Book - The Nervous System of Vertebrates (1907) 4
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Chapter IV. Nerve Elements and Their Functions
The chief functions of nerve elements are the origination and transmission of nerve impulses. A nerve impulse originates as the result of a stimulus. Although the stimulus may be one of several kinds, mechanical, chemical, thermal, photic, etc., it always consists in some change in the environment of the nerve cell. Whatever the character of the stimulus, it produces in the protoplasm of the nerve cell chemical and physical changes of such a character that they may be propagated or transmitted through the protoplasm from one part of the nerve cell to another. These changes within a nerve cell constitute a nerve impulse. Although it is called forth by a stimulus, the impulse is not that stimulus caught up and passed on, but is a new thing consisting in the activity of the nerve cell itself. Besides the origination and transmission of impulses it is believed that other functions may be performed by nerve elements. They may reinforce or strengthen the impulses during their transmission; they may store up weak impulses so as to discharge a stronger one after an interval (summation) ; such discharge may be repeated at more or less regular intervals (rhythmical nerve action) ; or the impulses may be more or less completely blocked or impeded in their transmission (inhibition). The consideration of these processes within nerve elements belongs to the field of general nerve physiology with which the present book does not deal.
Although the chemical and physical nature of a nerve impulse may and probably does differ in relation to the internal structure of the nerve cell and the character of the stimulus, the importance of the nerve impulse to the organism consists in the effect produced when the nerve impulse is delivered by the nerve element to some other tissue element. What tissue element shall receive the impulse; after how long a time it shall be delivered; whether it shall be delivered in full force, strengthened or inhibited, these things which are all-important in the life of the organism depend upon the form of the nerve elements and the manner in which they are arranged. While the work done by the nervous system is our primary interest, we can understand this only by first understanding the construction of the mechanism which does the work. On the other hand the work done must be held in mind in order that we may truly interpret the constituent parts of the mechanism. The study of the minute structure and physiology of the nervous systems of various classes of animals has shown that nerve elements usually transmit impulses in a given direction. This is equivalent to saying that the elements have a specific form, as otherwise all directions would be alike. In vertebrates as a rule nerve elements present structurally and functionally a polar differentiation; the two ends of the element differ in both form and function. The nerve element consists of a mass of protoplasm containing a nucleus, it is a cell. Usually the nucleus is immediately surrounded by a larger mass of protoplasm which is called the cell-body. From this cell-body there extend more or less slender strands of protoplasm called processes. In nearly all cases the processes are seen to be of two forms. Some are relatively thick, irregular and have numerous branches; one is relatively slender, uniform in diameter, gives off collateral branches and is profusely branched at the end of its course. The former bear a general resemblance to a bush or branch of a tree and are hence called dendrites or dendrons; the latter is more thread like and since it forms the axial and essential portion of a common nerve fiber it is called a neurite or axone. A nerve cell commonly possesses two or more dendrites but usually has only one neurite. The volume of all the dendritic processes may exceed by many times the volume of the cell-body, and even the volume of the neurite may be considerable when it is very long or has many end branches. In certain cases the cell-body is elongated and the dendrites arise from one end and the neurite from the other. Most nerve cells, however, do not so clearly illustrate polar differentiation in their form. The dendrites may arise irregularly from various parts of the cell-body and the neurite may take its origin from one of the dendrites at some distance from the cell-body. Figure 39 shows several forms of nerve cells with reference to polar differentiation.
With respect to the form and disposition of their neurites, two chief types of nerve cells are distinguished. In the first and more common type (type I) the neurite is long and serves to make a connection with some distant nerve cell, muscle cell, gland cell, etc. In the second type (type II) the neurite is short and divides into terminal branches in the near vicinity of its cell, serving to make connection with several other cells of the same nerve center (Fig. 39, E).
FIG. 39. Several types of nerve cells from the central and peripheral nervous system of vertebrates. A, spinal ganglion cells; B, a Purkinje cell from the brain of the sturgeon; C, a cell from the nucleus praeopticus; D, a granule from the cerebellum; E, a cell of type II from the tectum opticurn.
