind) form the olfactory tract. In many fishes and some reptiles the olfactory bulbs are drawn forward some distance from the forebrain so that the olfactory tract is a long cylindrical structure which contains a slender continuation of the forebrain ventricle. In cyclostomes, amphibia and mammals the bulb is closely applied to the forebrain so that the olfactory tract does not run for any considerable distance free from surrounding gray matter. In all cases the fibers of the olfactory tract enter one or other of several groups of cells which collectively should be called the olfactory lobe or area.
FlG. 98. Cells with short neurites in the olfactory bulb of mammals. Schematic figure from Cajal. A, Golgi cell; 5, cell with peripheral neurite; C, horizontal fusiform cell of the internal plexiform layer; D, cell with horizontal neurite; E and F, periglomerular corpuscles; a, collateral of mitral cell neurite; 6, collateral of a small brush cell.
The olfactory tract is the secondary central tract of the olfactory apparatus; the nuclei of the olfactory lobe are the secondary nuclei.
These secondary nuclei form a larger or smaller part of the forebrain in different vertebrates according to the importance of the olfactory organ. In cyclostomes the lateral lobes of the forebrain are divided by a vertical groove into anterior and posterior halves. The anterior half is the olfactory bulb ; the posterior half is mostly occupied by the lobe, which is not divided into nuclei but is a continuous mass. In selachians the bulb is distinct from the lobe and a longer or shorter olfactory tract is present. The olfactory lobes occupy the anterior, the lateral and a larger or smaller part of the dorsal wall of the forebrain. The olfactory organ is very important in selachians and the large development of the olfactory centers accounts for the relatively great size of the forebrain in those fishes. In bony fishes the olfactory organ is much less important and the centers are correspondingly small, being confined to the lateral and ventral part of the forebrain. The caudo-lateral part becomes especially differentiated and is known as the nucleus thaeniae. In amphibia and reptiles the olfactory lobe has a similar extent. In mammals, owing to the great development of the cerebral hemispheres, the olfactory lobe is relatively very small. It includes the tuberculum olfactorium and praecommissural body occupying a small area on the ventral and mesial aspect of the hemisphere in front of the commissures, and the pyriform lobe which extends along the ventral surface of trie corpus striatum at the junction with the lateral cortex.
The olfactory lobe may be divided into cephalic and lateral portions. The cephalic portion occupies the front wall of the forebrain, in front of the commissures. In lower vertebrates it includes two or more collections of cells which have been called the (median, lateral and dorsal) postolfactory nuclei. The lateral portion is an extensive nucleus covering the outer surface of the corpus striatum, a part of which is the nucleus thaeniae.
The cells of all of these nuclei are irregular stellate or spindle cells which show no definite orientation. The fibers which end in these nuclei are all olfactory tract fibers which come from the bulb of the same side or from that of the opposite side, crossing in the anterior commissure. The fibers which arise from the nuclei form two main tracts, the tractus oljacto-hypothalamicus and the tractus oljacto-habenularis. The fibers of the former tract arise from the cells of both portions, run backward through the lateral and ventral parts of the corpus striatum and at the junction of the forebrain and thalamus appear in a medial and a lateral bundle. These enter the diencephalon and end in the hypothalamus. The tractus olfacto-habenularis is smaller. It arises chiefly from the nucleus thaeniae and sometimes from the other olfactory nuclei including the nucleus praeopticus in the walls of the preoptic recess, and runs upward and backward through the epistriatum to the dorsal part of the diencephalon. There each tract wholly or partially crosses to the opposite side, the decussation of the two tracts forming what is known as the superior or habenular commissure. The tracts end in the nuclei habenulae. These two tracts are tertiary olfactory tracts.
FIG. 99. A transverse section of the brain of the sturgeon at the level of the anterior commissure.
