Arthur Sheridan Lea Sir Michael Foster.

A text book of physiology online

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muscular fibre itself is retained. Hence, in a muscle poisoned by
urari the contraction begins at that part of the muscular substance
which is first affected by the stimulus, and we may start a con-
traction in what part of the muscle we please by properly placing
the electrodes.

Some muscles, such for instance as the sartorius of the frog,
though of some length are composed of fibres which run parallel
to each other from one end of the muscle to the other. If such a
muscle be poisoned with urari so as to eliminate the action of the
ner\'es and stimulated at one end (an induction-shock sent through
a pair of electrodes placed at some little distance apart from each
other at the end of the muscle may be employed, but better
results are obtained if a mode of stimulation, of which we shall
have to speak presently, viz. the application of the " constant cur-
rent," be adopted), the contraction which ensues starts from the
end stimulated, and travels thence along the muscle. If two
levers be made to rest on, or be suspended from, two parts of such
a muscle placed horizontally, the parts being at a known distance
from each other and from the part stimulated, the progress of the
contraction may be studied

The movements of the levers indicate in this case the thicken-
ing of the fibres which is taking place at the parts on which
the levers rest or to which they are attached; and if we take
a graphic record of these movements, bringing the two levers to
mark, one immediately below the other, we shall find that the
lever nearer the part stimulated begins to move earlier, reaches its
maximum earlier, and returns to rest earlier than does the farther
lever. The contraction, started by the stimulus, in travelling along
the muscle from the part stimulated reaches the nearer lever some
little time before it reaches the farther lever, and has passed by
the nearer lever some Uttle time before it has passed by the
futher \erer ; and the farther apart the two levers are the greater
will be the diflTerence in time between their movements. In other
irords the contraction travels along the muscle in the form of a
wMve, each part of the muscle in succession from the end



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84 THE WAVE OF CONTRACTION. [Book i.

stimulated swelling out and shortening as the contraction reaches
it, and then returning to its original state. And what is true of
the collection of parallel fibres which we call the muscle is also
true of each fibre, for the swelling at any part of the muscle is
only the sum of the swelling of the individual fibres ; if we were
able to take a single long fibre and stimulate it at one end, we
should be able under the microscope to see a swelling or bulging
accompanied by a corresponding shortening, i.e. to see a con-
traction sweep along the fibre from end to end.

If in the graphic record of the two levers just mentioned
we count the number of vibrations of the tuning-fork which
intervene between the mark on the record which indicates the
beginning of the rise of the near lever (that is, the arrival of the
contraction wave at this lever) and the mark which indicates the
beginning of the rise of the far lever, this will give us the time
which it has taken the contraction wave to travel from the near to
the far lever. Let us suppose this to be 005 sec. Let us suppose
the distance between the two levers to be 15 mm. The con-
traction wave then has taken -005 sec. to travel 15 mm., that is
to say it has travelled at the rate of 3 meters per sec. And indeed
we find by this, or by other methods, that in the frog's muscles the
contraction wave does travel at a rate which may be put down as
from 3 to 4 meters a second, though it varies under different con-
ditions. In the warm blooded mammal the rate is somewhat
greater, and may probably be put down at 5 meters a second
in the excised muscle, rising possibly to 10 meters in a muscle
within the living body.

If again in the graphic record of the two levers we count, in
the case of either lever, the number of vibrations of the tuning-
fork which intervene between the mark where the lever begins to
rise and the mark where it has finished its fall and returned to the
base line, we can measure the time intervening between the
contraction wave reaching the lever, and leaving the lever on its
way onward, that is to say, we can measure the time which it has
taken the contraction wave to pass over the part of the muscle on
which the lever is resting. Let us suppose this time to be say
•1 sec. But a wave which is travelling at the rate of 3 m. a
second and takes 1 sec. to pass over any point must be 300 mm,
long. And indeed we find that in the frog the length of the
contraction wave may be put down as varying from 200 to
400 mm.; and in the mammal it is not very different.

Now the very longest muscular fibre is stated to be at most
only about 40 mm. in length ; hence, in an ordinary contraction,
during the greater part of the duration of the contraction the
whole length of the fibre will be occupied by the contraction
wave. Just at the beginning of the contraction there will be
a time when the front of the contraction wave has reached for



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Chap, n.] THE CONTRACTILE TISSUES. 85

instance only half-way down the fibre (supposing the stimulus
to be applied, as in the case we have been discussing, at one end
only), and just at the end of the contraction there will be a time
for instance when the contraction has left the half of the fibre
next to the stimulus, but has not yet cleared away from the other
half. But nearly all the rest of the time every part of the fibre
will be in some phase or other of contraction, though the parts
nearer the stimulus will be in more advanced phases than the
parts farther from the stimulus.

