Book - The Nervous System of Vertebrates (1907) 8
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Chapter VIII. Somatic Afferent Division. The Visual Apparatus
The general and special cutaneous systems serve for the reception of stimuli from the external world due to mechanical contact or pressure or to vibrations in fluids. Another set of stimuli of the greatest importance to the vertebrate animal, namely those given by light, seem to common observation not to affect these cutaneous organs. As a matter of fact, it has been shown that light stimuli do affect the endings of general cutaneous nerves in such a way as to produce characteristic reactions. If a light of suitable intensity be allowed to fall upon a frog whose eyes have been removed, the animal will turn its head toward the source of light and jump toward it. If the frog's skin also be covered from the light no such reaction takes place. Other amphibia, some fishes and reptiles are influenced by light which falls on the skin alone. In the case of ammocoetes, which lives buried in the mud, the skin of the tail is more sensitive to light than any other part of the body, including the eyes. This condition is useful to the animal, since it burrows head foremost and the sensitiveness of the tail ensures that it shall completely bury itself.
While light must be reckoned as one of the most important factors in the external world influencing the organism, it is evident that the free nerve endings in the skin are not an adequate means for the perception of light. It is beyond the province of this work to inquire how light has influenced the organism so as to produce an organ for its perception. It is, however, the business of comparative morphology to consider whether the organism responded to external influences by producing an entirely new structure or whether the organ produced was a modification of some structure already existing, and if the latter, what was its probable mode or course of evolution. The attempt to answer these theoretical questions should add interest to the study of the development and nerve connections of the visual apparatus. It was shown in an earlier chapter that the lateral eyes are developed as a pair of vesicles from the dorso-lateral wall of the brain in about the region of the second neuromere (Figs. 18-22). Each vesicle comes to be attached to the ventral wall of the brain by a hollow stalk. There now appears in the ectoderm a thickening which sinks in to form a pit and finally separates from the ectoderm as a closed sac. This then develops into the lens of the eye. Meanwhile the optic vesicle becomes cupshaped toward the outside, enclosing the lens, and the wall of the concave side thickens to form the retina, while the convex wall remains thin. For the further development and adult relations of the eye-ball and accessory structures the reader must be referred to the larger text-books of embryology and anatomy. The histogenesis of the retina proceeds in a manner similar to that of the spinal cord or brain. Supporting structures are formed (the so-called Miillerian fibers) which are to be compared with the spongioblasts of the brain, and among these are found neuroblasts which give rise to the nerve cells of the retina (Fig. 71 A). On the inner surface of the retina, that is next to the portion of the brain ventricle which extends into the optic vesicle, there are formed spindle-shaped cells whose one process extends to the ventricular surface and ends in
FIG. 71. Histogenesis and structure of the retina. From Cajal. (Die Retina der Wirbelthiere.)
A, a section through the retina of a mouse embryo of 15 mm. a,b,c, neuroblasts; d,e, epithelial (Miillerian) cells; /, nucleus of rod cell.
B, a section through the retina of a chick embryo of fourteen days, a, b, epithelial cells; c, rod cell; d, deep granule; e,n, cone cells; m, bipolar cell; s,u, amacrine cells.
C, a section of the retina of a dog. a, cones; b, rods; c and e, bipolar cells related respectively to rods and cones; /, giant bipolar cell; h, amacrine cell; ', centrifugal fiber; n, layer of ganglion cells.
FIG. 72. Cells from the retina of the chicken.