Another secondary nucleus in the forebrain is to be described. The central gray matter surrounding the forebrain ventricle, bounded in front by the postolfactory nuclei and covered laterally and ventrally by the lateral olfactory nucleus and the corpus striatum, is known as the epistriatum. The epistriatum is composed of pyramidal cells arranged in rows, the cell-bodies being near the ventricle and the dendrites directed toward the surface.
A large number of fibers of the olfactory tract which decussate in the anterior commissure end in the epistriatum. As has been mentioned (p. 181), the epistriatum receives also fibers from the hypothalamus, which probably in large part or wholly carry gustatory impulses. The epistriatum is thus related to both the sense organs which are concerned with the food relations of the organism. A large part of the epistriatum cells have short neurites which end hi the underlying corpus striatum (Fig. 149) from which the tractus strio-thalamicus goes backward. This constitutes a third path by which olfactory impulses may reach the diencephalon. As the result of the distribution of these three tracts the greater part of the gray matter surrounding the third ventricle is brought into the service of the olfactory organ. In selachians, where the olfactory apparatus reaches an unusually great development, the nuclei and commissures require some explanation. In all lower vertebrates the olfactory portion of the anterior commissure is more or less distinct from the portion which is formed of fibers coming forward from the hypothalamus. In Petromyzon the olfactory commissure is quite separate and is situated farther forward and upward on the front end of the forebrain (Fig. ioo). In selachians the dorsal and lateral olfactory nuclei extend up from the front and side walls to form the thick nervous roof of the forebrain which has been mentioned. No olfactory commissure has been seen in selachians but a large tract arises from the dorsal olfactory nucleus on either side and passes to the opposite side in the roof of the forebrain to end in the epistriatum. The decussation of these tracts seems to take the place of the larger part of the anterior commissure of other fishes, since the anterior commissure proper hi selachians is very small. It should be noticed that this is a tertiary olfactory tract and that it forms a decussation and not a true commissure. In some other fishes similar fibers are found which run from the olfactory nuclei on one side through the anterior commissure to the epistriatum of the other side. The peculiarities in selachians are the large size of the olfactory nuclei and of the tertiary tract and the consequent shifting forward of this tract (cf. further Chapter XVIII).
FIG. ioo. An outline of the median sagittal section of the forebrain of Lampetra.
The olfactory apparatus of fish-like vertebrates may be summarized as follows. The fibers of the olfactory nerve arise from the sense cells and end in the glomeruli of the olfactory bulb. The fibers from the cells of the bulb constitute the (secondary) olfactory tract which is distributed to the olfactory lobe and the epistriatum. A part of the fibers of the olfactory tract, including probably all those to the epistriatum, cross to the opposite side of the brain in a special part of the anterior commissure. From the nuclei of the lobe tertiary tracts go to the epistriatum through the anterior commissure, to the nuclei habenulae through the habenular commissure and to the hypothalamus for the most part without crossing. The epistriatum gives its fibers to the striatum which in turn sends a tract to the thalamus. From the thalamus and hypothalamus tracts are sent to the medulla oblongata and cerebellum where they make direct and indirect connections with the motor nuclei. From the nuclei habenulae a tract goes to the corpus interpedunculare and adjacent nuclei in the base of the mesencephalon, from which further connections with motor nuclei are made. See Figure 101 and the figures in Chapter XVIII.
In amphibia, reptiles, birds and mammals the olfactory apparatus differs from that described for fishes chiefly in the greater development of the tertiary centers within the forebrain which constitute the olfactory cortex of the cerebral hemispheres. The relations and homologies can be treated best in a separate chapter dealing with the hemispheres (Chapter XVIII).
The primary relations of the olfactory apparatus to the rest of the nervous system is one of the most difficult problems before us. It has appeared in previous chapters that all the parts of the nervous system concerned with the reception of stimuli affecting the bodily welfare of the organism in its surroundings are morphologically as well as physiologically related. The stimuli to which the sense organs respond are physical vibrations of some form (changes of pressure, sound- vibrations, light- vibrations), and all of the peripheral and central organs have probably been developed out of the general cutaneous nerves and centers as a common fundament. On the other hand, the gustatory system of nerves and centers is almost inextricably bound up with the general visceral. This condition is readily understood if it be true (p. 164) that the taste buds first appeared in the entodermal lining of the pharynx to which the general visceral fibers were already distributed.
