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mitted in the direction of its length, and being followed by relaxation. Again, in the Muscular structure of the Bladder and Uterus, powerful contractions are excited by irritation, and these produce a great degree of shortening but they do not alternate in the healthy state with any rapid and decided elongation; whilst, on the other hand, an irritation applied to one spot causes more extensive contractions, than are seen to occur as its immediate consequence in the preceding cases. In the Heart, the Muscular structure of a large part of the organ is thrown into rapid and energetic contraction, by a stimulus applied at any one point; and this contraction is speedily followed by relaxation. And in the fibrous tissue of the middle coat of the Arteries, the contraction takes place rather after the manner of that of the bladder and uterus, and a prolonged application of the stimulus is often necessary to produce the effect; but when the contraction commences, it produces a considerable degree of shortening, which takes place in other fasciculi than those directly irritated, and does not speedily give way to relaxation.

353. On the other hand, when the stimuli which excite muscular contraction are applied to the Nerve, which supplies a voluntary muscle composed of striated fibre, they produce a simultaneous contraction in the whole muscle; the effect of the stimulus being at once exerted upon every part of it. In the ordinary action of such muscles, the nervous system is always the channel through which they are called into play, whether to carry into effect the determinations of the mind. (§ 391), or to perform some office necessary to the continuance of life, such as the movements concerned in Respiration (§ 394). The nerves of the striated fibre are all derived at once from the brain or spinal cord. The ordinary actions of the non-striated fibre, on the contrary, are executed in respondence to stimuli applied directly to themselves. It is so difficult to excite contractions in it through the medium of its nerves, that many Physiologists have denied the possibility of doing so; and the nerves lose their power of conveying the influence of stimuli very soon after death, although the contractility of the muscles may remain for a considerable time. The nerves of the non-striated fibre are chiefly those belonging to the Sympathetic system; but, as will be shown hereafter (CHAP. XII.), those which excite motion are probably derived in reality from the Cerebro-spinal system, through the communicating branches which unite the two.

354. When a Muscle is thrown into contraction, its bulk does not appear to be at all affected. Its extremities approach, so that it is shortened in the direction of its fibres; but its diameter enlarges in the same proportion. It was formerly supposed that the ultimate fibres, in the act of contraction, threw themselves into zigzag folds; but this is now well-ascertained not to be the case. The fibre, like the entire muscle, preserves its straight direction in shortening, and increases in diameter. The fibrilla themselves, as already mentioned (§ 336), exhibit an evident change, in regard to the distances of their successive light and dark portions; and the fibre, which is made up of these, exhibits, in its contracted state, a very close approximation of the transverse striæ; to such an extent that they become two, three, or even four times as numerous in a given length, as they are in a

similar length of a non-contracted fibre. According to Mr. Bowman's observations, the contraction usually commences at the extremities of a fibre; but it may occur also at one or more intermediate points. The first appearance of contraction is a dark spot, caused by the approximation of the striæ, and this gradually extends itself, so as to involve a greater or less proportion of the length of the fibre. The approximation of the solid portions forces out the fluid, which was previously contained amongst the fibrilla; and this is seen to lie in bullæ or blebs beneath the myolemma, which is drawn up into wrinkles. 355. The successive stages of the act of contraction can only be thus observed when it takes place very slowly, as in the rigor mortis, or slow contraction after death, the phenomena of which will be presently noticed (§ 367). But the resulting change in muscular fibres, which have been made to contract by galvanism or any other stimulus is essentially the same. This may be best seen in transparent Entozoa, Crustacea, and others among the lower Articulated Animals, whilst alive. Again, in persons who have died from Tetanus, a considerable number of the fibres are found to have been ruptured by violent spasmodic action; the contractile force, called into action by the powerful stimulation of the nerves, having overcome the tendency of the fibre: and in such cases, the same approximation of the transverse striæ and proportional increase in the diameter of the fibre, are to be observed.

356. It appears that, even when considerable force of contraction is being exerted, the whole fibre is seldom or never in contraction at once; but that a continual interchange is taking place amongst its different parts some of them passing from the contracted to the relaxed state as shown by the separation of the transverse striæ,whilst others are taking up the duty, and passing from the relaxed to the contracted condition, as shown by the approximation of the striæ. But it is not only among the different parts of the individual fibres, that this interchange seems to take place. There is good reason to believe, that, when a muscle is kept in a contracted state, by an effort of the will, for any length of time, only a part of its fibres are in contraction at any one time; but that a constant interchange of condition takes place amongst them, some contracting while others are relaxing so that the entire muscle remains contracted, whilst the state of every individual fibre may have undergone a succession of alterations. When the ear is applied to a muscle in vigorous action, an exceedingly rapid, faint, silvery vibration is heard, which seems to be attributable to this constant movement in its substance.