a rod-shaped or cone-shaped structure, and From c a jai. a,6,centriwhose other process runs deeper into the retina and branches in relation with the dendrites of cells forming the inner nuclear layer (Figs. 71, 72). These cells in turn set up connections with the elements of the ganglionic cell layer. The fibers from the ganglionic cells form a fiber layer on the outer, concave surface of the retina. The fibers collect toward a point near the center of the retina, dip into its substance and pierce it to its inner surface. From this surface of the retina the fibers pass along the optic stalk to the brain. This arrangement is brought about during development by a change of form of the retina and shifting of position of the stalk. At first the stalk is at the ventral border of the retina and the fibers run to the ventral border and thence to the brain in the stalk. Later the retina grows in such a way as to bring the attachment of the optic tract nearer its center. Finally, in all vertebrates except cyclostomes and some fishes and urodule amphibia, the hollow stalk degenerates and leaves the optic tract as the only connection of the retina with the brain. It should be noticed now that this optic tract, although commonly called the optic nerve, differs from all peripheral nerves in several ways, (i) The cells from which its fibers arise are derived from the wall of the brain. (2) They form part of a many-layered nervous structure whose development and histology suggest comparison with the brain wall. (3) Although the fibers carry afferent impulses they enter the ventral wall of the brain. Indeed, it has long been recognized that this is not a peripheral nerve but a central nerve tract and that the retina is a part of the brain wall. When the optic tracts arrive at the ventral wall of the diencephalon (or before in bony fishes) they decussate, forming the optic chiasma. As a general rule the fibers from one retina pass to the opposite side of the brain, but in mammals a complete decussation of the optic tracts is rare. In different mammals a variable part of the fibers remain uncrossed and in man approximately one-third enter the same side of the brain as the eye from which they come. In vertebrates below the mammals most authors agree that the decussation is total. The beginnings of partial decussation are seen, however, in fishes. Golgi preparations of the chiasma of the sturgeon show a considerable number of fibers of the optic tracts giving off thick collaterals which enter the thalamus of the same side. The number of uncrossed fibers in any animal seems to depend upon the position of the eye. When the eyes are laterally placed as in most submammalian orders the uncrossed fibers are at a minimum. Among mammals, those whose eyes are lateral, so that the fields of vision do not overlap, have few uncrossed fibers; while those (apes, man) whose fields of vision largely overlap have a large part of the optic tracts uncrossed. The uncrossed fibers in man arise chiefly from the temporal part of the retina and run in a fairly well isolated bundle in the optic tracts and chiasma. Beyond the chiasma the optic tracts run up in the lateral part of the thalamus and end in the dorsal part of the thalamus and in the tectum opticum.
FIG. 73. Part of a section of the tectum opticum of the sturgeon, schematic.
Structure of the Tectum Opticum
In the lower fishes the tectum contains a large number of cells of several forms, most of which lie near the ventricle. The outer portion of the tectum is composed of fiber layers. Cells of both type I and type II are present. The cells of type II (Figs. 39 and 73) are slender spindle-shaped elements vertically placed near the ventricle, whose single thick dendrites rise toward the surface and branch profusely in the outer layers of the tectum. The neurite arises from some point of this dendrite far removed from the cell-body and branches very richly in the middle layers of the tectum in the immediate vicinity of the cell. All the other cells of the tectum may be described as cells of type I whose dendrites spread more widely and do not reach the outer surface of the tectum. Some of these cells have vertical spindle-shaped bodies and one or more branching dendrites, others have stellate bodies with several dendrites diverging widely, and still others are spindle-shaped but are placed tangentially and have two dendrites running parallel with the surface of the tectum. The neurites may arise from the cell-bodies or from any part of the dendrites, even from the tip of one of the tangential dendrites. The neurites arising from the cells of type I have various courses and destinations, (i) They cross to the opposite side, forming the dorsal decussation of the tectum. Whether any or all of these fibers leave the tectum after crossing is not known. (2) They enter the tractus tectobulbaris et spinalis and go either to the same or opposite side of the medulla oblongata and spinal cord. (3) They go down through the lateral wall of the mesencephalon or diencephalon to end in the inferior lobes, the tractus tecto-lobaris. Part of these fibers cross to the opposite side in the ansulate commissure in the ventral wall of the mesencephalon or in the postoptic decussation, or in both. (4) They go along the lateral border of the tectum to enter the cerebellum, the tractus tecto-cerebellaris. (5) They go out in the optic tract to end in the retina (Fig. 72). The last are called centrifugal fibers. The elements in the optic lobe of birds are shown in Fig. 74.
A special apparatus is described in connection with the tectum opticum of all classes of vertebrates which is supposed to serve at least in early life for direct and prompt reflexes in response to optic impulses. In the mesial part of the tectum is found a nucleus consisting of a variable number of cells which are usually of extraordinary size. This is known as the roof nucleus or nucleus magnocellularis tecti. The neurites of these cells are said to enter the ventricle and form the structure known as Reissner's fiber, which makes connections with cells in the gray matter of the spinal cord. By means of this apparatus it is supposed that aquatic animals are able to avoid obstacles and danger by movements more prompt than those which are directed by the more complex brain tracts.
