Thus it would appear that active contraction consists in a disturbance of that state of equili brium ordinarily existing in muscles when at rest ; that their different portions successively undergo momentary contractions, and that there is always a considerable part of each fibre un contracted. This will account for the remark able fact that detached fragments of the volun tary fibre will contract by two-thirds of their length, though an entire muscle, in its natural situation, cannot shorten by more than one third. This great capacity of contraction in the tissue would be without a purpose, if it were not that it only admits of momentary exertion, and therefore requires that in the organ succes sive parts should take up the act, and by so doing, render it, as a whole, continuous. In an active fibre the contracting parts are continually dragging on those in which the contractile force has just subsided, and which intervene between them and the extremities of the fibre. These are thereby instantly stretched, and come to serve the temporary purpose of a tendon ; but one which resists extension more by its passive contractility than by its mere tenacity. It is these parts which in tetanic spasm suffer lace ration ; which happens in consequence of the contraction excited by the vis nervosa, being then too powerful to be resisted by the passive contractility.
The preceding account of the minute changes occurring during contraction rests on data fur nished by the striped form of muscular fibre ; but there is nothing contained in it, which seems at variance with the little that is posi tively known regarding the contractions of the other form. The differences between the con tractions of the two varieties are almost cer tainly confined to the manner of exercise, and do not extend to the essential nature of the act. Though the unstriped fibre has not been stu died by the microscope during its active state, with the same success as the other, yet the similarity of the gross changes observed in it by the naked eye, to those seen in voluntary muscle, forbid us to doubt the identity of the phenomenon, in all that essentially constitutes it an act of contraction.
From the knowledge we possess, we are per haps entitled to hazard some further conjec tures respecting the differences in the mode of exercise of the contractile power in different cases. In whatever that mysterious power may consist, it would appear that the structural modifications of the two kinds of fibres are intimately connected with the manner in which it is capable of being exerted. Wherever the striped structure occurs, we witness an apti tude for quick, energetic, and rapidly repeated movements, while, where it is deficient, they are sluggish, progressive, and more sustained. The varieties in the character of contractions performed by striped muscles are very strik ing, especially that of the heart, as compared with the prolonged action of the voluntary muscles. In both there is an alternate mo mentary action and repose of every contractile particle, but in the heart the contraction is universal at one instant, and the repose equally universal at the next, while, in the prolonged action of the voluntary muscles, contractions of certain parts of each fibre always co-exist with repose of other parts.* The contractions of voluntary muscles differ greatly from one another in duration, energy, and extent. Nothing is more wonderful, if it be well considered, than the power the will possesses of regulating the amount of stimulus which it is able to give to the muscles, and that of transmitting it with uniformity during a given period. Dr. Wollaston t was of opi nion, that the phenomenon of the muscular sound affords a proof that the duration of a muscle's contraction depends on the application to it of a succession of distinct impulses; and this idea, according very nearly, as it does, with the later evidence of observation, appears, on the whole, the most satisfactory that has been ad vanced on this abstruse subject. lie also thought that the intensity of a contraction cor responds with the rapidity with which these impulses are transmitted to it, and this like wise may be, in part, true. But there is, in addition to this, in all probability, a difference in the intensity of the stimulus itself in dif ferent cases, producing a difference in the size of each wave, a difference in the amount of contractile energy exerted in each, and a dif ference in the rapidity with which the waves oscillate along the fibre. The extent of the
contraction (the duration and intensity being the same) will manifestly depend on the amount of the length of the fibre which is contracted at once. But we are ignorant whether this variation in amount is effected by a variety in the number of waves, or in the extent of the fibre engaged by each of them.
The ancients appear to have been quite ig norant of the nature of muscles. Plato and Aristotle attributed to them so trivial an use, as to think that, like fat, or a kind of clothing, they kept out heat in summer and cold in winter.* The nerves and tendons were con founded with the muscles, as they commonly are at this day, by the vulgar. Borelli, in his elaborate work, De mote animali,m,t thinks it requisite (in 1734) to adduce arguments against the doctrine that muscle and flesh are different, the former composed of an ag gregation of tendinous fibres, the latter a cer tain villous substance incrusted by the blood upon their exterior, a fact showing the ex tremely loose notions that prevailed on this subject even up to a comparatively recent pe riod. The fibres so obviously composing the essential part of muscle have been the subject of the most extraordinary speculations, pro bably ever since it was discovered that they were endowed with contractility, the property which, on a superficial aspect, seemed the most closely associated with life. And it is by no means surprising, that when the micro scope began to open a new world to view, it was applied with ardour to the investigation of this tissue. It is not easy to appreciate justly the accounts given of it by some of the earlier micrographers, in consequence of the indeter minate meaning of many of the terms they employed, and the imperfection of the means at their disposal for accurate definition and measurement of the objects they describe. Robert Hooke, however, had probably a cor rect general knowledge of the elementary fibres of voluntary muscle, and possibly even saw the fibrillae into which they often split; for we find him in 1678 speaking of the " fibres which seemed like a necklace of pearl in the micro scope." Previous to him no author appears to have examined them. But Leenwetillock,-1 his friend and correspondent, makes continual mention of his examinations of the muscular fibre of various animals. This acute and en thusiastic observer clearly recognized the im portant fact, that each elementary fibre is a perfect and separate organ in itself; he was astonished to find that in all animals, the largest as well as the smallest, these fibres are excessively minute ; he discovered the manner in which they are aggregated, and invested by areolar tissue ; and by boiling and drying a muscle and then making tranverse sections of it, he ascertained those of voluntary muscle to be polygonal and solid Ile described the cross lines, which he conceived to be on the surface only and to be the coils of a spiral thread. To this structure he attributed the active power of the fibre, comparing it to an elastic coil of wire. lie further saw the longitudinal lines visible on the elementary fibre, and considered them to be an evidence of a still minuter com position by fibrillae. All these points are well illustrated by figures, which leave no doubt of his meaning; but, as his results are scattered through a great number of letters, much of what he accomplished seems to have been over looked by later writers. Leeuwenhoek con cluded that in contraction the cross markings approximate, but I cannot discover that he speaks of having seen this. lie confounded the cross markings seen on tendon with those of muscle, and fell into the prevalent error of attributing contractility to the tendons. Mal pighi incidentally mentions the minute structure of muscle in only one passage of his works.! Ile appears to have seen the transverse stripes of the elementary fibre,and tohave alsolikened them to those of tendon. Contemporary with Leeuwen hoek was de Ileide,§ who, in 1698, published some observations on muscular fibre, describ ing and figuring the transverse markings. In 1741, Muys,11 in a voluminous work, with good plates, gave all that was previously known, and added many observations of his own. His book, however, is learned rather than profound. Ile separates the elementary fibres into the simple and reticulated, and seems to have con sidered the stripes to be the effect either of mi nute zigzags during contraction or of a spiral form of the fibrilla.