cells.

The brain is a soft and delicate structure, seated in the brain case and carefully protected; it consists essentially of two kinds of material, the nerve-cells, and the nerve-fibres. The nerve-cells, when duly nourished, are Nervethe makers of nerve-force; for their proper nutrition they need a good supply of blood in their vessels. A nerve- Nervefibre passes off from each cell and conveys the force fibres. generated in it, which is then called a nerve-current; Nervethere are millions of such cells in the structure of the current. brain. When the nerve-force generated by a nerve-cell is carried by a fibre to a muscle, say in the face, or in the limbs, this nerve-current causes the muscle to contract or shorten, and visible movement results, the movement being stimulated by the force sent from the nervecell. The movement seen indicates to us the time and Nervemuscular quantity of the discharge of force from the nerve-cell; movement. such movement is conveniently called a nerve-muscular

movement.

The substance of the brain is thus mainly made up The brain. of groups of nerve-cells, many of which are connected with one another by nerve-fibres, and many of them are connected with the muscles of the body and send nervecurrents to them, thus causing the movements of the members. The nerve-cell generates force as the outcome of its nutrition. While the brain is giving out force, it must be replenished by nutrition, or it will run. down, and be less capable of producing energy after a short time, it will then need food and rest.

We have spoken of the nerve-cells of the brain as Afferent

nerve

being connected with one another, and with the muscles fibres.

of the body which produce movements of its parts, it must now be explained that there are other nerve-fibres which connect the organs of special sense, the eye, and the ear, etc., and the skin all over the body, with the

Efferent

nerve.

fibres.

Visible movement.

cells of the brain, and convey currents of force from these parts respectively to the cells of the brain; such nerve-fibres are called afferent because they convey currents to the nerve-system; in distinction from these the fibres which convey currents from the nerve-cells to the muscles are called efferent. The fibres which pass in both directions are collected into bundles, or strings, and are commonly called the nerves of the body; the ingoing or afferent nerves convey stimuli to the brain, the outcoming or efferent nerves carry motor currents from the nervecells to the muscles.

I desire in this lecture to indicate certain characters of the brain as data for the theory of mental action (Psychosis), and explanation of methods of observation of facts, and methods of training the mind, which will be advanced further on.

The principal method by which we observe action in nerve-centres is by noting their action on muscles; a current passing from a nerve-centre to a muscle is followed by contraction of the muscle, and visible movement in the body; the muscle is the visible index of a nerve-current proceeding from the centre, indicating the time and to some extent the quantity of its action. One set of muscular contractions indicates action in one set of nerve-centres corresponding'. Further, the sequence of the muscular action is also the sequence of the action in the centres corresponding, and the antecedent of the action in the centres is the necessary antecedent of the result of the movement. This combined

1 The clinical investigations of Dr Hughlings Jackson, and the physiological inquiries of Dr Charles Beevor and Mr Victor Horseley, have shewn that while a certain movement is principally due to action in one particular nerve-centre, it may be produced in part by action in other centres.

action of a nerve-centre and muscle is conveniently termed a nerve-muscular act, and such acts are often stimulated through the organs of special sense.

tion of

Let me ask your attention to this diagram, it is not a Explanarepresentation of anatomical structure, but may serve our diagram.

Fig. 1.

The brain is represented by the shading as divided into areas A, B, C, D, E which can act more or less separately: each area or section of brain is represented as connected by nerve-fibres, with a muscle corresponding. Each section of brain may receive a stimulus from the eye or the ear. The representation is purely diagrammatical for the sake of clearness of description.

Nerve

present purpose in illustrating some physiological facts. The coloured areas of brain A, B, C, D, E, each receive fibres carrying impressions from the eye and the ear, so that they can separately be stimulated by sight and sound. Fibres pass from each brain area to the muscles a, b, c, d, e respectively, so that when A is stimulated the muscle a contracts, if the centre E be stimulated the corresponding muscle e contracts, and so on for each centre and muscle respectively, the muscle is the visible index of nerve-currents proceeding from its own centre. If we see the muscles a, b contract at the same moment that indicates that centres A, B act together. The nerve-centre must of course be well-supplied with good blood.

Our primary proposition concerning the stimulation centres of all vital acts in living things by force from without stimulated by sight them, applies to the action of nerve-centres, and is the and sound. basis of our theory of mental action. Action in the

Nervecentre at work.

body has been shewn to be dependent upon muscular contractions secondary to their stimulation by currents from the nerve-centres. The nerve-centres themselves are usually stimulated by sound and sight through the organs of sense.

The nerve-centre is the special seat in which we now desire to study action. In our explanation of the diagram of a nerve-muscular apparatus we speak of (1) A nerve-centre in which action is studied, and the cells which compose it. (2) Its supply of blood. (3) The forces stimulating it. (4) The outcome of action in it. We now have to consider the limit of capacity for action in the nerve-centre under given circumstances. The circumstances that may vary are the condition of the centre itself; the blood supply; and the stimulation from without, both in time, quantity, and kind. A child

that is starving from want of food, i.e. a child whose nerve-centres are deprived of due blood supply, does not give out any great amount of force, there is but low capacity for mental and nerve-muscular action. A nervecentre must be well supplied with good blood in order that it may be apt for action, and clearly impressionable to stimulation from without. In such case the limit of capacity for action is determined by food, and better or fuller feeding may be followed by more action.

stimula

tion.

The child may be amply fed, but live in a dull dark Want of house, with but few people about to talk to him. Food supply being good in kind, and the supply being regular in time and sufficient in quantity, the cells of the centre also being healthy; then up to a certain point the quantity of action in that centre will be in direct ratio to the stimulation.

material.

If the supply of blood to a muscle be constant, as in Supply of ordinary health, then increasing the stimulus is followed food by growth of its tissue. So if the branch of a tree be bent, causing a congestion of sap, the apples grow larger. If the nerve-centres be well supplied with blood, and have good internal capacity they will act more or less strongly in proportion to the amount of stimulation from without, received through the special senses, and probably such stimulation helps to make them grow. If you believe the nerve-centres to be healthy, and well supplied with good blood, then usually it is well to stimulate them through the senses, that they may grow and become healthy and more impressionable to the environment.

The amount of work or physical force that the brain Amount of braincan discharge in a given time is limited, but when we power. come to study the outcome of action of a group of centres then the position is changed. The value of the