In cyclostomes all the optic tract fibers end in the tectum opticum but in selachians a part of the tract ends in the diencephalon and in all higher forms an increasingly large part of the tract has this ending. The chief nucleus in which the fibers end is situated in the lateral wall of the thalamus and is known as the corpus geniculatum later ale. The morphology of the thalamic and midbrain optic centers and their tertiary connections in higher vertebrates will be discussed in a later chapter (Chap. XVI).
FIG. 74. A section of the optic lobe of a bird. From Cajal (Textura, etc.). The letters refer to extended descriptions in Cajal's text.
The question of the origin of the eye and of its relation to other sensory systems may now be reviewed. Obviously a close relation would not be expected between the eye and the nerves or organs of visceral sensation. Is the eye, then, related to cutaneous sense organs and has it been formed by modification of any preexisting organ, or is it a wholly new structure ? The facts show a remarkable similarity between the eye and the general cutaneous structures of other segments. The retina is derived from the dorso-lateral wall of the second neuromere. It therefore represents a dorso-lateral nucleus and as such corresponds to the cutaneous nuclei in the hindbrain and spinal cord. The optic tract which arises in the retina is a secondary brain tract for which the tectum opticum is the secondary nucleus. The secondary tract decussates in the ventral wall of the brain as do the secondary tracts from the cutaneous nuclei, and ends in the same center with those tracts, the tectum opticum. The tectum itself is a primary cutaneous center, since in several classes of vertebrates it receives a part of the sensory trigeminus nerve, and in the embryos of lower vertebrates the ophthalmicus profundus nerve arises from the roof of the midbrain. In lower vertebrates the mode of formation of the optic vesicle bears a significant resemblance to the mode of formation of the general cutaneous ganglia. In selachians, for example, both are formed as hollow outgrowths from the dorso-lateral wall of the neural tube.
If to these facts there be added now the physiological facts with which this chapter was begun, a remarkably strong body of evidence is presented for a relationship between the eye and the general cutaneous sensory system. The general cutaneous nerves are susceptible to light stimulation and the impulses carried to the brain produce reactions which correspond to those produced when the eyes are the vehicles of light perception. Whether the frog has the use of its eyes or only of its skin it turns its head toward the light and jumps toward it. Not only are the cutaneous nerves and retina both affected by light waves, but the nerve centers of the two are in part identical, so that the impulses arriving in the brain from either source produce the same reflexes.
From these facts it must be supposed that the general ectoderm was originally sensitive to light and that in ancestral vertebrates the sensitiveness became greatest in an area favorably situated on the top of the head. The sensitive elements in the skin are the free endings of the dendrites of nerve cells. When the central nervous system sank below the surface the cells whose dendrites were distributed to the skin were in part enclosed within the neural tube (p. 37). The greater part formed the neural crest. In connection with the front part of the brain no neural crest is formed and it must be supposed that in this region the whole of the nervous ectoderm was included in the neural tube. In this area then the cells which were especially sensitive to light became the rod and cone cells of the retina. Each of these is a bipolar cell whose two processes are comparable respectively with the dendrites and neurite of a typical nerve cell or with the peripheral and central processes of a spinal ganglion cell. With the growing thickness and opacity of the muscles and skeleton overlying the brain tube it became necessary for the retinal area to project toward the skin in order to receive light stimuli. This area contained both primary receptive cells and brain centers. As it was carried out from the brain and the optic vesicle was formed, the stalk of the vesicle consisted chiefly of the fibers of the optic tract. Since these fibers were destined to decussate in the ventral wall of the brain, it was advantageous for the stalk to shift ventrad and so allow the tract to become shorter. This is illustrated in an accompanying diagram (Fig. 75). Although this special visual organ has been developed, the general cutaneous endings retain their sensitiveness to light in varying degrees.
FIG. 75. A series of diagrams intended to illustrate the origin and mode of formation of the optic vesicle in vertebrates.
