Book - The Nervous System of Vertebrates (1907) 1
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Chapter I. The Study of the Nervous System
The nervous system is a complex system of organs which stands in close relations with all the other organs of the body. The study of the nervous system requires on the one hand an examination of the structure and mode of functioning of its component parts and on the other a consideration of its manifold relations to other organs. There must be considered (a) structural connections which provide for the reception and discharge of nerve impulses which play an essential part in the functioning of organs and in the correlation of their activities; and (b) the more general relations of place, the arrangement of nervous and other organs in the body. In both these respects the actual relations of the nervous system in the organism have been determined by the conditions and course of the evolution of the vertebrate organism, in which the adaptation of structure to the mode of life is the controlling principle; and by the conditions of embryonic development, in which special structures adapted to the embryonic mode of life and inherited structures adapted to ancestral modes of life play important parts. The student who wishes to gain a knowledge of the nervous system is of necessity thrown into this maze of complicated relationships, but fortunately he finds methods of investigation adapted to the problems to be solved.
While the student may direct his attention especially to the phase of structure or the phase of function, he should understand from the outset that great progress and permanent results are to be attained only when the study of structure and the study of function go hand in hand. While in general the detailed study of structure must precede the detailed study of function, the interpretation of nervous structures, the statement of problems and the planning of investigations for their solution gain most from more general considerations of function, namely, considerations of the conditions and mode of life of the organism.
The method by which the study of the nervous system was first undertaken is also the first to be used by the student of today, namely, dissection. The purpose of dissection is to show the general place relations of the parts of the nervous system to other organs and the apparent structural connections between the two. Even to the investigator of original problems the method is still useful, and as it must be practiced in preparation for physiological experimentation and operation for the study of degeneration phenomena, the importance of thorough training in dissection can scarcely be over-estimated. The refinement of dissecting microscopes and mechanical appliances in recent years has made dissection applicable to more and more minute objects and at the same time has extended its usefulness to problems which could be approached before only by more indirect and tedious methods. The great advantage of handling one's object and of seeing its parts in their three dimensions makes the method of dissection indispensable and warrants every effort to refine and extend it.
When the dissection of the nervous system had reached the height of its efficiency by the appliances known to the earlier workers, as in the studies of the nerves of fishes by Stannius, the introduction of the microtome and the method of studying sections opened the way for great advances especially in the study of the central nervous system and of development. When the sections were stained by carmine or hamiatoxylin, more or less complete pictures were obtained of the structures in each section. By carefully keeping all the sections into which an object might be cut and mounting them in their serial order, it became possible to find the limits and form of structures too small to be dissected and to trace bundles of fibers from section to section throughout their course. Although for special investigations this method is largely replaced by more exact methods, it is still useful for the study of the general morphology of the nervous system and the topographical relations of nerve centers and fiber tracts, and for the purpose of reconstructions by which these relations can be shown on an enlarged scale in wax models and the like. Series of sections prepared with such a stain as Delafield's haematoxylin, well differentiated, remain one of the best means of studying general anatomical relations, especially in the central nervous system.