357. Thus it appears that the prolongation of the contraction of a muscle through any length of time, is not opposed to the fact that, in the individual fibres, relaxation speedily follows contraction; but is only a peculiar manifestation of it. The ordinary movements of the Heart exhibit a different manifestation; its fibres contracting simultaneously, and relaxing together, instead of alternating amongst themselves like those of a voluntary muscle. The occasional zigzag arrangement of the fibres, which has been supposed to be their contracted state, is really dependent upon the approximation of their extremities,

in consequence of the contraction of some neighboring fibres, whilst their own condition is that of relaxation. It may be artificially produced by bringing together the two extremities of a fasciculus, after the irritability of the fibre has ceased; so that the flexure at determinate points must be owing simply to the physical arrangement of the parts, perhaps to the passage of nerves or vessels in a transverse direction.

358. We have now to consider the conditions which are requisite for the manifestation of Muscular Irritability. It has been already pointed out, how close is the dependence of the property upon the due nutrition of the tissue; but the property cannot be long exercised except under another condition, which is consequently of almost equal importance,the circulation of arterial blood through the substance of the muscle. The length of time during which the contractility remains, after the circulation has ceased, has been shown by Dr. M. Hall to vary inversely to the activity of the respiration of the animal. In cold-blooded animals, the standard of whose respiration is low, the contractility remains for many hours after death, even in the voluntary muscles; and the muscles of organic life retain it with great tenacity. Thus the heart of a Frog will go on pulsating for many hours after its removal from the body; and the heart of a Sturgeon, which had been inflated with air and hung up to dry has been seen to continue beating, until the auricle had become absolutely so dry as to rustle during its movements. An exceedingly feeble Galvanic current is sufficient to excite the muscles of these animals to contraction; so that Matteucci, in his experiments upon Animal Electricity, has been accustomed to use the prepared hind-leg of a Frog as the best indicator of the passage of an electric current. Among warm-blooded animals whose respiration is vastly more active, the duration of the irritability is proportionally abbreviated; and the muscles of Birds whose respiration is peculiarly energetic, lose this property at an earlier period after the cessation of the circulation, than do those of Mammals. From experiments on the bodies of executed criminals who were previously in good health, Nysten ascertained that, in the Human subject, the contractility of the several muscular structures, as tested by Galvanism, departs in the following time and order: -the left ventricle of the heart first; the intestinal canal at the end of 45 or 55 minutes; the urinary bladder nearly at the same time; the right ventricle after the lapse of an hour; the oesophagus at the expiration of an hour and a half; the iris a quarter of an hour later; and lastly, the ventricles of the heart, especially the right, which in one instance contracted 16 hours after death.

359. That the circulation of arterial or oxygenated blood through the muscles, is the essential condition of the continuance of their irritability appears from this,-that after the general death of the system, and even after the removal of the brain and spinal cord, the muscles will preserve their irritability, and the action of the heart itself will continue for a long time, provided that the circulation be kept up through the lungs by artificial respiration on the principles hereafter to be explained (§ 688). But if, whilst the general circulation continues, the circulation through a particular muscular part be inter

rupted, that organ will lose its contractility earlier than usual. Thus it has been shown by Mr. Erichsen, that, if the coronary arteries (supplying the substance of the heart) be tied in a dog or rabbit, after the animal has been pithed, and the circulation is being maintained by artificial respiration, the pulsation of the heart will only go on for about 23 minutes after the ligature has been applied, or about 33 minutes after the death of the animal; instead of continuing for 90 minutes, which it will do under other circumstances. Further if blood charged with carbonic acid instead of with oxygen, circulate through the muscles, their irritability is speedily impaired, and is even destroyed. This is best seen, when animals are killed by being caused to breathe an atmosphere highly charged with carbonic acid; the irritability of their muscles departing as soon as they are dead. In fact, the destruction of the irritability of the heart, by the circulation of venous blood through its substance, is one of the immediate causes of death. A similar effect is produced by the respiration of other gases, which are either poisonous in themselves, or which prevent the interchange of carbonic acid and oxygen, which ought to take place in the lungs. On the other hand, when animals have been made to respire oxygen, and their blood has been consequently highly arterialized, the contractility of their muscles is retained for a longer time than usual.