The structural differentiation of the cell is correlated with a functional polarity. For, the nerve impulse within a given cell, whatever its source, begins in the dendrites or cell-body and is transmitted toward and along the neurite until it is passed on to some other cell through the branches of the neurite. In the case of the spinal ganglion cell there are but two processes from the cell and both these are slender, like neurites. As shown in the last chapter (p. 50) the process which receives the impulse, the peripheral fiber, corresponds in form and time of development to the dendrite of a cell of the usual form (Fig. 39, A and B). In the retina and in the olfactory epithelium the sensory cells seem to have no dendrites but instead only rod-like or hair-like peripheral processes.
In the cells which are usually regarded as typical nerve cells in the central nervous system of vertebrates the impulse starting in the dendrites passes through the cell-body and along the neurite. Probably in most cells in the central nervous system, however, and in the unipolar spinal ganglion cells the neurite arises from a dendrite at a longer or shorter distance from the cell-body. In these cases the most direct route for the nerve impulse is to pass from the dendrites to the neurite without entering the cell-body. Although it has long been supposed that such is probably the course of the impulse, it has not been possible actually to demonstrate it in vertebrates. In the crab Carcinas, Bethe has found a ganglion in which it was possible to secure experimental proof of this hypothesis. In the ganglion which supplies a sensory nerve to the second antenna the sensory cells have a form analogous to that of the unipolar spinal ganglion cells of higher vertebrates. Each cell body gives off a single process which is very much elongated and divides into a dendrite which serves as a sensory fiber to the antenna and a neurite which makes connection with motor cells in another part of the ganglion (Fig. 40). Without explaining the arrangement in 'detail, it may be said that the unipolar sensory cells are so placed that by very careful manipulation with fine instruments it was possible to remove the cell-bodies without disturbing the point of division of the single process into dendrite and neurite or the course and terminations of these processes. By artificial stimulation of the antenna, now, it was found that the same muscular reflexes followed as in the normal animal. Evidently the nerve elements perform their functions in the usual way in spite of the cell-bodies being cut away. The phenomenon of summation was also witnessed. It was further observed, however, that after a few days the functional responses ceased, presumably because of the death of the nerve fibers whose cell-bodies had been cut away.
It was supposed by the older anatomists and physiologists that the dendrites of a nerve cell were merely protoplasmic expansions which served to absorb the nutrient materials necessary for the cell, and they were called " nutritive processes. " It was thought that the cell-body and neurite carried on all the specific nervous functions. Since it has been shown that the dendrites play apart in the nerve activities not less important than that of the cellbody and neurite, they have not on that account lost their importance for nutrition. Since they offer by far the greatest surface for absorption of nutriment, together with a sufficient bulk of protoplasm for the purpose, it is probable that absorption does take place chiefly through the dendrites. But the final result of Bethe's experiment shows that the cell-body, or more especially the nucleus which it contains, is necessary for the nutritive activities. In nerve cells as in other cells the nucleus plays an essential part in the metabolic processes. Deprived of its nucleus the nerve cell soon dies, although it may carry on its normal functions so long as the nutrient materials contained in the protoplasm suffice for the necessary metabolic activities.
FIG. 40. A cell from one of Bethe's figures to illustrate the experiment described in the text. The arrow shows where the single process was cut.
As each nerve cell has two poles, an in-coming and an outgoing, so the nervous system as a whole receives stimuli from or through other tissues and gives out stimuli to other tissues. The nerve elements are so arranged in the nervous s> stem that certain elements serve to receive all the stimuli, while others give out impulses to the rest of the body. The former cells may be called receptive cells; the impulses which they carry into the central nervous system, afferent impulses. The cells which give out impulses may be called excitatory cells; and their impulses efferent or out- going impulses. The afferent impulses do not always give rise to sensations. When they do, the impulses may properly be called sensory impulses and the fibers which carry them, sensory fibers. The efferent impulses go to glands as well as to muscles. Those which go to muscles may properly be called motor or excitomotor impulses; those which go to glands may be called excitoglandular impulses; and the fibers concerned may be given corresponding names.
The elements of the nervous system are so arranged that the dendrites of the receptive cells are directed toward the periphery and constitute what are commonly known as sensory nerve fibers. The neurites of the excitatory cells likewise extend to the periphery and constitute excito-motor and excito-glandular fibers. These two sets of fibers, together with numerous sense organs, constitute the peripheral nervous system, within the limits of which the cell-bodies of the receptive cells may also be included. A great number of other cells, which make up the central nervous system, are engaged in transmitting impulses from the receptive to the excitatory cells, and in distributing and coordinating the impulses in such ways as to produce through the excitatory cells definite responses to the stimuli received. The only collective term for all the various categories of cells performing such functions is the term central cells. The relations of the sympathetic system to the peripheral and central portions of the nervous system will be taken up in a special section (Chapter XIII).