Scarcely had this general method been developed when special methods were devised to gain more exact pictures of nerve structures, especially by taking advantage of the selective activity of the chemicals to be employed. First among such methods was the impregnation of nerve elements by metallic salts. Results obtained by this method were published by Golgi in 1875, Dut the method did not come into general use for a number of years. The reasons for this were that the procedure followed by Golgi was slow, requiring several months for its completion, and the impregnation was more or less uncertain, while the method of Weigert which was soon introduced promised results more quickly and with less labor. About 1890 new procedures which were both more rapid and more certain in their action were brought forward by Cajal, Cox and others, and since that time the method of metallic impregnation has been more and more widely used. This technique has given some of the most valuable contributions to our knowledge of the nervous system and is still among the most important methods for research. The advantage of the method lies in the fact that it gives incomplete pictures. Few elements are stained, often in great detail; and these are not obscured by the richness and confusion seen in complete pictures. The coloring of the tissue is not a true stain but an impregnation with a metallic salt. Although neuroglia, capillaries and other elements may at times be impregnated, the procedure peculiarly affects the nerve elements. Of these only a few are impregnated, the selection apparently being due in some way to the physiological state of certain elements. The elements which are thus isolated may be studied with great certainty and completeness, but the full study of all the elements in a given region requires a sufficiently large number of preparations to enable the observer to make up a composite picture approaching completeness. The method when used alone always has the disadvantage that the observer can never know when he has obtained an impregnation of all the elements present in the organ which he wishes to study. The peculiar value of the method lies in the fact that the nerve elements themselves are brought to view. Not only can the form of the cells and their dendrites be seen but the neurites can be traced directly from their origins to their endings. The polarity of form of the nerve elements is shown and the direction of flow of impulses can be inferred with little danger of error. The method is especially adapted to the study of the central nervous system and gives facilities for determining the functional relations of centers and tracts scarcely afforded by any other anatomical method. It is equally trustworthy for the peripheral nervous system wherever clear pictures can be secured, but the technical difficulties are greater on account of the conditions of sectioning and the formation of precipitates between the tissues.
The method of staining myelin sheaths commonly known as the method of Weigert, who introduced it in 1881, differs widely from the method of Golgi in that it stains non-nervous elements only. The picture given by this procedure is complete so far as the myelin sheaths are concerned. Since the gray matter and the non-myelinated tracts are unstained, the myelinated tracts can be traced with relative ease. The method is especially adapted to the central nervous system where it gives quickly and easily the course and general topographical relations of the fiber tracts. The method is limited in its usefulness because it does not give the origin or ending of neurites or the course of nonmyelinated tracts, fibers or collaterals. Since neurites may run a longer or shorter distance before receiving their myelin sheaths and since the terminal branches of neurites run an unknown distance after passing the end of their sheaths, the study of the sheaths may lead to erroneous conclusions as to the groups of cells from which given fibers arise or in which they end. Since in recent years it has become possible to apply the method to material fixed in formalin or in Flemming's fluid, even better preparations are obtained than formerly and with greater ease. When the usual Weigert preparations are treated with a sharp secondary protoplasmic stain, such as acid fuchsin, the method gives a much larger number of facts and more reliable pictures. In particular, cell bodies are stained and the origin and endings of neurites are sometimes shown sufficiently well to enable the observer to determine the direction of impulses carried by a given tract. Even when this result is not obtained, the double-stained preparations are preferable to ordinary haematoxylin sections for the study of general topographical relations. As a special method, however, the Weigert procedure if used alone and on adult brains, can be relied upon for little in the way of functional relationships. The method is applicable to the study of the course of peripheral nerves by means of sections, and is especially valuable for the analysis of nerve trunks into their components. In addition to the usual Weigert technique, it may be mentioned that with small animals or small brains excellent Weigert effects may be obtained by simple fixation with vom Rath's picro-aceto-platino-osmic mixture, with or without after-staining with acid fuchsin.
A third special method which has proved extremely valuable is the intra vitam staining with methylene blue introduced by Ehrlich in 1886. This differs from both the preceding in that it is an actual stain of nerve elements. The preparations are in general comparable to those obtained by the Golgi technique, since the pictures are incomplete pictures of the same character. The method gives in addition the internal structure of the nerve elements and is extremely useful for cytological study. It gives also more extensive and reliable information than does the Golgi method as to the details of fiber endings and the structural relations between nerve elements. The method is especially adapted to the study of the peripheral nervous system and to small isolated portions of the central nervous system. It has not yet been extensively used for the study of centers and fiber tracts. It reaches the height of its efficiency where the tissues stained are suitable for study in the living condition.
Of the great variety of cytological methods which may be applied to the nervous system two require mention as special nerve methods. The first of these is known as the Nissl method and is a selective stain of certain plastic materials within nerve elements which are doubtless either nutritive or excretory in their nature, or both. These materials constitute the so-called Nissl bodies whose form and volume change with changing physiological states of the nerve elements. The study of these elements is therefore of great importance to the physiologist and the pathologist.
