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5. The brain and spinal medulla, when studied by the naked eye, are seen to be composed of white matter and gray matter. The white matter forms very nearly twothirds of the entire cerebro-spinal axis. It is composed of medullated nerve-fibres embedded in neuroglial tissue. The gray matter is composed of nerve-cells with their dendrites and axons. Some of the axons are in the form of naked axis cylinders, whilst others have a coating of myelin. Intimately intermixed with these parts is the neuroglia, which isolates them more or less completely from each other.
THE NATURE OF THE BRAIN.
In the foregoing account it has been explained that the nervous system is composed of a series of afferent nerves bringing information from every part of the body into the central nervous system, from which efferent nerves pass out to the muscular and other active parts of the body, providing the means for translating such information into appropriate action. But it has been seen that the essential part of the central nervous system is the intercalated cells, which provide the means whereby the information brought in by any sensory nerve may be placed at the service of the whole body, and the response which it excites may be controlled and regulated by the condition of the rest of the body. The system of intercalated cells links together into one co-ordinated mechanism the whole nervous system, and, through it, every part of the body itself.
In some very primitive and remote ancestor of man (and in fact of the vast majority of animals) the front end of the nervous system became enhanced in importance to form a brain, which assumed a dominant influence over the rest. This was brought about in the first place by the fact that in an elongated prone animal moving forwards, the front end would naturally come first into relationship with any change in environment; and this earlier acquisition of information concerning the outside world would necessarily give the head end of the nervous system exceptional opportunities for influencing the rest of the nervous system. This predominance is further accentuated by the development in the head region of the organs of special sense, which provide mechanisms specially adapted to be influenced by light, sound, and such delicate chemical forms of stimulation as excite in ourselves sensations of smell and taste. As the information conveyed by these special senses, such as the scent of food or the visual impression of some enemy, must be able immediately to influence the movements of the whole body, it follows that a specially abundant system of intercalated elements link the central ends of these nerves of the special senses with the rest of the central nervous system. Moreover the predominant influence of the head end of the central nervous system implies that it must be provided with a specially large series of nerve-fibres, not only for the purpose of bringing this influence to bear upon the rest of the nervous system, but also of being itself brought into intimate relationship with the nervous system as a whole, seeing that sensory impulses are constantly pouring into every part of it.
Thus the head end of the central nervous system becomes the brain, which is characterised by a series of large irregular swellings, due to (a) the development around the insertion of each special sensory nerve of a mass, or group of masses, of intercalated cells which will enable the effects of the visual, acoustic, olfactory, gustatory or other sensations to influence the whole nervous system, and (b) the evolution of complicated systems of intercalated cells, which receive, and in a sense blend, impressions coming from all parts of the nervous system, and emit fibres which pass, directly or indirectly, to the various groups of motor nerve-cells and control their activities and, through them, the behaviour of the animal.
In the development of the human embryo this distinction between the head end and the rest of the central nervous system is indicated even before the medullary plate is completely folded up to form the neural tube. The widened
part represents the rudiment of the encephalon or brain; and the rest of the tube will become converted into the medulla spinalis.
If the attempt is made to analyse the meaning of the early broadening of the brain rudiment it will be found to be due in great measure to the fact that there is added to the margins of the medullary plate (see Fig. 442, E, p. 501) the material from which the sensitive part of the eye and the optic nerve will be developed; but soon after the neural tube is closed irregular swellings will make their appearance around the attachments of the nerves of smell, vision, hearing, and taste (Fig. 454),
Indicates the place where a tract crosses the median plane.
FIG. 454.-DIAGRAM REPRESENTING THE CONNEXIONS OF SOME IMPORTANT SENSORY AND MOTOR TRACTS IN THE BRAIN to which references are made in pages 513 to 517. Motor paths in red; sensory in other colours.
and also the great vagus nerve that is widely distributed to the viscera of the neck, thorax, and abdomen.
But there are other factors besides these irregularities of growth of its walls which add complexity to the form of the encephalon in the embryo. In the course of their growth both parts (encephalon and medulla spinalis) of the neural tube undergo great extensions in length, breadth, and thickness; but in the case of the spinal medulla it is the increase in length that is most distinctive, whereas in the encephalon, the irregular expansion in breadth and thickness is more obtrusive. Nevertheless, the brain elongates more rapidly than that part of its mesodermal capsule which ultimately becomes the brain-case or cranium; and hence it becomes bent to permit of its being packed in the limited length of the cranial cavity. But if it is admitted that these mechanical considerations are in a measure responsible for the three bends which develop in the embryonic encephalon, their situation and the forms they assume are determined by the irregularities of growth inherent in the brain itself.
Even at a time, during the second week, when the anterior (oral) end of the neural tube is still open (neuroporus anterior), a right-angled bend has already developed in the rudiment of the brain (cerebral vesicle). Slightly less than half of the length of the vesicle had projected beyond the upper (anterior) end of the notochord and became flexed ventrally round it (Fig. 455).
Anterior limit of mesencephalon
This bend is known as the cephalic flexure. The region of the brain vesicle in which it develops will later on become the mesencephalon or mid-brain; and even at the early stage of development now under consideration (Fig. 455) there is a slight narrowing of the tube (isthmus) that marks the boundary between the mid-brain and the rhombencephalon or hind-brain. Just beyond the end of the notochord there is an even fainter trace of a constriction indicating the line of demarcation between the mid-brain and the prosencephalon or fore-brain.
Shortly after the appearance of the cephalic flexure a similar bending occurs in the region where the encephalon becomes continuous with the medulla spinalis (Fig. 456, A). This is the cervical flexure.
But at this stage, or even earlier (Fig. 456), there has been developing a third bend which produces effects differing from those just mentioned. At the end of the second week a slight bulging can be detected on the ventral side of the hind
FIG. 455. LEFT LATERAL ASPECT OF AN EARLY HUMAN EMBRYO (after His's model, reversed).
