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 
oceurring 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 
‘eases. 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 
-agiven period. Dr. Wollaston+ was of opi- 
* By the expression ‘ universal at one instant,’ 
T 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. 
| + Philos. Trans, 1811. 
ain 
MUSCULAR MOTION. 
527 
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 
coutractile 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 motu animalium,+ 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 fibrille 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. 
+ Propos. ii. 
