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THE NERVOUS SYSTEM.
I. THE CENTRAL NERVOUS SYSTEM.
ORIGINALLY WRITTEN BY D. J. CUNNINGHAM, F.R.S.,
[In its original form this chapter represented perhaps the most characteristic work of the late Editor of this Text-book, which continues to bear his name, and is a lasting memorial of his personality and scientific attainments. By his lamented death the difficult task has fallen upon the reviser of making such considerable alterations as the rapid changes in the state of our knowledge of the nervous system have rendered unavoidable, while endeavouring at the same time to preserve unaltered the general character of his friend's work.]
ELEMENTS OF THE CENTRAL NERVOUS SYSTEM.
EVERY type of nervous system with which we are acquainted, from the simplest and most primitive, such as that of Hydra, to the most complex and highly elaborated mechanism
found in man, is composed essentially of three categories of elements. These are (1) sensory cells, so situated and so specialised in structure as to be capable of being affected by changes in the animal's environment, and of transmitting the effects of such stimulation, directly or indirectly, to (2) efferent nerve-cells, which influence the muscles or other active tissues, so that the stimulation may find expression in some appropriate action; and (3) intercalated nerve-cells, which regulate such responsive behaviour by bringing it
FIG. 439.-A DIAGRAM REPRESENTING THE ESSENTIAL FEATURES IN THE
under the influence of other sensory impressions and of the state and activities of
the body as a whole.
The study of a simple scheme representing the relationship that obtains between these three classes of elements in the extremely primitive animal, Hydra (Fig. 439), will make these fundamental facts plain. Changes in the animal's environment affect the extremities of the peripheral processes of the sensory cells (A, B, and C), which in Hydra are situated amongst the ordinary tegumentary cells: the effect is transmitted by the central processes of such cells (4, for example), either directly to the efferent cell, represented in the diagram by a motor nerve-cell, or more usually to an intercalated nerve-cell (a, b, or e). Into this (a) impulses stream from other intercalated cells (b and c), bringing the impulse from the sensory cell A under the influence of those coming from B and from more distant parts of the body through the intermediation of the intercalated cell c. The cells a, c, and d are connected with the motor nerve-cell. Thus, there is provided a mechanism whereby the conditions affecting other regions of the body, B and C, may influence the nature of the response which the stimulation of A evokeseither increasing or diminishing its effect or perhaps altering its character.
In this way the intercalated nerve-cells form a great co-ordinating mechanism, linking together all parts of the body in such a way that the activity of any part of the organism may be influenced by the rest, and thus be enabled to act in the interest of the whole.
Hence the nervous system becomes the chief means whereby the various parts of the body are brought into functional relationship one with the other, and coordinated into one harmonious whole.
Throughout the whole course of its subsequent evolution the nervous system is formed of these three kinds of elements; and the essential feature in its elaboration and increasing complexity is the multiplication of the intercalated cells, and their concentration, together with the motor nerve-cells, to form a definite organ, which we call the central nervous system.
During this process of development of the more complex forms of nervous ⚫ system, most of the sensory cells migrate from their primitive positions in the skin (Fig. 439); and, as the free extremity of the peripheral process retains its primitive relationship to the skin, such migration of the cell bodies necessitates a great elongation of their peripheral processes. Although these sensory cells thus move inwards into the deeper tissues of the body, the great majority of them do not become incorporated in the central nervous system, but become collected into groups, which form the ganglia of the sensory nerves.
In addition to its primary functions of (a) providing the means whereby the organism can be brought under the influence of its surroundings, and (b) coordinating the activities of the whole body, the nervous system also comes to perform other functions of wider significance.
In the course of its evolution the co-ordinating mechanism formed by the intercalated cells becomes so disposed in each animal that an appropriate stimulus applied to the sensory nerves can evoke a definite response, often of great complexity and apparent purposiveness. In other words, the nervous system becomes the repository of those inherited dispositions of its constituent parts which determine the instincts: and in the course of time it eventually provides also the apparatus by which individual experience and the effects of education can be brought to bear upon and modify such instinctive behaviour. In other words, from the nervous system is formed the instrument of intelligence; and the relatively great bulk and extreme complexity of that instrument-the brain-in man are in a sense the physical expression of human intellectual pre-eminence.
In conformity with its primary function of affording a means of communication with the outside world, almost the whole nervous system in the human embryo, as in other animals, is developed from the ectoderm, as has already been explained in the chapter dealing with General Embryology (p. 30 et seq.). In the most primitive Metazoa the sensory cells remain in the ectoderm (Fig. 439), but other ectodermal cells become converted into motor nerve-cells and intercalated nerve-cells, which wander into the underlying tissues (Fig. 439). In the human embryo there is an analogous process of development, but with the important difference that the various nervous elements do not wander into the mesoderm individually. A
definite patch of ectoderm is set apart to produce the greater part of the nervous tissues for the whole body; and all except the margins of this area sinks into the body, en masse.
motor nerve cell
In one area of ectoderm all the motor nerve-cells develop (Fig. 440, d), in another (c) only intercalated nerve-cells, in yet another (b) the sensory cells originate; and the rest forms the epidermis of the skin (a). With our knowledge of the fact that the sensory cells were originally distributed throughout the skin (Fig. 439), the idea naturally suggests itself that in man also the units of the sensory ganglia might be formed in situ in the ectoderm, and that the collection of sensory cells in the ganglia might possibly be brought about by the migration of such sensory cells inwards, while their peripheral processes elongate to permit such migration of the cell bodies without disturbing their original endings in the skin. But there is no evidence
440.-DIAGRAM REPRESENTING (IN BLACK) The Left Half
OF A TRANSVERSE SECTION OF A 2 MM. HUMAN EMBRYO. Superimposed upon it there is shown (in colours) the hypothetical primitive arrangement of the nervous elements derived from each part of the ectoderm.
to show, or even to suggest, that such a process takes place in the human embryo. The facts at our disposal seem to indicate that the sensory cells are derived from sharply circumscribed patches of ectoderm, and that the peripheral processes of these cells are distributed to the outlying area of ectoderm beyond them, from which the epidermis is eventually formed (Fig. 440).