The other method includes a considerable number of procedures which may be spoken of collectively as neuro-fibrillar staining methods. These stains affect the more stable colloidal substances in nerve elements and so are complementary to the Nissl method. Besides the methylene blue stain mentioned above a number of methods have been used for this purpose, of which the latest are certain modifications of the photographic procedure introduced by Cajal, Bielschowsky and others.
The methods thus far mentioned are anatomical methods chiefly applicable to the adult nervous system. When the nervous system is studied with especial reference to its development or by making use of any peculiar characters which it possesses during development, it is proper to speak of embryological methods. These include the study of the development of the nervous system by any of the procedures applicable to embryos and the study of histogenesis by various staining techniques, including those of Golgi and of Ehrlich mentioned above. A special embryological method of great importance, the method of Flechsig, consists in the study of the course and order of myelination of nerve tracts by means of the Weigert technique. This gives especially favorable opportunities for studying the course of fiber tracts because specific tracts can be studied at the time of appearance of their myelin sheaths, certain tracts frequently presenting themselves in entire isolation from others.
The anatomical study of the nervous system may be greatly extended by means of experiments which produce artificial differences between nerve tracts or nerve centers, which may then be brought to view by suitable staining. Most prominent among these methods is the study of fiber tracts which have been caused to degenerate in accordance with Waller's law. Negative pictures of such tracts are obtained when the sections arc treated by the usual Weigert stain, or positive pictures are obtained by means of the technique of Marchi. The method is applicable wherever localized and known centers can be destroyed or known tracts can be severed from their cells of origin. The fibers then degenerate in a direction away from their cells of origin, and the course of the fibers is brought to view by selective staining of the degeneration products of their myelin sheaths. Random operations may reveal the position of centers or tracts not before suspected. Many operations and preparations are needed in order to gain complete results. The time necessary for degeneration after the operation must be determined by extended trial and if not accurately determined, either no result or misleading results may follow. The staining technique is not always reliable. A source of serious error is found in the fact that fat globules resulting from the degenerative processes may be carried by blood or lymph currents to situations far removed from their place of origin, and when stained in those situations they give very misleading pictures. The method is extremely valuable as giving the gross facts regarding the course and functional relationship of myelinated fiber tracts. With regard to the place of ending of fiber tracts the method has the same limitations as the method of Weigert, but in even greater degree because the fibers may not have degenerated throughout their entire length or may have passed the proper condition for staining. The method is more reliable for the origin of the fibers studied, or rather for the direction of the impulses which they carry, provided due care is taken to distinguish between Waller's degeneration and Gudden's degeneration (see p. 91) which may also occur. The method requires great understanding and caution in its use but promises to be one of the most important of anatomical methods, and one which will greatly increase in usefulness.
Allied to the last is the study of secondary degenerations due to lesions, operative or accidental, in related centers. Nature performs many experiments of the greatest value to the neurologist, lesions in the central nervous system often offering opportunity through the study of primary and secondary degenerations to determine pathways of impulses otherwise difficult to follow.
Closely related to the embryological methods already mentioned is the study of the processes at work in the degeneration and regeneration of cut nerve fibers. This may be prosecuted on either embryos or adults and throws especial light upon the character of neurones, their histogenesis, their relations to accessory structures and the mode and meaning of histological differentiation within the nervous system.
Recent work upon the rise and histogenesis of nervous elements and organs has shown that the embryological methods may be extended by experimental procedures to a degree little dreamed of a few years ago. Many problems of development and differentiation in the nervous system are capable of re-examination by experimental embryological methods, which will especially throw light on the causes at work in these processes.
These may be either experimental or observational. In the one case nervous actions are studied under conditions determined by operative or other artificial means. In the other case actions are studied under natural conditions believed to be sufficiently known. Among the simpler forms of experiments are those to determine the course of impulses. By the help of operations, or otherwise, suitable stimuli are applied to definite sensory areas, to nerve trunks or roots, to certain nerve centers, etc., and the actions called forth are observed and studied. The method is widely applicable and has been used independently for the investigation of the pathways of impulses and hence of functional connections in the nervous system. For its intelligent use it presupposes thorough anatomical knowledge and the most reliable results are obtained by combining one or more anatomical methods with it. Conduction paths may be determined by this method independently and some indications obtained as to the number and arrangement of their several steps. These data may then be tested and confirmed anatomically.
