Elementary principles of medicine / [Robert Williams].

  • Williams, Robert
Date:
1847]
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Licence: Public Domain Mark

Credit: Elementary principles of medicine / [Robert Williams]. Source: Wellcome Collection.

    6/306 page 528
    Elemen- secretion of the organ or part it supplies. Now the tary Prin- probable mode of action of a nerve is as follows :— ciples ot Anatomy has shown that the brain and nerves are Me icine.^ f\brous. an(j a fibrous structure almost of necessity implies action or contraction. But, independent of this argument, there are many beautiful phenomena, espe- cially of sight, which seem to prove that vision and sensation generally result from a physical action or contraction of the nerves, whether of the eye or other organ. Thus, if we look steadily at a target composed of concentric circles of various colours, placed in a strong light, till the sight is somewhat fatigued, and then close the eyes, shaded by the hand placed at about an inch distance, we shall see, says Darwin, the most beautiful circles that imagination can conceive, and which are most resembled when a drop or two of oil is poured on a still lake on a fine day. These circular irides of colour, however, are not only different from those of the target, but are perpetually changing till the eye recovers its usual passive state. These adventitious colours are called the reverse, or supplemental colours. This and similar facts show that vision is not owing to the mechanical impulse of light, or to its chemical com- bination with the nerves of the retina; for in those cases the spectra formed on the eye would remain of the same colour, only growing fainter and fainter, till they at last disappeared altogether. The probable explanation, therefore, of these phenomena seems to be, that the retina is composed of many sets of nervous fibres, and that when the set whose contractions, for instance, cause the sensation of red is fatigued, it is relieved by the action of an antagonist set whose con- tractions cause the sensation of green, in the same manner as we relieve the muscles of the arm by changing its position and bringing a different set of muscles into action. Another circumstance, also, which seems to demonstrate that the contraction of the nerves consti- tutes vision, is, if we press the ball of the eye at its external angle a luminous appearance is observed; while, if the eye be struck a smart blow, we all know flashes of fire are perceived. Now, the sensation of fire thus produced is entirely the result of mechanical causes, light or other natural stimulant being altogether absent. It follows, then, that vision is produced by contractions of the fibres of the optic nerve; and this mode of action, proved to be true of the sense of sight, may be equally demonstrated of the nerves of the other senses, and also of those of the different organs of secretion. It results, therefore, if a healthy action of the nerves (the great moving and secreting powers of the body) be essential to health, that every morbid action of the nerve must be a cause of disease. The contraction of a nerve, however, although it may account for sensation and secretion, is hardly sufficient to explain the great power imparted to and exercised by the muscles; for if we consider how soft and tender and little coherent the brain and nervous fibres are, and, also how easily the muscular fibre is torn after death, something more is wanting than mere muscular con- traction to explain the infinitely greater power which the muscular fibre is capable of exciting during life than after death. Sir Isaac Newton was of opinion that the cohesion of bodies depends on the presence of elastic fluids; and this opinion is strengthened, if not completely established, by modern discoveries, for a continued stream of electric fluid will enable a magnet to support a mass of iron, of any weight, for an almost indefinite time. It seems probable, therefore, that Elemen- nervous contraction is followed by the extrication of a taryPrin- fluid which is the cause of vital cohesion of parts, and of ciples of the wonderful force which the muscles exert in moving Medicine, and raising bodies. ' It is singular that the electric fluid, so powerful an agent in producing cohesion of inanimate substances, is also the great agent of chemical composition and of decomposition; and likewise that this fluid is evolved by mere change of motion in the particles of matter. It seems highly probable, therefore, from analogy, that a nervous fluid is extricated in like manner by a molecular motion or contraction of the nerves, and that this fluid is the cause, not only of nervous and muscular cohesion, but is also the great agent of the vital compositions and decompositions which are incessantly going on in every part of the body. Again, when we observe that heat is given off by most inorganic substances after a smart blow, which approximates their particles, we may almost infer that nervous contraction may be one of the means which nature employs to regulate the temper- ature of the body. If the theory of a nervous contraction, followed by the extrication of a nervous fluid, be established, the one may be taken as the measure of the other, and the term “ nervous sensibility” may be used to express their con- joint effect. This nervous sensibility, it would appear, then, in health, is accumulated by repose and exhausted by action; so that, at times, it may be considerably in excess or in defect. If, for instance, we sit in the dark, the sensibility of the retina is so greatly increased that the eye is actually pained by the admission of a strong light. On the contrary, if we look at the sun, the sensibility of the retina is so absolutely exhausted by the intensity of its rays, that for a time we are blinded to every other object. This property of accumulation and of exhaustion of nervous sensibility is often ex- tremely sudden, or otherwise exists in a most marked degree in disease. Thus, nothing is frequently more unexpected in its attack than a paroxysm of wild insanity, of epilepsy, of hysteria, or of tetanus; while fainting, or the last stages of fever, are familiar examples of collapse, and of the rapidity with which the nervous energy is exhausted. The hypothesis of a nervous fluid seems to lead to the inference that this elementary principle may be rapidly communicated from one part to another. In health, the action of the brain, in determining a greater quantity of nervous fluid to a particular part, is quite remarkable. Let a black spot, for instance, about the size of a tadpole, be made on a sheet of white paper; that spot, if looked at attentively, will be seen in a few seconds surrounded by an areola of light; a circum- stance which can only arise from the retina being rendered more sensible by the increased quantity of nervous energy communicated to it from the brain by the power of attention. In like manner, it is well known that the times of the action of the bowels, the times of eating, and so on of the other functions, may in a considerable degree be accelerated or retarded in proportion as we direct our attention towards those purposes. It is this power of the brain over distant parts which renders the nervous and hypochondriacal person so prone to exacerbations and remissions; for, by constantly brooding over his complaint, a flood of cerebral or nervous energy is directed to the diseased part, and all its morbid sensibilities are immediately
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