[III]. [1]. It is not easy to assign a cause, why those animal movements, that have once occurred in succession, or in combination, should afterwards have a tendency to succeed or accompany each other. It is a property of animation, and distinguishes this order of being from the other productions of nature.
When a child first wrote the word man, it was distinguished in his mind into three letters, and those letters into many parts of letters; but by repeated use the word man becomes to his hand in writing it, as to his organs of speech in pronouncing it, but one movement without any deliberation, or sensation, or irritation, interposed between the parts of it. And as many separate motions of our muscles thus become united, and form, as it were, one motion; so each separate motion before such union may be conceived to consist of many parts or spaces moved through; and perhaps even the individual fibres of our muscles have thus gradually been brought to act in concert, which habits began to be acquired as early as the very formation of the moving organs, long before the nativity of the animal; as explained in the Section [XVI. 2]. on instinct.
[2]. There are many motions of the body, belonging to the irritative class, which might by a hasty observer be mistaken for associated ones; as the peristaltic motion of the stomach and intestines, and the contractions of the heart and arteries, might be supposed to be associated with the irritative motions of their nerves of sense, rather than to be excited by the irritation of their muscular fibres by the distention, acrimony, or momentum of the blood. So the distention or elongation of muscles by objects external to them irritates them into contraction, though the cuticle or other parts may intervene between the stimulating body and the contracting muscle. Thus a horse voids his excrement when its weight or bulk irritates the rectum or sphincter ani. These muscles act from the irritation of distention, when he excludes his excrement, but the muscles of the abdomen and diaphragm are brought into motion by association with those of the sphincter and rectum.
SECT. [XII].
OF STIMULUS, SENSORIAL EXERTION, AND FIBROUS CONTRACTION.
[I]. Of fibrous contraction. [1]. Two particles of a fibre cannot approach without the intervention of something, as in magnetism, electricity, elasticity. Spirit of life is not electric ether. Galvani's experiments. [2]. Contraction of a fibre. [3]. Relaxation succeeds. [4]. Successive contractions, with intervals. Quick pulse from debility, from paucity of blood. Weak contractions performed in less time, and with shorter intervals. [5]. Last situation of the fibres continues after contraction. [6]. Contraction greater than usual induces pleasure or pain. [7]. Mobility of the fibres uniform. Quantity of sensorial power fluctuates. Constitutes excitability. [II]. Of sensorial exertion. [1]. Animal motion includes stimulus, sensorial power, and contractile fibres. The sensorial faculties act separately or conjointly. Stimulus of four kinds. Strength and weakness defined. Sensorial power perpetually exhausted and renewed. Weakness from defect of stimulus. From defect of sensorial power, the direct and indirect debility of Dr. Brown. Why we become warm in Buxton bath after a time, and see well after a time in a darkish room. Fibres may act violently, or with their whole force, and yet feebly. Great exertion in inflammation explained. Great muscular force of some insane people. [2]. Occasional accumulation of sensorial power in muscles subject to constant stimulus. In animals sleeping in winter. In eggs, seeds, schirrous tumours, tendons, bones. [3]. Great exertion introduces pleasure or pain. Inflammation. Libration of the system between torpor and activity. Fever-fits. [4]. Desire and aversion introduced. Excess of volition cures fevers. [III]. Of repeated stimulus. [1]. A stimulus repeated too frequently looses effect. As opium, wine, grief. Hence old age. Opium and aloes in small doses. [2]. A stimulus not repeated too frequently does not lose effect. Perpetual movement of the vital organs. [3]. A stimulus repeated at uniform times produces greater effect. Irritation combined with association. [4]. A stimulus repeated frequently and uniformly may be withdrawn, and the action of the organ will continue. Hence the bark cures agues, and strengthens weak constitutions. [5]. Defect of stimulus repeated at certain intervals causes fever-fits. [6]. Stimulus