This is true when a muscle of parallel fibres is stimulated
artificially at one end of the muscles, and when therefore each
fibre is stimulated at one end. It is of course all the more true
when a muscle of ordinary construction is stimulated by means of
its nerve. The stimulus of the nervous impulse impinges, in this
case, on the muscle fibre at the end-plate which, as we have said,
is placed towards the middle of the fibre, and the contraction
wave travels from the end-plate in opposite directions toward
each end, and has accord'ngly only about half the length of the
fibre to run in. All the more therefore must the whole fibre be
in a state of contraction at the same time.

§ 54. We may now turn to the question, What takes place in
a muscular fibre when a contraction wave sweeps over it ?

Optical Changes, Although undoubtedly the optical features
of a muscular fibre change while it is contracting, it is very diffi-
cult to make an authoritative statement as to what those changes
are. In the first place a contraction wave, even when it is travel-
ling with relative slowness, travels so rapidly that the individual
features cannot be seized by the eye. We are confined to con-
clusions drawn from the study of short local contractions, local
thickenings and shortenings which may be obtained in the hving
fibre and fixed by the action of osmic acid vapour or by other
means ; and it has to be assumed that these local bulgings give a
true picture of a normal contraction wave by which, as we have
just seen, the whole length of a fibre is occupied at the same time.
la the second place the minute structure of a muscular fibre has
been and still is the subject of fierce disputa

If we adopt the view that the fibre is made up of dim
hands or discs of dim substance alternating with bright bands
or discs of bright substance, with transverse markings in the
middle of each bright band forming a line "intermediate" be-
tween the two adjacent dim bands, we may, according to some
observers, say that during a contraction there seems to be an
interchange between the dim and bright bands so that, in ordinary
l^t, atthe height of the contraction, in the broadest part of one
of the bulgings just spoken of, the previously obscure " interme-
date line " becomes a conspicuous dark band, the interval between
two f ocb changed intermediate hues becoming relatively and uni-



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86 CHEMISTRY OF MUSCLE. [Book i.

f ormly bright ; in other words there is a sort of reversal of the
situation, what was bright becoming, in its middle at least, dark,
and what was dim becoming relatively bright. When the fibre
is examined under polarized light, by which the dim bands are
shown to be largely composed of doubly refractive, anisotropic
material and the bright bands chiefly of singly refractive, isotropic
material, it is found that the above apparent reversal is not based
on any reversal of the refractive material, the anisotropic (dim)
band remains anisotropic, and the isotropic (bright) band remains
isotropic. But while both bands become broader (across the fibre)
and thinner (shorter along the length of the fibre), the anisotropic
band does not become so much thinner as does the isotropic band,
in other words the dim doubly refractive band or disc increases in
bulk at the expense of the bright singly refractive band. And
this accords with another feature of the fibre during contraction ;
namely, that the sarcolemma, which in the fibre at rest presents
a quite even line, is then indented at the middle of the bright
band at about the position of the intermediate line, and bulges
out opposite the dim band, that is opposite the enlarged aniso-
tropic disc.

It is useless, however, to dwell on these matters until the minute
structure of the fibre has been more clearly and satisfactorily made
out than it is at present. A contraction is obviously a transloca-
tion of molecules of the muscle substance and may, very roughly,
be compared to the movement by which a company, say of one hun-
dred soldiers ten ranks deep, with ten men in each rank, extends
out into a double line of two ranks with fifty men in each rank.
The movement of translocation is obviously, in striated muscle, a
very complicated one, but how the striation helps the movement
we do not at present really know. All we can say is that when
swift and rapid contraction is needed, the contractile tissue em-
ployed puts on in nearly all cases the striated structure.



The Chemistry of Muscle.

§ 55. We said, in the Introduction, that it was difi&cult to
make out with certainty the exact chemical differences between
dead and living substance. Muscle however in dying undergoes
a remarkable chemical change, which may be studied with com-
parative ease. We have already said that all muscles, within a
certain time after removal from the body, or, if still remaining part
of the body, within a certain time after ' general ' death of the
body, lose their irritability, and that the loss of irritability, which
leven when rapid, is gradual, is succeeded by an event which is
somewhat more sudden, viz. the entrance into the condition known
as rigor mortis. The occurrence of rigor mortis, or cadaveric rigid-



