there remains an inferior form of the sensibility of the three lower senses-Touch, Taste, and Smell. By stimuli applied to these senses, reflex movements may be excited. Thus, the hemispheres are not the exclusive seat of consciousness, but they are doubtless the seat both of Intelligence and of nearly all the innumerable shades and varieties of Sensation and Emotion. The attempt to localize the mental functions in special portions of the cerebral mass, has been thwarted by observations of a remarkable kind. The phrenologists noticed cases where the destruction or disease of one hemisphere was unaccompanied with the entire loss of any function; the inference being that the hemispheres were duplicate bodies performing the same office, like the two eyes, or the two halves of the nostrils. But cases have been recorded of disease of large portions of the brain in both hemispheres at once, without apparent loss of function; which would require us to extend still farther the supposition of a plurality of nervous tracks for a single mental aptitude. Functions of the Cerebellum. 21. The experiments made upon the Cerebellum, and the inferences founded upon its comparative size in different animals, have led some physiologists to assign to it the function of harmonizing and co-ordinating the locomotive and other movements. Flourens removed the cerebellum from pigeons by successive slices. During the removal of the superficial layers there appeared only a slight feebleness and want of harmony in the movements, without any expression of pain. On reaching the middle layers, an almost universal agitation was manifested, without any sign of convulsion; the animal performed rapid and ill-regulated movements; it could hear and see. After the removal of the deepest layers, the animal lost completely the power of standing, walking, leaping, or flying. The power had been injured by the previous mutila FUNCTIONS OF THE CEREBELLUM. 47 tions, but now it was completely gone. When placed upon his back, he was unable to rise. He did not, however, remain quiet and motionless, as pigeons deprived of the cerebral hemispheres do; but evinced an incessant restlessness, and an inability to accomplish any regular or definite movement. He could see the instrument raised to threaten him with a blow, and would make a thousand contortions to avoid it, but did not escape. Volition and sensation remained-the power of executing movements. remained; but that of co-ordinating these movements into regular and combined actions was lost. 'Animals deprived of the cerebellum are in a condition very similar to that of a drunken man, so far as relates to their power of locomotion. They are unable to produce that combination of action in different sets of muscles which is necessary to enable them to assume or maintain any attitudes. They cannot stand still for a moment, and in attempting to walk, their gait is unsteady, they totter from side to side, and their progress is interrupted by frequent falls. The fruitless attempts which they make to stand or walk are sufficient proof that a certain degree of intelligence remains, and that voluntary power continues to be enjoyed.' (TODD and BOWMAN, I., p. 359.) When the cerebellum is cut away at the top, the animal moves backward. When one side is cut away, the animal rolls over to the other side; the eye of the sound side is turned outwards and downwards, the other eye inwards and upwards. Sometimes a vertiginous action ensues, as if the body were revolved on a spit. The inference drawn from these experiments-that the cerebellum is the exclusive seat of combined movements-is denied by Dr. Brown-Séquard. He says 'I have ascertained that it is by the irritation they produce on the various parts of the base of the brain that the diseases of the cerebellum, or its extirpation in animals, cause the disorder of movements which has been considered as depending upon the absence of a guiding power. In fact, the least irritation of several parts of the brain with only the I point of a needle, may generate very nearly the same disorder of movement that follows the extirpation of the cerebellum. have thus been led to conclude that, after this extirpation, or after the destruction by disease of a large or small part of this nervous centre, it is not its absence, but some irritative influence upon the parts of the encephalon that remain unaltered which causes the irregularity of movements (Lectures, p. 79). This line of criticism has the defect of proving too much; it would lead to the conclusion that the cerebellum has no function. The views of Flourens have been recently supported by M. Vulpian; who, after comparing numerous facts, has shown that, although disease or deficiency of the cerebellum is not uniformly attended with utter incapability of locomotion, yet there is a want of steadiness, and a great liability to stumble, in such instances. The safest inference at present seems to be, that the cerebellum is not the sole organ concerned in rhythmical or combined movements, but concurs with some of the other ganglia in upholding this function. The remark above made, regarding the plurality of nervous tracks for the higher cerebral aptitudes, may be extended to the inferior department of the combined or associated movements. Of the Nerve Force, and the course of Power in the Brain. 