At the beginning of the second week the nervous system of the human embryo is represented by two thickened plates of ectoderm lying parallel the one to the
FIG. 441. THE DORSAL ASPECT OF A VERY EARLY HUMAN EMBRYO (after von Spee).
other, alongside the median axis of the embryo (Fig. 441), which is occupied by a shallow furrow.
Upon a diagram (Fig. 440), representing a transverse section through one-half of such an embryo (the uncoloured part), colours corresponding to those employed in
Fig. 439 have been placed to indicate the nature of the elements that are known to develop in relation with each area of the ectoderm at a later period in the history of the embryo: b represents an area which later will form the crista neuralis, from which the sensory cells will be developed. The peripheral processes of these cells will pass into the skin (a) and their central processes into the area cd, which will become part of the neural tube. In the area c intercalated cells will develop to receive the incoming sensory nerves; and in the area d the motor nerve-cells (as well as other intercalated cells) will be formed.
When it is recalled that all the elements of the primitive nervous system of Hydra are modified ectodermal cells, and, moreover, that when the intercalated and motor nerve-cells wander into the deeper tissues the protoplasm of the whole nervous network remains in uninterrupted continuity (Fig. 439), it is instructive to note that in the primitive human nervous system the rudiment of the epidermis of the skin is linked to the medullary plate by the patch of ectoderm from which the sensory ganglia will be formed.
In the discussion of the inter-relationships of the various constituent elements of the nervous system, there will be occasion to refer to this matter again. But while we are studying Fig. 440 it is important to emphasise the fact that in accordance with the commonly accepted ideas it is taught that the area b becomes completely severed from a and c, and shortly afterwards fibres are budded off from the cells in the area b to form the sensory nerves linking a to c, thus re-establishing a connexion which existed a few days earlier. This suggests the possibility that the connexions between these three series of elements may not have been completely sundered during the intermediate phase of development. Early in the second week in the human embryo the axial groove separating the two bands of thickened ectoderm (Fig. 441) that form the medullary plate becomes deepened by the tilting-up of the lateral margins of the two bands. This process becomes accentuated during the next day or two until a deep cleft is formed, the walls of which consist of the thickened ectoderm and the floor of the thinner ectoderm (floor-plate) joining them together. Before the end of the week the dorsal edges of these thickened plates become joined in the region which will develop into the neck; and during the third week the sealing of the lips of the neural groove extends upwards (headwards) and downwards (tailwards), so that the neural tube becomes completely closed by the end of that week. The extreme anterior (head-) end and the dorsal aspect of the caudal extremity of the tube are the last parts to close, the latter being, as a rule, a little later than the former. When the tube is in the stage of being patent only at its two ends, the openings are known as the neuroporus anterior and neuroporus posterior, respectively.
In the process of closing, the extreme dorsal edge of the medullary plate becomes excluded, in the greater part of its extent, from participation in the constitution either of the neural tube or of the skin, and forms a column of cells lying between the two. This is the neural crest (Fig. 442, A, B, and C; x and y represent the places where the apparent sundering occurs).
It is commonly supposed that the neural crests do not extend the whole length of the neural tube. Nevertheless, peculiar ectodermal areas, which ultimately give origin to sensory nerves, are found at the junction of the medullary plate with the skin in those regions where the neural crest is supposed to be lacking. At the extreme anterior end of the neural tube the margins of the anterior neuropore become thickened to form crest-like patches; but when the tube closes these areas do not separate from the skin (at x, Fig. 442, D), as the rest of the neural crest does. They remain part of the skin and become the olfactory areas, in which sensory cells, precisely like those found in Hydra (Fig. 439), develop.
A little farther on the caudal side of the olfactory region a very large crestlike mass of ectoderm fails to separate from the medullary plate as it closes, and becomes a constituent part of the neural tube (Fig. 442, E). It develops into the optic diverticulum from which the cells of origin of the optic nerve are formed.
In several other regions sensory nerves originate from cells of ectodermal, and possibly even entodermal, areas which do not form parts of the neural crest, as that term is usually understood. The nerves of hearing and taste are developed
in a way that seems at first sight utterly abnormal, until it is remembered that they afford examples of very primitive methods of nerveformation.
The essential part of the organ of hearing is an ectodermal sac (otic vesicle) that develops as a diverticulum on the side of the head, from a thickened patch of ectoderm, which in the lower vertebrates forms part of a more extensive area, known as the dorsolateral placode. Some of the cells of this area seem to become transformed into nervecells, which migrate into the space between the otic vesicle and the neural tube (Fig. 443) and form the acoustic ganglion.
At the upper margins of
the branchial clefts a series of olfactorium
FIG. 442.-DIAGRAMS OF TRANSVERSE SECTIONS REPRESENTING
nerve-cells may arise from these placodes, from which the nerves of taste originate
FIG. 443.-RECONSTRUCTION OF THE GANGLIA OF THE FACIAL, ACOUSTIC, GLOSSO-PHARYNGEAL, AND VAGUS NERVES OF A HUMAN EMBRYO 5 MILLIMETRES LONG (ABOUT THREE WEEKS OLD).
The epithelium of three branchial clefts and the otic vesicle is represented diagrammatically; and the supposed mode of origin of the gustatory nerve-cells (and their fibres) from the epibranchial placodes is indicated in blue, and of the acoustic nerve-cells from the otic vesicle in purple.