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380

CONVULSIVE FORMS OF REFLEX ACTION.

excited by the irritation which results from the pressure of the tooth as it rises against the unyielding gum (§ 174). They are often occasioned, too, by the presence of indigestible or injurious substances, or of intestinal worms, in the alimentary canal; and will cease as soon as this is properly cleared out. Again, in Tetanus or "lockjaw" resulting from a lacerated wound, the irritation of the injured nerve is the first cause of the convulsive action; and a similar local irritation is often the origin of Epileptic fits, in which the convulsion is accompanied by loss of consciousness. When these complaints prove fatal, it is usually by suffocation,-the muscles of respiration being fixed by the convulsive action, in such a manner that air cannot pass either in or out.

474. In other forms of convulsive disorders, however, the cause of irritation may directly affect the Spinal Cord, instead of being conveyed to it by the nerves from a distance. This seems to be the case, for example, in Hydrophobia; which terrible complaint is probably due to a poison introduced into the blood by the bite of the rabid animal, and conveyed by the circulating current to the nervous centres. So, when the poison termed Strychnia has found its way into the circulation, the whole Spinal Cord is thrown into such an excitable state, that the slightest stimulus produces the most violent convulsive movements, which succeed one another in extraordinary variety. And the teething-convulsions of infants often depend more upon a peculiar excitable state of the spinal cord, which results from atmospheric impurity, and is removed by change of air, than they do upon the irritation of the gums.-By knowing, as he now does, the part of the nervous system on which these convulsive disorders depend, the Physician is enabled to apply his remedies with much greater precision than heretofore, and to form a much more accurate estimate of the danger which attends them. Functions of the Ganglia of Special Sense.-Consensual Actions.

475. It has been seen that the nerves of special sense— those of smell, sight, and hearing-terminate in ganglionic centres peculiar to themselves, which are lodged within the skull, and form part of what is commonly termed the brain, though distinct both from the Cerebrum and the Cerebellum. These Sensory Ganglia are almost the only representatives of

GANGLIA OF SPECIAL SENSE-INSTINCTIVE ACTIONS. 381

the brain in the Invertebrated animals; and in Fishes they bear a very large proportion to the other parts, their relative size gradually diminishing as we ascend the scale towards Man.

Now when we study the actions of these lower tribes of animals, we find that those which evidently depend upon sensation, especially the sense of sight, are very far from being of the same spontaneous or voluntary character as those which we perform. We judge of this by their unvarying nature, the different individuals of the same species executing precisely the same movements, when the circumstances are the same, and this evidently without any choice, or intention to fulfil a given purpose, but in direct respondence to an internal impulse. Of this we cannot have a more remarkable example than is to be found in the operations of Bees, Wasps, and other social Insects; which construct habitations for themselves upon plans which the most enlightened human intelligence could not surpass; yet which do so without hesitation, confusion, or interruption, the different individuals of a community all labouring effectively for one common purpose, because their impulses are the same (Chapter XIV.)

476. In higher animals we may often notice the effect of similar promptings, by which the various species are guided in their choice of food, in the construction of their habitations, in their migrations, &c.: but these are frequently modified to a certain degree by the intelligence which they possess. The closure of the pupil when the eye is exposed to a strong light, and its dilatation when the light diminishes (§ 534), afford a very marked example of this "consensual" class of movements, which differ from the simply-reflex in requiring the stimulus of sensations, but which are, like them, quite independent both of the reason and of the will. A still more striking illustration, however, is furnished by the mode in which a little Fish, termed the Chaetodon rostratus, obtains its food. Its mouth is prolonged into a kind of beak or snout, through which it shoots drops of liquid at insects that may be hovering near the surface of the water, and rarely fails in bringing them down. Now, according to the laws of Optics, the insect, being above the water whilst the eye of the fish is beneath it, is not seen by it in its proper place; since the rays do not pass from the insect to the fish's eye in a straight line (§ 528).

382

CONSENSUAL ACTIONS IN MAN.

The insect will in fact appear to the fish a little above the place which it really occupies; and the difference is not constant, but varies with every change in the relative positions of the fish and the insect. Yet the wonderful instinct with which the fish is endowed, leads it to make the due allowance in every case; doing that at once, for which long course of experience would be required by the most skilful Human marksman, under similar circumstances.