long applied ceases to act a second time. [7]. If a stimulus excites sensation in an organ not usually excited into sensation, inflammation is produced. [IV]. Of stimulus greater than natural. [1]. A stimulus greater than natural diminishes the quantity of sensorial power in general. [2]. In particular organs. [3]. Induces the organ into spasmodic actions. [4]. Induces the antagonist fibres into action. [5]. Induces the organ into convulsive or fixed spasms. [6]. Produces paralysis of the organ. [V]. Of stimulus less than natural. [1]. Stimulus less than natural occasions accumulation of sensorial power in general. [2]. In particular organs, flushing of the face in a frosty morning. In fibres subject to perpetual stimulus only. Quantity of sensorial power inversely as the stimulus. [3]. Induces pain. As of cold, hunger, head-ach. [4]. Induces more feeble and frequent contraction. As in low fevers. Which are frequently owing to deficiency of sensorial power rather than to deficiency of stimulus. [5]. Inverts successive trains of motion. Inverts ideas. [6]. Induces paralysis and death. [VI]. Cure of increased exertion. [1]. Natural cure of exhaustion of sensorial power. [2]. Decrease the irritations. Venesection. Cold. Abstinence. [3]. Prevent the previous cold fit. Opium. Bark. Warmth. Anger. Surprise. [4]. Excite some other part of the system. Opium and warm bath relieve pains both from defect and from excess of stimulus. [5]. First increase the stimulus above, and then decrease it beneath the natural quantity. [VII]. Cure of decreased exertion. [1]. Natural cure by accumulation of sensorial power. Ague-fits. Syncope. [2]. Increase the stimulation, by wine, opium, given so as not to intoxicate. Cheerful ideas. [3]. Change the kinds of stimulus. [4]. Stimulate the associated organs. Blisters of use in heart-burn, and cold extremities. [5]. Decrease the stimulation for a time, cold bath. [6]. Decrease the stimulation below natural, and then increase it above natural. Bark after emetics. Opium after venesection. Practice of Sydenham in chlorosis. [7]. Prevent unnecessary expenditure of sensorial power. Decumbent posture, silence, darkness. Pulse quickened by rising out of bed. [8]. To the greatest degree of quiescence apply the least stimulus. Otherwise paralysis or inflammation of the organ ensues. Gin, wine, blisters, destroy by too great stimulation in fevers with debility. Intoxication in the slightest degree succeeded by debility. Golden rule for determining the best degree of stimulus in low fevers. Another golden rule for determining the quantity of spirit which those, who are debilitated by drinking it, may safely omit.
[I]. Of fibrous contraction.
[1]. If two particles of iron lie near each other without motion, and afterwards approach each other; it is reasonable to conclude that something besides the iron particles is the cause of their approximation; this invisible something is termed magnetism. In the same manner, if the particles, which compose an animal muscle, do not touch each other in the relaxed state of the muscle, and are brought into contact during the contraction of the muscle, it is reasonable to conclude, that some other agent is the cause of this new approximation. For nothing can act, where it does not exist; for to act includes to exist; and therefore the particles of the muscular fibre (which in its state of relaxation are supposed not to touch) cannot affect each other without the influence of some intermediate agent; this agent is here termed the spirit of animation, or sensorial power, but may with equal propriety be termed the power, which causes contraction; or may be called by any other name, which the reader may choose to affix to it.
The contraction of a muscular fibre may be compared to the following electric experiment, which is here mentioned not as a philosophical analogy, but as an illustration or simile to facilitate the conception of a difficult subject. Let twenty very small Leyden phials properly coated be hung in a row by fine silk threads at a small distance from each other; let the internal charge of one phial be positive, and of the other negative alternately, if a communication be made from the internal surface of the first to the external surface of the last in the row, they will all of them instantly approach each other, and thus shorten a line that might connect them like a muscular fibre. See Botanic Garden, p. 1. Canto I. 1. 202, note on Gymnotus.