Upper limit of
FIG. 456.-TWO STAGES IN THE DEVELOPMENT OF THE HUMAN BRAIN (after His).
brain (Fig. 455): during the next four weeks this steadily becomes accentuated and forms the pontine flexure. The convexity of the bend is directed ventrally, differing in this respect from both of the other flexures. This difference in direction has a profound influence upon the form which the hind-brain assumes. If a plastic tube is bent a strain is thrown upon the wall in the concavity
of the flexure. If this wall is strong and resisting, like the floor-plate of the neural tube (in the cases of the cephalic and cervical flexures) the bending does not affect the outline of the tube (in section) very materially. But when the strain is thrown upon the thin roof-plate during the development of the pontine flexure it is not strong enough to resist; it becomes stretched and allows the side walls of the neural tube to splay laterally in precisely the same manner as occurs when a rubber tube is bent towards a side which has been split (or weakened) longitudinally (Fig. 457). This mechanical factor determines the form assumed by the hind-brain at the end of the first month; and gives its cavity, the fourth ventricle, a lozenge or rhomboid form, when seen from its dorsal aspect through the thin translucent roof. For this reason the hind-brain is known as the rhombencephalon.
The rhombencephalon forms at first more than half of the encephalon, and as it expands it appears to become marked off from the rest by a constriction (the isthmus rhombencephali).
The insertion of the nervus acusticus in the neighbourhood of the outsplayed lateral angle of the rhombencephalon leads to the profound transformation of the metencephalon. The nervus acusticus conveys into the hind-brain impulses which are stimulated by movements of fluid in the closed sac developed from the otic vesicle (Fig. 443, p. 501). The truly acoustic function of this apparatus is called into activity when the movements of this fluid are caused by waves of sound transmitted to it from the outside world. But it is obvious that motion may also be set up in this fluid by changes in position of the body itself; in other words, movements in the fluid of the otic vesicle may stimulate nerves to convey to the brain information concerning the position and movements of the body itself. A great mass of nerve-cells develops around the insertion of the nervus acusticus (that part of it, however, which is called vestibular and is not concerned with the function of hearing) to make use of this information for the regulation of the movements of the body in balancing or equilibration. To enable this terminal vestibular nucleus the better to perform this function of equilibration, depending as it does upon the co-operation and adjustment of the movements of vast numbers of widely separated muscles, nerve tracts coming from muscles and skin areas of all parts of the body make their way into this vestibular nucleus; and it expands and forms a great excrescence which is known as the cerebellum. And as this cerebellum has to adjust the activities of all the muscles of the body it necessarily becomes the great organ of muscular co-ordination, and as such it is made use of by those parts of the brain which have to initiate and control complex actions such as skilled movements. It will be shown in the subsequent account how the
FIG. 457.-PROFILE VIEW OF THE BRAIN OF A HUMAN
The various cerebral nerves are indicated by numerals.
cerebellum becomes linked to the mesencephalon to co-ordinate the movements of the body which are excited by this part of the encephalon; and later how it becomes associated with the prosencephalon, when the latter becomes responsible for the acquisition and control of the most highly skilled actions. For the latter purpose a great pathway of nerve-fibres is laid down to connect the fore-brain with the cerebellum: the terminal stage of this connexion is situated upon the ventral (anterior) aspect of the metencephalon in the form of a great mass of transverse fibres. At one time these strands of nerve-fibres were looked upon as a bridge between the two hemispheres of the cerebellum: hence the name pons was applied to them. This term is now applied not only to the fibres themselves, but also to the upward prolongation of the medulla oblongata, to the surface of which they are applied.
The subdivision of the rest of the encephalon into mesencephalon and prosencephalon develops later and is less fundamental than the primary demarcation between them and the rhombencephalon.
The visual apparatus is connected with both the mid- brain and the forebrain, but at first more intimately with the former, to which nerve pathways are established to convey from the spinal medulla and medulla oblongata sensory impressions of touch and hearing. From the alar laminæ of the mesencephalon there are developed four little hillocks (colliculi)-corpora quadrigemina-to receive these
A, Brain as seen in profile. B, Median section through the same brain. M, Mamillary eminence; Tc, Tuber cinereum; Hp, Hypophysis (hypophyseal diverticulum from buccal cavity); Opt, Optic stalk; TH, Thalamus; Tg, Tegmental part of mesencephalon; Ps, Hypothalamus; Cs, Corpus striatum; FM, Foramen inter ventriculare; L, Lamina terminalis; RO, Recessus opticus; Ri,
FIG. 458. THE BRAIN OF A HUMAN EMBRYO IN THE FIFTH WEEK varied impressions and to enable them to influence the actions of the whole body. Special nerve paths are laid down from the corpora quadrigemina (Fig. 454) to the spinal medulla Recessus infundibuli. to enable the mid-brain to control the motor nuclei of the muscles of the trunk and limbs. These are called the fasciculi tectospinales (tectum being a synonym for corpora quadrigemina). A group of intercalated cells known as the nucleus ruber develops upon each side of the mesencephalon for the purpose of establishing connexions between the cerebellum and the mid-brain. When an impulse passes out of the mid-brain by the tectospinal bundle to excite some movement of the body, the red nucleus provides the link by which the cerebellum can co-ordinate the actions of the muscles involved. By means of a fasciculus rubrospinalis it can bring its influence to bear directly upon the nuclei of motor nerves in the brain and spinal medulla (Fig. 454).
The prosencephalon is at first, and in some of the lower fishes remains, the most insignificant of the three brain vesicles, but in the human brain (as also in that of most other vertebrates, though in varying degrees) a pair of enormous