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.

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

tion of

diagram.

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

centres

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 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.

power.

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

Sponta

neous

action of

nervecentres.

work done depends more upon the special groups of cells acting as stimulated by surrounding forces, than it does upon the actual quantity of nerve-force expended, that is to say, the value of brain work depends more upon the impressionability of the brain for co-ordination than upon the amount of physical force discharged. Hence good mental training diminishes the amount of subsequent brain wear.

Spontaneous action of nerve-centres.

I use this term as a label for the subject now before us, but do so under protest of its inaccuracy, and simply for present convenience. I here refer to the tendency to action in nerve-centres, as indicated by movements, when no stimulus has immediately preceded. We saw the so-called spontaneous movements of the circumnutating radicle of a pea. Such movement in the apex of the radicle is said to be spontaneous because it is not immediately stimulated by incident forces or controlled by them-so also with regard to movements in man, many are said to be spontaneous because their stimuli are not immediately antecedent. Spontaneous action of nerve-centres is common in infancy, and returns under depressing circumstances in adult age, as in conditions of exhaustion and irritability. This appears to be a form of lessened impressionability, in such cases, the centre cannot be readily impressed or stimulated by the forces acting upon it.

In observing movements of the body as signs of action of parts of the brain, it is most important to note what circumstances appear to excite the action; if we see no forces exciting the action we call such action spontaneous.