CHAP, vii.] VARIETIES OF CONTRACTION. 185 



the other form. The differences between the contractions of the 

 two varieties are almost certainly confined to the manner of exer- 

 cise, and do not extend to the essential nature of the act. Though 

 the unstriped fibre has not been studied by the microscope,, during 

 its active state, with the same success as the other, yet the simila- 

 rity 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 is essential to it as an act of contraction. 



From the knowledge we possess, we are perhaps entitled to 

 hazard some further conjectures 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. 

 Whenever the striated structure occur, we witness an aptitude 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 

 striated muscles are very striking, especally that of the heart, as 

 compared with the prolonged action of the voluntary muscles. In 

 both, there is an alternate momentary 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. Dr. Wollastonf was of 

 opinion, that the phenomenon of the muscular sound affords a proof 

 that the duration of a muscle's contraction depends on the applica- 

 tion to it of a succession of distinct impulses ; and this idea, accord- 

 ing very nearly, as it does, with the latter evidence of observation, 

 appears, on the whole, the most satisfactory that has been advanced 

 on this abstruse subject. He also thought that the intensity of a 

 contraction corresponds with the rapidity with which these impulses 

 arc transmitted to it ; and this likewise 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 different cases, producing a 



By the expression " universal at one instant," we 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 apparently momentary, is progressive. 

 t Philos. Trans. 1811. 