360. Hence we may conclude the presence of oxygen in the blood to be one of the conditions of muscular contraction; although it is much less essential in the case of cold-blooded, than in that of warm-blooded animals. It is interesting to remark, that the muscles of hybernating warm-blooded Mammals are reduced for a time to the level of those of cold-blooded animals; their contractility being retained almost as long as that of the latter;-thus confirming the general principle already stated, as to the relation between the amount of respiration, and the duration of the irritability.

361. The Muscles, as we have seen, are largely supplied with blood; and the flow of blood into them increases with the use that is made of them. The demand for nutrition is obviously augmented, in proportion to the activity of the exercise of the Muscular system; for the slightest observation suffices to show, that a much smaller amount of nourishment is sufficient to sustain the body in its normal condition, when the Muscular system is not actively exercised, than when it is in energetic operation. The quantity which is ample for an individual leading an inactive life, is far too little for the same person in the full exercise of his muscular power.-Again, there is evidence derived from observation of the relative amount of the solid matters excreted from the body under different circumstances (§ 731), that a waste or disintegration of the muscular tissue takes place, whenever it is actively employed; and this in a degree strictly proportional to the amount of force which it is called upon to exercise. In fact, it would appear that this waste is a necessary consequence of the exercise of the muscle; every act of contraction involving the death and decomposition of a certain amount of tissue. And as the presence of oxygen is always necessary for the decomposition of organic substances, so we do find that the penetration of the muscular tissue by oxygenated blood is essential to the manifes

tation of its contractile power.-Every act of contraction, then, may be said to involve the death of a certain amount of muscular tissue; and on the principles formerly laid down (CHAP. I., Sect. 3), we may look upon the development of contractile power as an expenditure of the vital force which that tissue previously possessed, and which ceases to exist as such, when the elements of the tissue enter into new combinations.

362. On the other hand, the muscular substance is repaired by an act of nutrition, at the expense of the fibrine supplied to it by the circulating fluid. There are certain muscles, as the heart, and the muscles of respiration, whose action is necessarily constant; and their reparation must take place as unceasingly as their waste. In these muscles no sense of fatigue is ever experienced. But in the muscles which are usually put in action by the will, this is not the case. Any prolonged exertion of them induces fatigue; and this fatigue is an evidence of their impaired condition, and of the necessity of rest to impart to them a renewal of vigor. The rest of muscles is essential to the recovery of their powers; and this recovery is due to the nutritive operations, which then take place unchecked, and which repair the losses previously sustained. The permanently-increased flow of blood to a muscle, which takes place when it is continually being called into vigorous action, is thus on the one hand occasioned by the demand for oxygenated blood created by its use, whilst on the other hand it tends to increase the power of the muscle by an augmentation of its nutrition. Hence it is, that, the more a muscle is exercised, the more vigorous and more bulky does it become. This is equally the case, whether the exercise of the muscle be voluntary or not. We see examples of it in the arms of the smith and in the legs of the opera-dancer; and we have a still more striking manifestation of it in those cases, in which an obstruction to the exit of urine through the urethra, has called for increased efforts on the part of the bladder, the continuance of which gives rise to an extraordinary augmentation in the thickness of its muscular coat.

363. Thus we see that the property of Irritability is a vital endowment peculiar to muscular tissue, and dependent for its existence upon due nutrition of that tissue; that it may be called into exercise by certain stimuli, applied either to the muscle itself, or to the nerve supplying it, provided that the muscle be also permeated with oxygen; that it may be exhausted by repeated stimulation, but is then recovered by rest, provided that there be no obstacle to the nutrition of the muscle; that the nutrition of the muscle is impaired by continued repose, and that its irritability diminishes in the same proportion; that the nutrition is increased by frequent use, and that the power of the muscle then augments in like degree; and finally, that the departure of muscular power, which ensues upon the general death of the system, is dependent in part upon the cessation of the supply of oxygen, and in part upon changes in the composition of the muscle itself, which are no longer compensated by the functions that keep it in its normal condition during life. The rapidity of these changes is the greatest in warmblooded animals, in which also the muscular irritability is most depen

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