CHAPTER II

THE NERVOUS SYSTEM AND ITS FUNCTIONS.

(Summary of Results.)

1. THE Brain is the principal, although not the sole, organ of mind; and its leading functions are mental.

The proofs of this position are these:—

(1) The physical pain of excessive mental excitement is localized in the head. In extreme muscular fatigue, pain is felt in the muscles; irritation of the lungs is referred to the chest, indigestion to the stomach; and when mental exercise brings on acute irritation, the local seat is the head.

(2) Injury or disease of the brain affects the mental powers. A blow on the head destroys consciousness; physical alterations of the nervous substance (as seen after death) are connected with loss of speech, loss of memory, insanity, or some other mental deprivation or derangement.

(3) The products of nervous waste are more abundant after mental excitement. These products, eliminated mainly by the kidneys, are the alkaline phosphates, combined in the triple phosphate of ammonia and magnesia. Phosphorus is a characteristic ingredient of the nervous substance.

(4) There is a general connexion between size of brain and mental energy. In the animal series, intelligence increases with the development of the brain. The human brain greatly exceeds the animal brain; and the most advanced races of men have the largest brains. Men distinguished for mental force have, as a general rule, brains of an unusual size. The average weight of the brain is 48 oz. ; the brain of Cuvier weighed 64 oz. Idiots commonly have small brains.

(5) By specific experiments on the brain and nerves, it is shown that they are indispensable to the mental functions.

2. The Nervous System, as a whole, is composed of a central mass, or lump, and a system of branching or ramifying threads, designated the nerves.

The central mass, or lump, is called the cerebro-spinal axis, or centre, because contained in the head and backbone, being a large roundish lump (in the head), united to a slender column or rod (in the spine).

The nerves are the silvery threads proceeding from the central lump, and ramifying to all parts of the body. As there is a circle of action between the brain and the bodily organs, one-half of the nerves carry influence outwards, the other half inwards.

3. The nervous substance is composed of two elements, described as the white matter and the grey matter.

The white matter is made up of minute fibres. The grey matter contains fibres, together with small bodies, termed cells, or corpuscles.

By slicing through a brain, we may observe the two kinds of substance. The interior mass is a pale, waxy white; the circumference shows an irregular cake of ashy grey colour.

Microscopically viewed, the two elements of the nerve substance are (1) fibres, and (2) little bodies called cells or corpuscles. The white matter is made up of fibres; the grey matter contains cells intermingled with fibres.

One remarkable peculiarity of the nerve fibres is their exceeding minuteness. Their thickness ranges from the Tooth, the sooth, 15.booth, 30.ooth, to the 100.000th of an inch. In a rod of nervous matter, an inch thick, there might be, from ten to one hundred millions of fibres. Such minuteness and corresponding multiplication of fibres must be viewed with reference to the variety and complicacy of the mental functions.

A second fact is their position. This is always a completed connexion between the extremities of the body and the cells of the grey matter, or else between one cell and another of the central lump; there are no loose ends. The fibres are thus a connecting or conducting material.

The cells or corpuscles are rounded, pear shaped, or irregular little bodies, and give origin each to two or more fibres. They are on a corresponding scale of minuteness. They range as high as the th of an inch, and as low as the booth. A little cube of grey matter, a quarter of an inch in the side, might contain one hundred thousand cells.

These corpuscles are richly supplied with blood (so are the nerve fibres), and are supposed to be Centres of nervous energy or influence, or, at all events, parts where the nervous

FUNCTIONS OF THE SPINAL CORD.

7

energy is re-inforced. Hence the masses of grey matter are spoken of as constituting the Nerve Centres.

A second function attaching to the corpuscles supplies a key to the plan of the brain. They are Grand Junctions or Crossings, where the fibres extend and multiply their connexions. The fibres coming from all parts of the body, enter sooner or later into the corpuscles of the grey substance, and, through these, establish forward and lateral communications with other fibres, which communications are required for grouping and co-ordinating sensations and movements in the exercise of our mental functions.

4. The Central nervous mass, or Cerebro-Spinal Axis, is composed of parts, which may be separately viewed, and to which belong separate functions.

I. The SPINAL CORD is the rod or column of nervous substance enclosed in the back-bone. It is chiefly made up of white matter, but contains a core of grey substance.

The Spinal Cord is supposed to terminate at the edge of the hole in the skull where the column enters to join the brain. At this point, it is expanded both in width and in depth, and receives additions of grey matter. The expanded portion, about 14 inch in length, is called the medulla oblongata, and is a body of great importance, being the centre of important

nerves.

The functions of the Spinal Cord are known to be these-First, It is the main Trunk of all the nerves distributed to the body generally (the head excepted). Its destruction or severance at any part puts an end to all communication with the members supplied with nerves below the point of severance; whence follow paralysis and loss of feeling.

