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muscles be coarse or fine in their obvious texture. According to Mr. Bowman their average size is somewhat greater in the male than in the female, being in the former and in the later, or more than a fourth smaller.'-Ib.

'As to the structure of fibres, it has been ascertained that each is made up of a larger number of extremely fine filaments or fibrils, inclosed in a tubular sheath.' 'It would seem that the elementary particles of which the fibril is made up, are little masses of pellucid substance presenting a rectangular outline, and appearing dark in the centre.' The length of the elementary particles is estimated by Mr. Bowman at 400 of an inch. He finds that their size is remarkably uniform in mammalia, birds, reptiles, fishes, and insects.'-1b.

Nerves of Voluntary Muscles.-The nerves of a voluntary muscle are of considerable size. Their branches pass between the fasciculi, and repeatedly unite with each other in form of a plexus, which is for the most part confined to a small part of the muscle, or muscular division in which it lies. From one or more of such primary plexuses nervous twigs proceed and end by finer or terminal plexuses, formed by slender bundles consisting of two or three primitive tubules each, some of them separating into single tubules.-Ib.

'By means of the microscope these fine nervous bundles and single tubules may be observed to pass between the muscular fibres, and after a longer or shorter course, to return to the plexus. They cross the direction of the muscular fibres directly or obliquely, forming wide arches; and on their return they either rejoin the larger nervous bundles from which they set out, or enter into other divisions of the plexus. The nervous filaments, therefore, do not come to an end in the muscle, but form loops or strings among its fibres.'-Ib.*

I refrain from entering into the description given of the involuntary muscles,-those of the heart, intestines, bronchial tubes, iris, middle coat of the arteries, &c.-as being less important for the object of the present work. It will, how

The active connexion between the nerves and the muscles would seem to consist in an electrical current passing from the one to the other. The numerous experiments of Du Bois Reymond and others in this subject, scarcely permit any other conclusion.

ever, be interesting to hear what the same authority has said on the Sensibility of muscle, and also on the Contractility, or the source of its power as a mechanical prime mover.

3. Sensibility. This property is manifested by the pain which is felt when a muscle is cut or lacerated, or otherwise violently injured, or when it is seized with spasm. Here, as in other instances, the sensibility, properly speaking, belongs to the nerves which are distributed through the tissue, and accordingly, when the nerves going to a muscle are cut, it forthwith becomes insensible. It is by means of this property, which is sometimes called the muscular sense,' that we become conscious of the existing state of the muscles which are subject to the will, or rather of the condition of the limbs and other parts which are moved through means of the voluntary muscles, and we are thereby guided in directing our voluntary movements towards the end in view. Accordingly, when the muscular sense is lost, while the power of motion remains,-a case which, though rare, sometimes occurs, the person cannot direct the movements of the affected limbs without the guidance of the eye.'

On this passage I would remark that the two sensibilities described differ very much in their character. The sensibility to injuries is a fact distinct from those feelings of the state of voluntary muscles that serve to guide the movements in working for ends. The one is the passive, and the other the active, sensibility of muscle.

4. Irritability or Contractility. In order to cause contraction, the muscle must be excited by a stimulus. The stimulus may be applied immediately to the muscular tissue, as when the fibres are irritated by a sharp point; or it may be applied to the nerve or nerves which belong to the muscle; in the former case the stimulus is said to be "immediate," in the latter "remote." The nerve does not contract, but it has the property, when stimulated, of exciting contractions in the muscular fibres to which it is distributed, and this property, named the "vis nervosa" (true nervous force), is distinguished from contractility, which is confined to the muscle. Again, a stimulus may be either directly applied to the nerve of the muscle, as when that nerve is itself mechanically irritated or galvanized; or it may be first made to

THE STIMULI TO MUSCLES.

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act on certain other nerves, by which its influence is, so to speak, conducted in the first instance to the brain or spinal cord (or perhaps even to some subordinate nervous centre) and thence transferred or reflected to the muscular nerve.

3.

'The stimuli to which muscles are obedient are of various kinds; those best ascertained are the following, viz. :—1. Mechanical irritation of almost any sort, under which head is to be included sudden extension of the muscular fibres. 2. Chemical stimuli, as by the application of salt or acrid substances. Electrical; usually by means of a galvanic current made to pass through the muscular fibres, or along the nerve. 4. Sudden heat or cold. These four may be classed together as physical stimuli. Next, mental stimuli, viz.—1. The operation of the will, or volition. 2. Emotions, and some other involuntary states of the mind. Lastly, there still remain exciting causes of muscular motions in the economy which, although they may probably turn out to be physical, are as yet of doubtful nature, and these, until better known, may perhaps, without impropriety, be called organic stimuli; to this head may be also referred, at least provisionally, some of the stimuli which excite convulsions and other involuntary motions which occur in disease.'-p. clxxvii.