In order to understand the functional relations of the several kinds of nerve cells to one another and to the rest of the organism, a certain type of relatively simple actions which are performed by all animals may be examined. If a frog be touched or pinched it will usually make a single prompt movement which may result in drawing the part concerned out of danger. If a flash of light suddenly falls on the eye of a fish, or if a swiftly moving shadow crosses the water, the fish may make a quick dart due to a single stroke or a few strokes of the tail. Further movements of either frog or fish may be indirectly connected with the stimulus mentioned, but it is the initial, relatively simple movement which concerns us here. We ourselves make similar movements under certain conditions. In our sleep we may move a hand to brush away a fly without being conscious of the act. We commonly toss about more or less in our sleep, and quite unconsciously. Certain waking movements are also of the same simple class, as when the eye-lid is suddenly closed to shut out a flying insect which is not consciously seen, or when sudden coughing is caused by some object entering the trachea. Probably many simpler mechanical operations, such as walking, fall at least at times under the same category of simple actions.
If these simple actions are studied experimentally under proper conditions we can determine what nerve elements are engaged and how they act. First, if in a frog or other lower animal the brain be entirely destroyed, the sort of actions mentioned are still performed with normal efficiency. Only the spinal cord and the peripheral elements connected with it are necessary. In the simplest case a single receptive cell whose body is situated in the spinal ganglion, receives the stimulus through the terminal branches of its dendrite in the skin and transmits an impulse along its centrally directed neurite. Within the spinal cord lateral branches are given off from the neurite and one of these collaterals carries the impulse to one or more excitatory cells lying in the ventral horn of the cord. These cells send impulses out along their neurites in the ventral root of a spinal nerve to certain muscles whose contraction produces the observed movement. Thus only receptive and excitatory cells are engaged in the whole action from the reception of the stimulus to the movement in response. This is illustrated in the left half of Fig. 41, A. The movement is called a reflex movement, the whole act including the nervous processes is a reflex act and the chain or series of nerve cells concerned is known as the reflex chain or reflex arc. In this simplest case the reflex chain has but two links. Probably only the simplest movements, such as a jerk or twitch, are produced by so simple a chain.
Usually the afferent impulse instead of being handed over directly to an excitatory cell, is spread more widely through the spinal cord by means of a larger number of branhcesof the incoming neurite and by means of central cells (Fig. 41, B, D). The arrangement of these cells will be treated later (Chapter XIV) but it may be said that at least one set of cells usually intervenes between the receptive and the excitatory cells. The function of these cells is to spread the impulse so that it will affect a larger number of excitatory cells and so provoke a more ample or more intense movement in response. The extent to which the muscles of the body may be called into movement by such a reflex mechanism depends chiefly upon the strength and the duration of the stimulus. A larger number of links may be introduced into the reflex chain and the excitatory impulses may pass out over several nerves and give rise to coordinated contractions of many muscles. In this case we may speak of a complex reflex. A large number of the ordinary actions of the body are carried out in this way without the intervention of the brain. A large part of the brain also, even in man, may take part in such reflexes without voluntary effort or effect in consciousness. When the brain is involved, however, even in the frog or other lower animal, the actions are likely to become so long continued, so complex and varied that they are difficult to analyze into their simple constituents and to describe as reflexes. Nevertheless in all these cases essentially the same events are taking place. Receptive cells when stimulated transmit impulses which, after passing through a larger or smaller number of central cells, are sent out by excitatory cells to provoke contraction of muscles. If we speak of the cells through which an impulse is transmitted as the path of the impulse, the path in this case instead of being simple and direct is more complex, and the movements resulting may be much more complex and the means by which the end is attained more indirect than when the spinal cord alone is involved.
FIG. 41. Diagrams intended to show several forms of reflex chains in the nervous system of vertebrates. A, somatic sensory and motor reflex; B, reflex by way of tract cells; C, visceral sensory and motor reflex; D, a diagram of the spinal cord and nerve roots from the side, showing the same elements as in A, B and C. d. c. t., direct cerebellar tract.