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Chap, n.] THE CONTRACTILE TISSUES. 87

ity, as it is sometimes called, which may be considered as the token
of the death of the muscle, is marked by the following features.
The Uving muscle possesses a certain translucency, the rigid muscle
is distinctly more opaque. The living muscle is very extensible
and elastic, it stretches readily and to a considerable extent when
a weight is hung upon it or when any traction is applied to it,
but speedily and, under normal circumstances, completely returns
to its original length when the weight or traction is removed ; as
we shall see however the rapidity and completeness of the return
depends on the condition of the muscle, a well-nourished, active
muscle regaining its normal length much more rapidly and com-
pletely than a tired and exhausted muscle. A dead, rigid muscle
is much less extensible and at the same time much less elastic ;
the muscle now requires considerable force to stretch it, and when
the force is removed, does not, as before, return to its former
length. To the touch the rigid muscle has lost much of its
former softness, and has become firmer and more resistant. The
entrance into rigor mortis is moreover accompanied by a shorten-
ing or contraction, which may, under certain circumstances, be
considerable. The energy of this contraction is not great, so that
any actual shortening is easily prevented by the presence of even
a slight opposing force.

Now the chemical features of the dead, rigid muscle are also
strikingly dififerent from those of the living muscle.

§ 56. If a dead mvscle, from which all fat, tendon, fascia, and

connective tissue have been as much as possible removed, and

which has been freed from blood by the injection of * normal' saline

solution, be minced and repeatedly washed with water, the washings

will contain certain forms of albumin and certain extractive bodies,

of which we shall speak directly. When the washing has been

continued until the wash-water gives no proteid reaction, a large

portion of muscle will still remain undissolved. If this be treated

with a 10 p. c. solution of a neutral salt, ammonium chloride being

the best, a large portion of it will become dissolved ; the solution

however is more or less imperfect and filters with difficulty. If the

filtrate be allowed to fall drop by drop into a large quantity of

distilled water, a white flocculent matter will be precipitated.

This floccalent precipitate is myosin. Myosin is a proteid, giving

the ordinary proteid reactions, and having the same general

elementary composition as other proteids. It is soluble in dilute

saline solutions, especially those of ammonium chloride, and may

he classed in the globulin family, though it is not so soluble as

panglobuUn, requiring a stronger solution of a neutral salt to

disBolve it; thus while soluble in a 5 or 10 p. c. solution of such a

mlL it is f^^ ^^^^ soluble in a 1 p. c. solution, which as we have

§eeD reBdily dissolves paraglobulin. From its solutions in neutral

Im solution it is precipitated by saturation with a neutral



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88 CHEMISTRY OF MUSCLE. [Book i.

salt, preferably sodium chloride, and may be purified by being
washed with a saturated solution, dissolved again in a weaker
solution, and reprecipitated by saturation. Dissolved in saline
solutions it readily coagulates when heated, i.e. is converted into
coagulated proteid, and it is worthy of notice that it coagulates
at a comparatively low temperature, viz. about 56° C. ; this it will
be remembered is the temperature at which fibrinogen is coagu-
lated, whereas paraglobulin, serum albumin, and many other pro-
teids do not coagulate until a higher temperature, 75° C, is reached.
Solutions of myosin are precipitated by alcohol, and the precipitate,
as in the case of other proteids, becomes by continued action of the
ftlcohol altered into coagulated insoluble proteid.

We have seen that paraglobulin, and indeed any member of
the globulin group, is very readily changed by the action of dilute
acids into a body called acid albumin, characterised by not being
soluble either in water or in dilute saline solutions but readily
soluble in dilute acids and alkalis, from its solutions in either of
which it is precipitated by neutralisation, and by the fact that the
solutions in dilute acids and alkalis are not coagulated by heat.
When therefore a globulin is dissolved in dilute acid, what takes
place is not a mere solution but a chemical change ; the globulin
cannot be got back from the solution, it has been changed into
acid-albumin. Similarly when globulin is dissolved in dilute
alkalis it is changed into alkali albumin ; and broadly speaking
alkali albumin precipitated by neutralisation can be changed by
solution with dilute acids into acid albumin, and acid albumin by
dilute alkalis into alkali albumin.

Now myosin is similarly, and even more readily than is
globulin, converted into acid albumin, and by treating a muscle
either washed or not, directly with dilute hydrochloric acid, the
myosin may be converted into acid albumin and dissolved out
Acid albumin obtained by dissolving muscle in dilute acid used to
be called si/ntonin, and it used to be said that a muscle contained
syntonin ; the muscle however contains myosin, not syntonin, but
it may be useful to retain the word syntonin to denote acid albumin
obtained by the action of dilute acid on myosin. By the action
of dilute alkalis, myosin may similarly be converted into alkali
albumin.

From what has been stated above it is obvious that myosin has
many analogies with fibrin, and we have yet to mention other
striking analogies ; it is however much more soluble than fibrin,
and speaking generally it may be said to be intermediate in its
character between fibrin and globulin. On keeping, and especially
on drying, its solubility is much diminished.

