MUSCULAR MOTION. 



527 



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 f was of opi- 



* By the expression ' universal at one instant,' 

 I do not mean absolutely so, for observation and the 

 presence of the muscular sound both declare that 

 the contraction, even of the heart, though so ap- 

 parently momentary, is progressive. 



t Philos. Trans. 1811. 



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. He 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 mutn animaliumfi thinks 

 it requisite (in 1731) 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, arid 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- 



* Vesalius, vol. i. p. 182. 

 t Propos. ii. 



