morphological difference between a dendrite and an axon disappears, and van Gehuchten's functional distinction alone remains characteristic, viz., that the axon is cellulifugal and conducts impulses away from the cell, whilst the dendrites are cellulipetal and conduct impulses towards the cell.

It is, however, more in accordance with the facts to regard the sensory neurones as genetically quite distinct from the rest of the nervous system (see p. 498).

Neuroglia. The neuroglia is the supporting tissue of the cerebro-spinal axis. It may be considered to include two different forms of tissue, viz., the lining ependymal cells and the neuroglia proper. We place these under the one heading, seeing that they have a common developmental origin.

The ependymal cells are the columnar epithelial cells which line the central canal of the spinal medulla and the ventricles of the brain. In the embryonic condition a process from the deep extremity of each cell, traverses the entire thickness of the neural wall, and reaches the surface. It is not known whether this process

exists in the adult.

The neuroglia proper is present in both the white and the gray matter of the cerebro-spinal axis. It constitutes an allpervading basis substance, in which the various nerve elements are embedded in such a way that they are all bound together into a consistent mass, and are yet all severally isolated from each other. Neuroglia consists of cells and fine filaments. The fibrils are present in enormous numbers, and by their interlacements they constitute what appears to be a fine feltwork. At the points where the fibrils intercross may be seen the flattened glial cells. Whilst the neuroglia is for the most part intimately intermixed with the nerve elements, there are, in both brain and spinal medulla, certain localities where it is spread out in more or less pure layers. Thus, upon the surface of the brain and of the spinal medulla there is such a layer; likewise beneath the epithelial lining of the central canal and of the cavities of the brain there is a thin stratum of neuroglia.

FIG. 453. SECTION THROUGH

THE CENTRAL

CANAL OF THE SPINAL MEDULLA OF A HUMAN
EMBRYO, SHOWING EPENDYMAL AND NEUR-
OGLIAL CELLS (after v. Lenhossek).
A, Ependymal cell.

B, Neuroglial cell.
[Note that the dorsal (posterior) aspect is below.]

The ependymal cells are derived from the original neuro-epithelial cells of the early neural tube, and in all probability the neuroglia proper has a similar origin. They both, therefore, are products of the ectoderm. Summary.-1. The cerebro-spinal nervous system is composed of two parts, viz., (a) a central part, consisting of the brain and spinal medulla, with the efferent nervefibres which pass out from them; (b) the ganglionic part, with the afferent nerve-fibres. 2. Each of these parts has a different origin, and is composed of neurones which possess characteristic features.

3. The ganglionic neurones are derived from the primitive cells of the neural crest, and have each one process, which divides into two. Of these the central division enters the cerebro-spinal axis, whilst the peripheral division becomes connected with a peripheral part. The central fibres from the ganglionic cells in the region of the spinal medulla form the dorsal or posterior roots of the spinal nerves. The cells of origin of these posterior roots are outside the spinal medulla, and carry impulses into its substance.

4. The cerebro-spinal neurones are derived from the neuroblasts in the wall of the early neural tube. Certain of these furnish efferent nerve-fibres, which issue from the spinal medulla in separate bundles termed the anterior or ventral roots of the spinal nerves. In the case of the cerebral nerves, however, with the exception of the trigeminal and facial nerves, the efferent fibres are not thus separated from the afferent fibres at their attachment to the brain.

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),

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).

TELENCEPHALON

Anterior neurophore

PROSENCEPHALON

Anterior limit of mesencephalon

Skin

Recessus

infundibuli

Recessus mamillaris

Cephalic flexure"

Upper limit of
rhombencephalon

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.

(after His's model, reversed).

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. 456.-Two STAGES IN THE DEVELOPMENT OF THE HUMAN BRAIN (after His).
A, Brain of an embryo of the third week. B, Brain of an embryo of five weeks.

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 development of the pontine flexure subdivides the rhombencephalon into two parts, one joined to the spinal medulla, the myelencephalon, and the other, joined to the rest of the brain, the metencephalon.

AIN

REBRAL HEMISPHERE

In the myelencephalon develop the nuclei of the nerves that regulate the activities of the heart, lungs, and a considerable part of the alimentary canal, and also the receptive nuclei of the nerves of taste. It is known as the medulla oblongata.

MID

BRAI

AIN

CEPHALIC

FLEXURE

XI

XII
VIII CERVICAL

PONTINE

--FLEXURE

FLEXURE

HIND-BRAIN

SPINAL

MEDULLA

000000

FIG. 457.-PROFILE VIEW OF THE BRAIN OF A HUMAN
EMBRYO OF TEN WEEKS (His).

The various cerebral nerves are indicated by numerals.
A, Cerebral diverticulum of hypophysis cerebri.
B, Buccal diverticulum of hypophysis cerebri.

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