It is evident that there is no limit to the extent to which this conception of the reflex activity may be carried. So far as the nervous activities are concerned they are always of the same general type as that represented in the reflex. The term reflex is properly used only when a stimulus, through the medium of the nervous system, provokes a responsive action in the organism. But, given a nerve impulse of any kind, aroused in any way, the series of events in the nervous system is similar to that of a reflex act. The impulse may pass from cell to cell, the motor response may be inhibited, and the impulse traveling into the sensory areas of the cerebral cortex, a sensation may result. This may mark the beginning of a new series of events within the cerebral cortex, associations, memory, thought processes. In all cases we are dealing with the origination of nerve impulses and their transmission over definite paths which may be anatomically studied. It is these anatomical pathways, themselves determined by experience, hereditary and individual, which determine the course taken by nerve impulses and the responses of the organism to stimuli. Our object therefore is to trace out these pathways of impulses, to arrange them into systems of nerve elements with reference to their appropriate stimuli and responses, and to give an orderly account of these systems.
The Neurone Theory
The cell theory of Schleiden and Schwann (1838-39) was a statement of the general conclusions of anatomists and embryologists up to that time regarding the mode of construction of the animal and plant organism. In brief, it was to the effect that every higher animal or plant is made up of many individual organisms known as cells. The cell was regarded as the unit of structure. Each cell carried on its own processes and had its own life history, and at the same time joined with others to produce the structure and actions of an organism of higher order. The cells of each tissue were like one another and the differences between tissues depended upon the characters of the cells constituting each. As the knowledge of various animal and plant tissue was increased, this theory was extended and amplified and remained for fifty years the best expression of our knowledge. In 1891 the cell theory was stated in a special form as it applied to the nervous system. This statement of the cell theory of the nervous system has since been known as the neurone theory of Waldeyer, who formulated it. The theory may be stated as follows. The nervous system consists of cells each of which (i) arises from a single embryonic cell, processes of which grow out to form the neurite and dendrites; (2) remains as an independent cell in adult life, making connections with other cells only by contact of its processes with the processes or cell-bodies of other cells. The constituent elements of the nervous system were called by Waldeyer neurones. As the result of later discussion of this theory two further points have been added to it; (3) the structural and functional polarity of the neurone; and (4) that all parts of the neurone constitute a trophic unit for whose continued metabolic activity the presence of the nucleus is necessary. When the neurone is cut in two in any way only that part which retains the nucleus is capable of long continued functional existence.
FIG. 42. A portion'of the subepithelial nervous plexus in the palate of the frog. From Prentiss.
Even before this theory had been expressed the cell theory itself was being severely criticized and it may be said that since the year 1890 it has undergone important modifications. Although we may still speak of many- celled animals and may regard cells in a general way as units of structure, we can no longer consider cells as unit organisms which join in the formation of an organism of a higher order. Cells are not always completely bounded by cell walls and in many cases adjacent cells are directly continuous with one another by means of strands of protoplasm. Moreover, in the course of embryonic development the number, form, position and multiplication of cells do not determine the course of differentiation. The cells are not controlling organisms which possess the iniative and directive power in these processes, but are perhaps only mass divisions made necessary by the metabolic relations of the nucleus and cytoplasm. The course of developmental differentiation is determined by a more fundamental organization which exists in the whole protoplasm of the organism before its division into cells, and by the interactions between the organism and its environment during development. The cells are rather the plastic material of differentiation. As the processes of differentiation complete themselves, however, cells and groups of cells become the means or organs for the performance of certain functions. Organs are cell- complexes and the function performed by an organ is the mass result of the functioning of its constituent cells. Thus it may be stated as a general rule that each mass of protoplasm containing a nucleus is essentially a structural and functional unit of the organ. It is the nucleated mass of protoplasm which maintains itself as a structural entity although it may be connected by plasm strands with its neighbors, and which performs its specific part of the function of the organ.
FIG. 43. A portion of the network about the walls of a small vessel in the palate of the frog. From Prentiss.
These considerations apply to the nerve cell as well. It has been clearly shown that in many cases nerve cells are in continuity with one another. In the case of some peripheral plexuses (Figs. 42, 43), dendrites are fused with dendrites into an intricate network. In other cases neurites are fused with the protoplasm of other nerve cells instead of merely ending in contact with them. That part of the neurone theory which states that nerve cells make connections with one another only by contact is definitely disproved. Other criticisms which have been brought against the neurone theory are rather in the nature of extensions and corrections of the conception of neurones.
