make its way through some part or other of the medulla oblongata, the importance of this organ becomes obvious.

21. It is a singular fact that when one side of the brain is diseased or injured, the effects are visible on the other side of the body. Thus when, as not unfrequently happens, a blood-vessel gives way in the right cerebral hemisphere, leading to a destruction of nervous matter there, the result is that the left arm, and left leg, and left side of the body generally are paralysed, that is, the will has no longer any power to move the muscles of that side, and impulses started in the skin of that side cannot awaken sensations in the brain. Hence, it is said that between the brain and the peripheral organs there is a complete crossing or decussation of efferent (voluntary) and afferent (sensory) impulses. We have already seen (§ 14) that a certain amount of crossing of impulses of both kinds takes place all along the spinal cord; but the chief decussation seems to take place in the medulla oblongata, and is probably largely, though not wholly, effected by means of what is called the decussation of the anterior pyramids (see Fig. 84). Here, large bundles of fibres coming chiefly from the lateral columns of the spinal cord (which as we have seen (§ 14) seem to be the chief channels for the conduction of sensory and motor impulses along the cord), rise up to the front and cross over to the other side.

But there is also a decussation of impulses in the case of the nerves arising from the medulla above the decussation of the pyramids. Thus, in the case quoted above of a blood vessel bursting in the right cerebral hemisphere, the left side of the man's face is paralysed as well as the left side of his body, that is to say, impulses cannot pass to and from his brain and the left facial and fifth nerves. The impulses along these nerves cross over, decussate, and reach the right side of the brain.

It sometimes happens, however, that disease or injury may affect the medulla oblongata itself, on one side only (e.g. the right), above the decussation of the pyramids, in such a way that the fifth and facial nerves affected in their course before they decussate, that is to say, on the same side as the injury. The man then,

are

while still paralysed on the left side of his body, is paralysed on the right side of his face.

22. The functions of most of the parts of the brain which lie in front of the medulla oblongata are, at present, very ill understood; but it is certain that extensive injury, or removal, of the cerebral hemispheres puts an end to intelligence and voluntary movement, and leaves the animal in the condition of a machine, working by the reflex action of the remainder of the cerebro-spinal axis.

We have seen that in the frog the movements of the body which the spinal cord alone, in the absence of the whole of the brain, including the medulla oblongata, is capable of executing, are of themselves strikingly complex and varied. But none of these movements arise from changes originating within the organism, they are not what are called voluntary or spontaneous movements; they never occur unless the animal be stimulated from without. Removal of the cerebral hemispheres is alone sufficient to deprive the frog of all spontaneous or voluntary movements; but the presence of the medulla oblongata and other parts of the brain (such as the corpora quadrigemina, or what corresponds to them in the frog, and the cerebellum) renders the animal master of movements of a far higher nature than when the spinal cord only is left. In the latter case the animal does not breathe when left to itself, lies flat on the table with its fore-limbs beneath it in an unnatural position; when irritated kicks out its legs, and may be thrown into actual convulsions, but never jumps from place to place; when thrown into a basin of water falls to the bottom like a lump of lead, and when placed on its back will remain so, without making any effort to turn over. In the former case the animal sits on the table, resting on its front limbs, in the position natural to a frog; breathes quite naturally; when pricked behind jumps away, often getting over a considerable distance; when thrown into water begins at once to swim, and continues swimming until it finds some object on which it can rest; and when placed on its back immediately turns over and resumes its natural position. Not only so, but the following very striking experiment may be performed with it. Placed on a small board it remains perfectly motionless so long as the board is

horizontal; if, however, the board be gradually tilted up so as to raise the animal's head, directly the board becomes inclined at such an angle as to throw the frog's centre of gravity too much backwards, the creature begins slowly to creep up the board, and, if the board continues to be inclined, will at last reach the edge, upon which when the board becomes vertical he will seat himself with apparent great content. Nevertheless, though his movements when they do occur are extremely well combined and apparently identical with those of a frog possessing the whole of his brain, he never moves spontaneously, and never stirs unless irritated.

Thus the parts of the brain below the cerebral hemispheres constitute a complex nervous machinery for carrying out intricate and orderly movements, in which afferent impulses play an important part, though they do not give rise to clear or permanent affections of consciousness.