In Amphioxus special light percipient cells are contained in the front end of the brain and throughout the spinal cord. These have been shown to be similar to the light percipient cells in the ganglia of certain worms. Whether there is any relation between the light percipient cells of Amphioxus and the lateral eyes of vertebrates is uncertain.
FIG. 76. A sketch showing the relations of the two epiphyses in vertebrates.
In addition to the lateral eyes two other organs of light perception are present in vertebrates, the degenerate or reduced pineal eyes. Although in most vertebrates only one of these structures is present, at least in the adult, both are present in adult cyclostomes, and in some reptiles one in front of the other. The position and innervation of the organ found in other classes of vertebrates show that sometimes it is the anterior and sometimes the posterior organ which is present. The anterior organ sends its nerve fibers into the nucleus habenulae or adjacent center. Those from the posterior organ go to the region of the posterior commissure and probably enter the tectum opticum.
These relations are illustrated by Figure 76. Although degenerate in all vertebrates, yet one or the other of these organs is capable of light perception, at least in cyclostomes and some reptiles, and perhaps in some fishes and amphibia. The structure of the organ is apparently much simpler than that of the retina, but in some forms the presence of rods or cones has been described. There is evidence that the pineal eyes are not median structures. The organ is never quite median in its adult form and in the embryo paired organs begin to develop and only one .persists. In selachians there have been described in early stages a series of accessory optic vesicles" following the true optic vesicles. The pineal eyes were probably originally paired organs serially homologous with the lateral eyes, and the three pairs represent the cutaneous sensory system in those segments of the head in which the cutaneous nerves and ganglia are wanting (p. 61).
Demonstration of Laboratory Work
- Study the development of the optic vesicle, lens and optic tract in some vertebrate embryos. Compare the histogenesis of the retina with that of the brain wall.
- Histology of the retina in Golgi preparations.
- Trace the course of the optic tracts and compare the chiasma with the decussation of internal arcuate fibers in the medulla oblongata
- Study the structure of the tectum opticum in Golgi sections of the brain of a fish and the frog.
- In Golgi or Weigert sections study the tracts arising in the tectum and colliculus.
Barker, L. F. : The Nervous System and its Constituent Neurones. New York. 1899. Chapter LIII especially.
Boveri, T.: Ueber die phylogenetische Bedeutung der Sehorgan des Amphioxus. Zool. Jahrb., Suppl. Bd. 7. 1904.
Cajal, S. R.: Die Retina der Wirbelthiere. Wiesbaden. 1894.
Eycleshymer, A. C. : The Development of the Optic Vesicles in Amphibia Jour. Morph., Vol. 8. 1890.
Froriep, A.: Die Entwickelung des Auges der Wirbelthiere. Hertwig's Handbuch der Entwickelungslehre. 1905.
Hesse, R.: Die Sehorgane des Amphioxus. Zeit. f. wiss. Zool., Bd. 63. 1898.
Johnston, J. B. : Das Gehirn und die Cranialnerven der Anamnier. Merkel u. Bonnet's Ergebnisse, Bd. u. 1902.
Johnston, J. B.: The Morphology of the Vertebrate Head. Jour. Comp. Neur., Vol. 15. 1905.
Johnston, J. B.: The Radix mesencephalica trigemini. Anat. Anz., Bd. 26. 1905.
Kerr, J. Graham: The Development of Lepidosiren paradoxa. Part 3. The Development of the Skin and its Derivatives. Quart. Jour. Mic. Sci., Vol. 46. 1902.
Locy, W. A. : Contribution to the Structure and Development of the Vertebrate Head. Jour. Morph., Vol. n. 1895.
Parker, G. H. : The Skin and Eyes as Receptive Organs in the Reactions of Frogs to Light. Amer. Jour, of Physiol., Vol. 10. 1903.
Parker, G. H.: The Stimulation of the Integumentary Nerves of Fishes by Light. Amer. Jour, of Physiol., Vol. 14. 1905.
Sargent, Porter E.: The Optic Reflex Apparatus of Vertebrates for Short-circuit Transmission of Motor Reflexes through Reissner's Fiber. Part i. Fish -like Vertebrates. Bull. Mus. Comp. Zool. Harvard Coll., Vol. 45. 1904.
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