FIG. 101. A diagram of the olfactory conduction paths in the sturgeon. E, epitriatum. The tractus strio-thalamicus is not lettered. It is placed horizontally in the center of the figure.
The olfactory organ is closely related in function with the gustatory. The stimuli affecting both are changes in the chemical character of the substances present in the indifferent water by which the organs are constantly bathed. In both cases the nerve impulse is probably aroused by the entrance into the sense cell of a small modicum of the stimulating substance. Since there is this similarity in the stimuli and in the purpose which the gustatory and olfactory organs serve, the question whether there is any morphological relation between the two becomes of considerable interest.
A direct comparison of the olfactory organ and the taste buds is impossible, for the reason that the sense cells of the olfactory organ themselves give rise to the fibers which carry their impulses to the central nervous system, while the fibers which carry taste impulses arise from ganglion cells situated in the cranial nerve ganglia. The olfactory sense cells are to be grouped together with the ganglion cells of general visceral and gustatory fibers as primary visceral receptive cells, while the rod and cone cells of the retina and the ganglion cells of the general cutaneous and acusticolateral fibers are primary somatic receptive cells. The cells of the two categories are broadly homologous, as primary receptive cells possessing neurites. The taste cells and neuromast cells are accessory sense cells without neurities. Turning to the central nervous system, a certain connection between the gustatory and olfactory apparatus is at once evident since the chief tertiary tracts of both systems enter the hypothalamus. When the structural relations of the correlating centers of the brain are examined in the following chapters, evidence will appear that all of the gustatory and olfactory centers belong to a continuous zone of the brain of which the visceral lobe in the medulla oblongata is a part. It is necessary that organs which function as visceral sensory organs should be related to the same set of central nuclei.
Demonstration or Laboratory Work
- Study the development and histology of the olfactory organ in fish, amphibian or chick embryos by the method of Golgi.
- Study the histology of the olfactory bulb in the brain of a fish or frog. Golgi method.
- Study the arrangement of the secondary nuclei and tracts in dissections of the brains of a selachian and a frog and in serial sections by the Weigert and Golgi methods.
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Catois, E. H.: Recherches sur Phistologie et I'anatpmie microscopique de Pencephale chez les poissons. Bull. sci. de la France et de la Belgique, Tome 36. 1901.
Disse, J.: Riechschleimhaut u. Riechnerv u. s. w. Merkel u. Bonnet's Ergebnisse, Bd. n. 1901.
Edinger, L.: Untersuchungen iiber die vergleichende Anatomine des Gehirns. I. Das Vorderhirn. Frankfurt a. M. 1888. III. Neue Studien iiber das Vorderhirn der Reptilien. 1896.
Van Gehuchten, A. : Contribution a Petude de la muqueuse olfactive chez les mammiferes. La Cellule, 1891.
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Johnston, J. B.: The Olfactory Lobes, Forebrain and Habenular Tracts of Acipenser. Zool. Bull., Vol. i. 1898.
Johnston, J. B.: The Brain of Petromyzon. Jour. Comp. Neur., Vol. 12. 1902.
Kappers, C. U. A.: The Structure of the Teleostean and Selachian Brain. Jour. Comp. Neur. and Psych., Vol. 16. 1906.
Koelliker, A.: Gewebelehre. 6te Aufl., Bd. 2.
Rubashkin, W.: Ueber die Beziehungen des Nervus Trigeminus zur Riechschleimhaut. Anat. Anz., Bd. 22. 1903.
Schultze, M.: Untersuchungen iiber den Bau der Nasenschleimhaut, namentlich die Struktur und Endigungsweise der Geruchsnerven beim Menschen und den Wirbelthieren. Abhandl. d. Naturf . Gesellsch. zu Halle, Bd. 7. 1862.
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