FIG. 44. Two ganglion cells of the nervous network in the intestinal wall of the leech, Pontobdella, showing neurofibrillae passing through the cells. From Bethe after Apathy.
The relationships of the neurofibrillae have been represented as antagonistic to the neurone theory. Although the existence of fibrillae in the protoplasm of nerve cells has been known for nearly fifty years, it is only in the last few years that they have been extensively studied and described. They have been found in many classes of animals and in many kinds of cells in the nervous system of invertebrates and vertebrates, so that their existence as structures characteristic of nervous tissue is quite certain. Further, it has been clearly shown that when two nerve cells are connected by strands of protoplasm the neurofibrillae may extend from one cell into the other. Neurofibrillae have been described as running through two, three or more cells without interruption. With regard to the origin, structural character and function of these fibrillae, further study is needed. It is said that they are formed outside of nerve cells and grow into them, that they may extend beyond the limits of the protoplasm of nerve cells as in nerve-muscle endings, and that they are the medium of conduction of nerve impulses, while the nerve cells perform only the incidental function of nutrition. Indeed the neurofibrillae are regarded by some as quite new structures in the nervous system in addition to nerve cells; structures which are more intimately and essentially concerned in specific nervous functions than are the nerve cells themselves.
Certainly in the present state of knowledge this view is extreme. It seems more reasonable to regard the neurofibrillae as a dense portion of the colloid substances in the cytoplasm of nerve cells, whose definite form and arrangement are conditioned upon the intimate structure of the protoplasm as a whole. Thus if the protoplasm has the structure of a foam, then the colloid substances forming the walls of the vesicles and filling the solid angles, in elongated strands such as dendrites and neurites would certainly appear as threads. In more rounded cell-bodies the colloid substance would be more irregularly arranged and there would be the appearance of branching and crossing of threads, as is actually the case with the neurofibrillae. When the dendrites of two cells fuse together the neurofibrillae would of course continue from one cell into the other. In the most minute end branches of neurites the plasm may be so slight in amount that in preparations in which the colloid substance alone is sharply stained, the slender branches would appear to be composed of a neurofibrilla alone. Finally, in nerve-muscle endings it is conceivable that the neuroplasm and sarcoplasm should be fused together and that the neurofibrillae should continue into the sarcoplasm. This is at present the most probable interpretation of the appearances presented by the neurofibrillae. Whether the fibrallae constitute a special conducting substance in the nerve cell is doubtful. It is scarcely conceivable that any one substance in the nerve cell can originate and conduct a nerve impulse without interaction with other substances in the cell. However, if the nerve impulse and its transmission are phenomena in the production of which the various substances in the cell cooperate, then the neurofibrillae by reason of their density and their staining properties may reasonably claim our attention as indicators of the course taken by impulses.
The process of regeneration of injured nerves, although it may not coincide with the processes of normal growth of nerve elements, is instructive as to the nature of the neurone. The course of regeneration of a cut nerve seems to be as follows. The part distal to the cut, i.e. away from the cell-body, degenerates in agreement with the observations of Waller. The fibers proximal to the cut remain in a healthy condition and grow along the line of the degenerated distal part. Whether the growth of the fibers is sufficient to replace the whole length of the part cut away has not been positively demonstrated but is presumed in the absence of evidence to the contrary. When the proximal ends are not prevented from growing down along the degenerated nerve the naked extensions of the axis cylinders may be traced for a considerable distance. The degeneration of the distal part of the axis cylinders leads to changes in the sheaths. The myelin is resorbed and the sheath cells, or some of them, form strands of plasm which are to be regarded as in the nature of embryonic tissue. When the proximal end of the nerve is tied to one side so that its fibers cannot grow out along the nerve bed, it is said (Bethe) that the embryonic strands develop neurofibrillae and become able to conduct impulses. In adult animals the strands do not acquire this power and in any case connection with the central end of the nerve is necessary; without it the strand formed from sheath cells degenerates. The conditions of the experiments of Bethe are not such as to show what takes place in the course of normal regeneration. It is not shown that the strands derived from sheath cells form part of the nerve when the proximal stump is undisturbed. The later researches of Cajal are opposed to this "autogenetic regeneration" of Bethe and give positive evidence that the nerve is regenerated by outgrowth of the proximal stumps of the cut fibers.