The study of the rate, direction and mode of conduction of impulses in neurones and neurone- chains has been carried on by stimulation under normal and experimental conditions. Opportunity is offered for the extension of this field in the direction of experimentation with chemical conditions combined with cytological study to determine the character and mode of action of nerve protoplasm.
Numerous operative experiments have been directed toward the study of the function of various parts of the brain and of other nervous organs by stimulation, extirpation, poisoning, etc. Examples of such work are found in the numerous researches on the function of the cerebellum and upon the localization of function in the cerebral cortex. Other studies take account of the modifications of normal action following upon the destruction of any conduction path or center- anatomically known. Still others regard the modifications of responses to stimuli after the application of selective poisons such as curare and nicotin, which affect respectively the motor end plates and certain elements of the sympathetic system.
The functions of sense organs have been studied by experiments to determine the conditions of their functioning, the character of stimuli proper to each kind of sense organ, the range of stimulation, etc. Examples are seen in recently renewed studies upon the ear and lateral line organs and upon the gustatory organs of fishes.
The methods by observation may or may not be experimental in the sense that natural conditions are controlled or simplified by the observer without physical interference with the organism or its nervous mechanisms. This form of study is especially developed in the field of animal behavior and physiological psychology.
The main object in the study of the nervous system is to discover what functional relationships are provided for by each of its parts. The words " functional relationships" imply actions and organs which act. The nervous system is in relation both structurally and functionally with all parts of the organism. All activities are directly or indirectly dependent upon the proper functioning of the nervous system. The study of the nervous system implies a knowledge of the whole structure and the whole life of the organism. This breadth of view and attention to the functional side are two factors essential to the right attitude in the study of the nervous system which have been scarcely well enough appreciated.
Although the present book, as an introduction to the study of the nervous system, will deal chiefly with structure it is hoped that the student will be led into full sympathy with the functional point of view in dealing with anatomical facts.
One of the most serious questions before the student of the nervous system at present is, what names and terms will serve best for clear description. Although there is an unfortunate variance in the usage of different writers certain terms are in more or less common use and it is assumed that the reader is familiar enough with the anatomy of vertebrate animals to understand the meaning of these terms. For example, to indicate the two ends of a vertebrate animal the terms front and hind, anterior and posterior, cephalic or rostral and caudal ends are used, and for the relation of direction toward the head or toward the tail' the convenient terms cephalad and caudad are frequently used. So the terms right and left, dextral and sinistral sides and dextrad and sinistrad express corresponding relations, while the term median means in the middle vertical plane which divides the body into right and left halves. The terms mesial and lateral, mesad and laterad are similar in meaning to those already mentioned. For the upper and lower surfaces of vertebrate animals the terms dorsal and ventral are regularly used and these together with the other terms mentioned will be applied in the following pages to man in the same sense as to other vertebrates.
Since most of the description of the nervous system is taken from microscopic sections it is necessary to understand the terms used to designate sections cut in different planes. Sections made at right angles to the longitudinal axis of the body are called transverse sections. In many cases, as in embryos with the head bent ventrad, sections transverse to the trunk will not be transverse in the head region. Unless otherwise stated transverse sections are understood to be at right angles to the longitudinal axis in the region under consideration. Sections taken at right angles to transverse sections and passing symmetrically through corresponding organs of the right and left side, such as the limbs, would be parallel with the horizontal plane in animals whose position is prone. In man such sections would be approximately parallel with the vertical face of the frontal bone. The terms horizontal and frontal may be used interchangeably to indicate such sections. Sections taken at right angles to both of these and parallel with the median vertical plane of the body are called sagittal sections, since hi man such sections are parallel with the sagittal suture of the skull.
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Cite this page: Hill, M.A. (2021, March 3) Embryology Book - The Nervous System of Vertebrates (1907) 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Nervous_System_of_Vertebrates_(1907)_1
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