477. Though the Intelligence and Will of Man in a great degree supersede his consensual impulses, in the same manner as they hold in subordination his reflex movements (§ 471), yet we have many indications of the direct operation of sensations in determining respondent movements. Of this kind are the start produced by a loud sound, particularly if unexpected; the closure of the eyes to a dazzling light, or on the sudden approach of a body that might injure them; the production of sneezing by a dazzling light; the provocation of laughter by tickling, or by some sight or sound to which no distinct ludicrous idea or emotion attaches itself; and the excitement of vomiting by highly disagreeable sensations, as the sight of a loathsome object, an offensive smell, a nauseous taste, or by tickling the back of the mouth with a feather.1 None of these "consensual" movements can be excited without the consciousness of the subject of them; and this circumstance marks them out as belonging to a different category from the "reflex" movements performed through the instrumentality of the Spinal Cord.-In some convulsive disorders, the attacks are excited by causes that act through the organs of sense: thus, in Hydrophobia we observe the immediate influence of the sight or sound of liquids; and in many Hysteric subjects, the sight of a paroxysm in another individual is the most certain means of its induction in themselves.

478. But we may trace the agency of the Sensory Ganglia

1 This is the most ready way of exciting vomiting, when it is desired to free the stomach from poisons or unwholesome articles of food; but care must be taken not to apply the feather so low down as to cause it to be grasped by the muscles concerned in the act of swallowing; for its irritation, instead of producing vomiting, will then occasion an act of deglutition (§ 195), which may draw the feather from the hand of the operator, and carry it down into the stomach of the patient.

IMPORTANCE OF GUIDING SENSATIONS.

383

in Man and the highest Vertebrata, not merely in their direct and independent operation on the Muscles, but also in the manner in which they participate in all voluntary action. For it is now well established, that the Will cannot bring about any definite movement, except under the guidance of sensations, derived either from the muscles themselves, or through some channel of information which indicates what the muscles are doing. It is for want of the guiding sensations afforded by the ear, that persons who are born deaf are also dumb, the will not being able to make use of the muscles concerned in vocalization; and where, by long training, some imperfect power of speech has been acquired, it has been gained by attention to the sensations arising from the muscular exertion of the organs themselves. It is by the guiding influence of the visual sensations, that the movements of the two eye-balls are made to correspond; and, in children born completely blind, it may be observed that the eyes roll about without any harmony, though a very slight perception of light is sufficient to bring their motions into consent. So, again, if the arm or the leg be so paralysed that its sensibility is lost whilst its muscles are still under the power of the will, that power can only be exerted to occasion movement by the assistance of the sight; a mother, for example, so affected, being only able to hold her infant upon her arm so long as she looks at it; and a man, whose legs are thus paralysed, being only able to sustain himself in standing or walking by constantly looking at his legs.

479. It seems to be obviously through the shorter channel afforded by the Sensory Ganglia, that those actions are performed, which, though originally directed by Intelligence and Will, come by frequent repetition to be so completely automatic as to resemble the instinctive actions of the lower animals. Thus it is within the experience of almost every one, that he occasionally walks through the streets with his mind intently and continuously engaged on some train of thought, without the least attention to, or even consciousness of, the direction he is taking; yet he avoids obstacles, and follows his accustomed course, obviously under the guidance of his visual sense, whilst the movements of his limbs are kept-up by reflex action (§ 471); and on awaking, as it were, from his reverie, he may find that he has thus been automa

384 tically conducted to a place very different from that to which he had intended going. So, again, we may read aloud, or play on a musical instrument, without being at all aware of what we are about, the whole attention being absorbed by some engrossing thoughts or feelings within. And

HABITUAL ACTIONS:-FUNCTION OF CEREBELLUM.

it seems to be in this manner that the movements of Somnambulists are guided; their Cerebrum being, as it were, cut-off from communication with the outer world, and their Sensory Ganglia acting independently of it.

Function of the Cerebellum.-Combination of Muscular Actions.

480. Much discussion has taken place of late years respecting the uses of the Cerebellum; and many experiments have been made to determine them. That it is in some way connected with the powers of motion, is now generally admitted. Its size in the different tribes of Vertebrated animals bears a pretty close correspondence with the variety and energy of the movements performed by them; being greatest in those animals which require the constant united effort of a large number of muscles to maintain their usual position, whilst it is least in those which require no muscular exertion for this purpose. Thus in animals that habitually rest and move upon four legs, there is but little occasion for any organ to combine and harmonize the actions of their several muscles; and in these the Cerebellum is small. But among the more active predaceous Fishes (as the Shark),-Birds of most powerful and varied flight (as the Swallow, which not only flies rapidly, but executes the most complicated evolutions in pursuit of its Insect prey with the greatest facility),—and Mammals which can maintain the erect position and use their extremities for other purposes than support and motion,-we find the Cerebellum of much greater size: whilst in Man, who surpasses all other animals in the number and variety of the combinations of muscular movement which he is capable of executing, it attains its largest dimensions and its greatest complexity of structure.

481. From experiments upon all classes of Vertebrated Animals, it has been found that, when the Cerebellum was removed, the power of walking, springing, flying, standing, or maintaining the equilibrium of the body, was destroyed. It did not seem that the animal had in any degree lost volun

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