Secondly, It has the functions of a Centre; in other words, it completes a circle of nervous action, so that certain movements, in answer to stimulants, can be kept up by means of it alone. This property is allied with the inside core of grey matter. A decapitated frog will draw up and throw out its limbs when the skin is pinched or irritated.

Taking together the Spinal Cord and the Medulla Oblongata, we find that by their means a certain class of living actions are maintained, called automatic, and also reflex actions. These are involuntary actions; they are maintained without any feeling, intention, or volition, on our part. They are enumerated as follows:

:

(1) Movements connected with the process of Digestion.

The first operation upon the food in the mouth-the chewing or masticating-is voluntary, and requires the co-operation of the brain. When the morsel passes from the tongue into the bag of the throat, it is forced down the gullet by a series of contractions and movements which are involuntary; we have no feeling of them, and no control over them. The contact of the food with the surface of the alimentary tube impresses certain nerves distributed there; influence is conveyed to a nervous centre (in some part below the brain, probably the medulla oblongata, together with the sympathetic ganglia), and the response is manifested in the contracting of the muscular fibres of the alimentary tube.

(2) The movements connected with Respiration. The breathing action is sustained by a power withdrawn from our will, although voluntary muscles are made use of. In taking in breath, the lungs are expanded by the muscles of the chest; in expiration, the chest is compressed, and the air forced out, by the abdominal muscles. The medulla oblongata is the centre for sustaining this process.

The acts of coughing and sneezing are reflex acts, operated through the lungs. The irritation of the very sensitive surfaces of the throat and bronchial tubes, and of the lining membrane of the nose, originates, through the medulla ob. longata, a powerful discharge of nervous force to the expiratory muscles, and the air is forced out with explosive violence. Sucking in infants is a purely reflex act.

(3) Certain reflex movements are connected with the Eyes. The act of winking is stimulated by the contact of the eye with the inner surface of the upper eyelid, and serves to distribute the tears, or eye-wash, and clean the ball. There is also a reflex action of the light in opening and closing the pupil of the eye.

(4) There is a tendency, of a purely reflex nature, to move the muscles of any part, by a stimulus specially applied to that part. In the decapitated frog, the pinching of a foot leads to the retractation of that foot. An object placed in the open hand of any one asleep, stimulates the closure of the hand. Touching the cheek of a child makes it laugh. In tasting anything, the sensation, while awakening a general expression of feeling, more especially excites the muscles of the mouth. The same applies to smell; a bad odour produces a contortion of the nose. In these effects of the more special senses, the influence may not be limited to the spinal cord, but it illustrates the kind of reflex action referred to, an action which the cord

FUNCTIONS OF THE CEREBRUM.

9

is capable of sustaining. This whole class has sometimes been called sensori-motor actions.

(5) The effect denominated the tension, tone, or tonicity of the muscles. It is a fact, that in the profoundest slumber there is still a certain degree of contraction in the muscles; only after death are they wholly relaxed. Now, experiments seem to show that this remaining contraction is maintained through the agency of the spinal cord; it disappears with the destruction of the cord.

II. The BRAIN, or Encephalon, is the rounded or oval lump of nervous matter filling the cavity of the skull. It is a complex mass, but there are certain recognized divisions, with probable difference of function.

Commencing from below, and continuous with the Spinal cord, is the Medulla Oblongata, which has been already noticed.

Next is the Pons Varolii, or ring-like protuberance, so called because it embraces like a ring the main stem of the brain, continued upwards from the medulla oblongata. It contains white, or fibrous matter, running partly up and down, and partly in a transverse direction, with diffused grey matter. As regards the white portion, it serves as a track of communication from below upwards, and from one half of the cerebellum (which adjoins it) to the other half. As regards the grey matter, it must perform some of the functions of a centre, in reflecting and multiplying nervous communications. No more special explanation can be given of its functions.

The Cerebral Hemispheres, sometimes called the brain proper, constitute the highest and by far the largest part of the human brain. This mass is egg-shaped, but with a flattened base; the big end of the egg being behind. There is a complete division into two halves, right and left, by a deep fissure all round, leaving only a connecting band of white matter. The surface is not plain, but moulded into numerous smooth and tortuous eminences, called convolutions, which are separated by furrows of considerable, though variable depth. The convoluted surface consists of a cake of grey matter, somewhat less than half an inch thick, and very much extended by the convoluted arrangement. Inside of this cake, the hemispheres are made up of white matter, with the exception of certain small enclosed masses, which contain considerable portions of grey matter.

These last-named bodies, called the lesser grey brain, are regarded as the medium of connexion hemispheres above, and the great stem below.

centres of the between the Probably in