23. There can be no doubt that the cerebral hemispheres are the seat of powers, essential to the production of those phenomena which we term intelligence and will; and there is experimental and other evidence which seems to indicate a connection between particular parts of the surface of the cerebral hemispheres, and particular acts. Thus irritation of particular spots in the anterior part of a dog's brain will give rise to particular movements of this or that limb, or of this or that group of muscles; and the destruction of a certain part of the posterior lobes of the cerebral hemispheres is said to cause blindness. But the exact way in which these effects are brought about is not yet thoroughly understood; and even if it should be ultimately proved beyond all doubt, that the central endorgan of vision (Lesson VIII. § 28) consists of certain nerve-cells lying in a particular part of the posterior surface of the cerebral hemisphere, and that the central end-organ of hearing consists of other nerve-cells lying elsewhere on the cerebral surface, it will still leave us completely in the dark as to what goes on in the cerebral hemispheres when we think and when we will.

There is no doubt that a molecular change in some part of the cerebral substance is an indispensable antecedent to every phenomenon of consciousness. And it is

possible that the progress of investigation may enable us to map out the brain according to the psychical relations of its different parts. But supposing we get so far as to be able to prove that the irritation of a particular fragment of cerebral substance gives rise to a particular state of consciousness, the reason of the connection between the molecular disturbance and the psychical phenomenon appears to be out of the reach, not only of our means of investigation, but even of our powers of conception.

24. Even while the cerebral hemispheres are entire, and in full possession of their powers, the brain gives rise to actions which are as completely reflex as those of the spinal cord.

When the eyelids wink at a flash of light, or a threatened blow, a reflex action takes place, in which the afferent nerves are the optic, the efferent the facial. When a bad smell causes a grimace, there is a reflex action through the same motor nerve, while the olfactory nerves constitute the afferent channels. In these cases, therefore, reflex action must be effected through the brain, all the nerves involved being cerebral.

When the whole body starts at a loud noise, the afferent auditory nerve gives rise to an impulse which passes to the medulla oblongata, and thence affects the great majority of the motor nerves of the body.

25. It may be said that these are mere mechanical actions, and have nothing to do with the operations which we associate with intelligence. But let us consider what takes place in such an act as reading aloud. In this case, the whole attention of the mind is, or ought to be, bent upon the subject-matter of the book; while a multitude of most delicate muscular actions are going on, of which the reader is not in the slightest degree aware. Thus the book is held in the hand, at the right distance from the eyes; the eyes are moved from side to side, over the lines and up and down the pages. Further, the most delicately adjusted and rapid movements of the muscles of the lips, tongue, and throat, of the laryngeal and respiratory muscles, are involved in the production of speech. Perhaps the reader is standing up and accompanying the lecture with appropriate gestures. And yet every one of these muscular acts may be performed with utter

unconsciousness, on his part, of anything but the sense of the words in the book. In other words they are reflex acts.

26. The reflex actions proper to the spinal cord itself are natural, and are involved in the structure of the cord and the properties of its constituents. By the help of the brain we may acquire an infinity of artificial reflex actions, that is to say, an action may require all our attention and all our volition for its first, or second, or third performance, but by frequent repetition it becomes, in a manner, part of our organization, and is performed without volition,

or even consciousness.

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As everyone knows, it takes a soldier a long time to learn his drill-for instance, to put himself into the attitude of attention" at the instant the word of command is heard. But, after a time, the sound of the word gives rise to the act, whether the soldier be thinking of it, or not. There is a story, which is credible enough, though it may not be true, of a practical joker, who, seeing a discharged veteran carrying home his dinner, suddenly called out Attention!" whereupon the man instantly brought his hands down, and lost his mutton and potatoes in the gutter. The drill had been thorough, and its effects had become embodied in the man's nervous structure.

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The possibility of all education (of which military drill is only one particular form) is based upon the existence of this power which the nervous system possesses, of organizing conscious actions into more or less unconscious, or reflex, operations. It may be laid down as a rule, which is called the Law of Association, that if any two mental states be called up together, or in succession, with due frequency and vividness, the subsequent production of the one of them will suffice to call up the other, and that whether we desire it or not.

The object of intellectual education is to create such indissoluble associations of our ideas of things, in the order and relation in which they occur in nature; that of a moral education is to unite as fixedly the ideas of evil deeds with those of pain and degradation, and of good actions with those of pleasure and nobleness.

27. The sympathetic system consists chiefly of a double chain of ganglia, lying at the sides and in front of the