For the continued performance of its normal functions all parts of the neurone are necessary. The fact that a neurone may continue to function for some days after the cell-body containing the nucleus has been cut away shows that impulses follow the shortest path from dendrites to neurite. The further fact that the dendrites and neurite die after a few days proves the trophic unity of the neurone. The Wallerian degeneration is evidence of the same. It appears also that injury to one of the processes of the neurone may start destructive changes which the neurone is unable to combat and the whole neurone may degenerate as the result of the cutting of its neurite (Gudden's degeneration). It is quite unnecessary to think that all parts of the neurone must enter at once into the chemical and physical changes which constitute any single act of the neurone. The notion that the cell-body acts as a ganglionic center with reference to its dendrites and neurite is no part of the neurone theory. Facts opposed to such a conception were known long before that theory was formulated, namely the unipolar form of spinal ganglion cells and the origin of neurites from the dendrites of many cells in the brain.
The neurone theory as an expression of our general ideas regarding the structure and functioning of the nervous system may be tentatively re-stated as follows.
- The nervous system is formed of cells each of which is derived from a single cell in the course of development. From each cell grow out one or more processes which are comparable in a general way to the pseudopodia of unicellular animals. Such cells may be called neurones.
- Although in their primitive condition, as commonly in invertebrates, neurones do not show a functional differentiation of their processes, typical neurones in vertebrates present a structural and functional polarity. Each neurone consists of dendrites, cell-body and neurite; and the impulse passes through the parts in the order named, or passes from dendrites to neurite without passing through the cell-body. Certain peripheral plexuses in which the dendrites of adjacent neurones are fused into an intricate network require further study with reference to this statement.
- When the neurite of one neurone has a functional connection with some part of another neurone, the plasm of the two neurones at the point of connection may be fused. Whether such a continuity of structure between the neurones of a chain or pathway is universal, or how frequent it is, must be determined by further study.
- In the protoplasm of neurones the apparent fibrillar structure is a constant and normal feature. The real nature of the neurofibrillae in the living cell, the mode of their formation, and the structural changes which they may undergo during the different phases of functional activity of the neurone are not yet understood. When a fusion between two neurones takes place the neurofibrillae take part in it.
- The whole neurone is necessary for continued functional existence. Any part of the neurone which is cut away from the nucleated portion degenerates. The mode of repair of such injury is by growing out of the cut fiber.
- The form and position of the neurones and especially the disposition and connections of their processes determine the pathways of impulses and hence the work done by the nervous system.
The practical value of the neurone theory is found in the last statement. The linking together of neurones in pathways is determined in part by inheritance of the general plan of arrangement and in part by the experience of the individual. The nervous system considered as a complex of neurones variously linked together into functional systems, is at once the mechanism by which certain work is done and a record of the experience of the individual and the race. A knowledge of how neurones are linked together in functional systems is necessary for the pathologist and psychologist. The factors which determine the manner of linking of neurones are the chief interest of the social psychologist, the educator and the social reformer.
Demonstration of Laboratory Work
- Study in Golgi preparations the form and structural polarity of nerve cells of types I and II in various parts of the nervous system.
- Study the spinal reflexes of the frog by the methods in use in the physiological laboratory.
- Study the structure of nerve cells by the method of Nissl and by the methylene blue and other methods for neuronbrillae.
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Bethe, Albrecht: Studien iiber das Centralnervensystem von Carcinus maenas u. s. w. Arch. f. mik. Anat., Bd. 44. 1895. Bd - 5 1 - l8 9 8 Bethe, A.: Allgemeine Anatomic und Physiologic des Nervensystems. Leipzig. 1903.
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Held, H.: Beitrage zur Structur der Nervenzellen und ihre Fortsatze. Arch. f. Anat. u. Physiol., Anat. Abth., 1897.
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Neal. H. V.: The Development of the Ventral Nerves in Selachii. Mark Anniversary Volume. 1903.
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Whitman, C. O.: The Inadequacy of the Cell Theory of Development. Jour. Morph., Vol. 8. 1893.
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Cite this page: Hill, M.A. (2021, March 3) Embryology Book - The Nervous System of Vertebrates (1907) 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Nervous_System_of_Vertebrates_(1907)_4
- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G