Of the stimuli thus enumerated the most interesting to us are the mental stimuli. These are described as of two kinds; the Emotions-or the influence of the Feelings-and the Will. A third kind is the Spontaneous force to be presently discussed. There is one other property of muscle, alluded to in the previous chapter, which is described as follows:

5. Tonicity or Tonic Contraction.- Although in muscles. generally, contraction is succeeded by complete relaxation, there are various muscles which, after apparently ceasing to contract, remain in a state of tension, and have still a certain tendency to approximate their points of attachment, although this tendency is counterbalanced by antagonistic muscles, which are in the same condition, and the limb or other moveable part is thus maintained at rest. This condition of muscle is named "tonicity," or the "tonic state." It is no doubt a species of contraction, as well as the more conspicuous and powerful action with which it alternates; but it is employed merely to maintain equilibrium, not to cause motion, and it is not temporary but enduring-con

tinuing during sleep when volition is in abeyance, and occasioning no fatigue. It appears to be excited through the medium of the nerves, though independently of the will, for when the nerves are cut it ceases, and then the muscles nearly become flaccid : the stimulus which acts on the nerves is not known.'

PROOFS OF SPONTANEOUS ACTIVITY.

6. We have now to consider the evidence that there is for the existence of a class of movements and actions, anterior to, and independent of, the sensations of the senses. This question, brought on here to settle a point of precedence or arrangement, has a far wider import, and will re-appear on various occasions in the course of the subsequent exposition.

The proofs principally relied on are the following:

(1.) The already mentioned fact of the Tonicity of muscles. This fact I regard as proving the existence of a central stimulus in the nervous system. The tonicity does not, indeed, amount to actual movement; still, it is only a lower degree of the same thing: and what one centre does in a low degree, another may do in a higher; the peculiar mode of operation is established as a fact of the nervous mechanism.*

(2.) The permanent closure of certain of the muscles— those named sphincters-is an effect of the same nature as the tonicity, but displaying a more energetic stimulus still,

Some physiologists would ascribe the tonicity, not to the exclusive influence of the centres, but to the existence of a constant stimulation proceeding from the extremities by the incarrying nerves. They allege in support of this view, that when all the sensory roots of the spine are cut, the tonicity disappears. This, however, would not affect the general doctrine in question. Granting that the muscular stimulus is in one sense reflex, and arises from a perennial irritation of the incarrying fibres, this constant irritation is not what we usually understand by stimulation from without. It is a current arising out of some constant condition of the sensitive tissues, and not out of visible and remitted applications to the parts. A constant stimulus is no stimulus at all. The real point is-given a certain intensity of outward stimulation, the resulting movements will vary according to the condition of the nerves and nerve centres; the same stimulus finding at one time a feeble, and at another time an energetic, response.

PROOFS OF SPONTANEOUS ACTIVITY.

such as we can refer only to central influence.

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It cannot

be referred to any impression from without. Neither can it be wholly ascribed to the muscle's own contractility, seeing that the destruction, or paralysis, of certain of the centres leads to the total relaxation of those muscles.

The singular rotatory movements, arising from uni-lateral section of the pons varolii and other ganglia, suggest, in a particularly marked manner, the existence of a high permanent charge of nervous power, ordinarily disguised by being in a state of equilibrium.

(3.) It is not altogether irrelevant, to cite the activity maintained by involuntary muscles, as showing the existence of a mode of power originating with the nerve centres. Nervous influence is required for maintaining the circulation of the blood, the movement of the food along the alimentary canal, &c., all which points to an inward evolution of force, although modified by stimulation in the several organs. It may be said that, when the movements are once commenced, the completion of one may be a stimulus to the succeeding; still the question would recur-by what force does the heart begin to beat?

Thus the notion of an initiative existing in the nerve centres is borne out by the tonicity, by the action of the sphincters, by the still more energetic movements of rotation, and by the analogy of the involuntary muscles. Seeing that the spinal cord and the other inferior ganglia are found capable of originating muscular contractions, we are entitled to suppose that the larger masses of the brain may be the sources of a much more abundant and conspicuous activity than these examples afford. The proofs that follow are intended to put in evidence the existence of such movements.

(4) In wakening from sleep, movement precedes sensation. If light were essential to the movements concerned in vision, it would be impossible to open the eyes. The act of wakening from sleep can hardly be considered in any other view, than as the reviving of the activity by a rush of nervous power to the muscles, followed by the exposure

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