22. The structure of the nervous substance, and the experiments made upon the nerves and nerve centres, establish beyond doubt certain peculiarities as belonging to the force that is exercised by the brain. This force is of a current nature; that is to say, a power generated at one part of the structure is conveyed along an intervening substance, and discharged at some other part. The different forms of Electricity and Magnetism have made us familiar with this sort of action. In a voltaic cell, energy is generated and transmitted along a wire with inconceivable rapidity to any place where the conductor reaches. This portable, or current, character of the nerve force is what enables movements, distant from one another in the body, to be associated together under a common stimulus. An impression of sound-a musical note, for example, is carried to the brain; the result is a responsive action and excitement extending to the voice, mouth, eyes, head, &c. This multiplex and various manifestation implies a system of connexion among the centres of action, whereby many strings can be touched from one point; a connexion due to the conducting nerves that pass and repass from centre to centre, and from the centres to the muscular apparatus over the body. Supposing the corpora quadrigemina to be a centre for the sense of vision, an impression passing to this centre propagates a movement towards many other centres,— to the convoluted hemispheres upwards, to the cerebellum behind, and to the medulla oblongata and spinal cord beneath; and through these various connexions an extensive wave of effects may be produced, ending in a complicated chain of movements all over the framework of the body. Such a system of intercommunication and transmission of power is therefore an essential part of the bodily and mental structure. 23. The experiments of Du Bois Reymond, show that there is a community of nature between the nerve force and common electricity. Electric currents are constantly maintained in the nerves and muscles, their character being changed during sensation and muscular contraction. The direction of these currents has been minutely examined by Du Bois Reymond, and he lays down a number of general principles regarding them. The following are some of his conclusions: 'The muscles and nerves, including the brain and spinal cord, are endowed during life with an electro-motive power.' 'This electro-motive power acts according to a definite law, which is the same in the nerves and in the muscles, the law of the antagonism of the longitudinal and the transverse sections. The longitudinal surface is positive, and the transverse section negative.' 'Every minute particle of the nerves and the muscles must be supposed to act according to the same law as the whole nerve or muscle.' The total currents are, in fact, the combined effect of these currents circulating round the ultimate particles. 'The current in muscles when in the act of contraction, and in nerves when conveying motion, or sensation, undergoes a sudden and great negative variation of its intensity.' 'It has not been ascertained whether, in the act of contraction, the muscular current is only diminished, or wholly vanishes, or whether it changes its direction.' Thus the proper nerve force-that is to say, the currents in the nerves during sensation and movement is so far in unison with electricity, that it neutralizes and reverses genuine electrical currents proved to exist in the nerves and muscles in their condition of rest. This is the utmost that can be said in the present state of our knowledge. Even granting that the force conveyed along the nerves during the mental processes were identical with voltaic electricity, the character of the nerve substance would create some points of contrast between the phenomena of vital action and a common voltaic battery. The conducting power of nerve fibre is attended with nervous waste, and the substance has to be constantly renewed from the blood, which is largely supplied to the nerves, although perhaps not so largely as to the vesicles. If now we compare this liability to waste and exhaustion with the undying endurance of an electric wire, we shall be struck with a very great contrast. The wire is doubtless a more compact, resisting, and sluggish mass; the conduction requires a certain energy of electric action to set it agoing, and in the course of a great distance becomes faint and dies away. The nerve, on the other hand, is stimulated by a slighter influence, and propagates that influence, with increase, by the consumption of its own material. The wire must be acted on at both ends, by the closure of the circuit, before acting as a conductor in any degree; the nerve takes fire from a slight stimulus like a train of gunpowder, and is wasted by the current that it propagates. If this view be correct, the influence conveyed is much more beholden to the conducting fibres, than electricity is to the copper wire. The fibres are made to sustain or increase the force at the cost of their own substance. |