Of the substances which are left in washed muscle, from which
all the myosin has been extracted by ammonium chloride solution,
little is known. If washed muscle be treated directly with dilute



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;



Chap, n.] THE CONTRACTILE TISSUES. 89

hydrochloric acid, a large part of the material of the muscle passes,
as we have said, at once into syntonin. The quantity of syntonin
thus obtained may be taken as roughly representing the quantity
of myosin previously existing in the muscle. A more prolonged
action of the acid may dissolve out other proteids, besides myosin,
left after the w^ashing. The portion insoluble in dilute hydro-
chloric acid consists in part of the gelatine yielding and other
substances of the sarcolemma and of the connective and other
tissues bet^ween the bundles, of the nuclei of these tissues and of
the fibres themselves, and in part, possibly, of some portions of
the muscle substance itself. We are not however at present in a
position to make any very definite statement as to the relation of
the myosin to the structural features of muscle. Since the dim
bands are rendered very indistinct by the action of 10 p.c. sodium
chloride solution, we may perhaps infer that myosin enters largely
into the composition of the dim bands, and therefore of the fibrillse;
but it would be hazardous to say much more than this.

§ 57. Living muscle may be frozen, and yet, after certain
precautions will, on being thawed, regain its irritability, or at all
events will for a time be found to be still living in the sense that
it has not yet passed into rigor mortis. We may therefore take
living muscle which has been frozen as still living.

If living contractile mtcscle, freed as much as possible from
blood, be frozen, and while frozen, minced, and rubbed up in a
mortar with four times its weight of snow containing 1 p.c. of
sodium chloride, a mixture is obtained which at a temperature
just below 0** C. is sufl&ciently fluid to be filtered, though with
difficulty. The slightly opalescent filtrate, or muscle plasma as it
18 called, is at first quite fluid, but will when exposed to the
ordinary temperature become a solid jelly, and afterwards separate
into a elot and serum. It vrill in fact clot like blood plasma, with
this difference, that the clot is not firm and fibrillar, but loose,

Snular, and flocculent. During the clotting the fluid, which
ore was neutral or slightly alkaline, becomes distinctly acid.
The clot is myosin. It gives all the reactions of myosin obtained
from dead muscle.

The serum contains an albumin very similar to, if not identical
with, serum albumin, a globulin differing somewhat from, and
ooAj^Iating at a lower temperature than paraglobulin, and which
to distinguish it from the globulin of blood has been called myo-
gtobnlin, some other proteids which need not be described here,
and varioas ' extractives ' of which we shall speak directly. Such
mnacles bb are red also contain a small quantity of haemoglobin
and poesibly, another allied red pigment

Thus while dead muscle contains myosin, albumin, and othet
PfoCetds extractives, and certain insoluble matters, together with
iebdoons Bnd other substances not referable to the musclo



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90 RIGOR MORTIS. [Book i.

substance itself, living muscle contains no myosin, but some
substance or substanc3S which bear somewhat the same relation to
myosin that the antecedents of fibrin do to fibrin, and which give
rise to myosin upon the death of the muscle. There are indeed
reasons for thinking that the myosin arises from the conversion of
a previously existing body, which may be called myosinogen, and
that the conversion takes place, or may take place, by the action
of a special ferment, the conversion . of myosinogen into myosin
b3ing very analogous to the conversion of fibrinogen into fibrin.

We may in fact speak of rigor mortis as characterised by a
clotting of the muscle plasma, comparable to the clotting of blood
plasma, but differing from it inasmuch as the product is not fibrin
but myosin. The rigidity, the loss of suppleness, and the dimin-
ished translucency appear to be at all events largely, though
probably not wholly, due to the change from the fluid plasma to the
solid myosin. We might compare a living muscle to a number of
fine transparent membranous tubes containing blood plasma. When
this blood plasma entered into the * jelly' stage of clotting, the
system of tubes would present many of the phenomena of rigor
mortis. They would lose much of their suppleness and translucency,
and acquire a certain amount of rigidity.

§ 58. There is however one very marked and important
difference between the rigor mortis of muscle and the clotting
of blood. Blood during its clotting undergoes a slight change
only in its reaction ; but muscle during the onset of rigor mortis
becomes distinctly acid.

A living muscle at rest is in reaction neutral, or, possibly from
some remains of lymph adhering to it, faintly alkaline. If on the
othar hand the reaction of a thoroughly rigid muscle be tested, it
will be found to be most distinctly acid. This development of an
acid reaction is witnessed not only in the solid untouched fibre but
also in expressed muscle plasma ; it seems to be associated in some
way with the appearance of the myosin.

The exact causation of this acid reaction has not at present
been clearly worked out. Since the coloration of the litmus pro-
duced is permanent, carbonic acid, which as we shall immediately
state, is set free at the same time, cannot be regarded as the active
acid, for the reddening of litmus produced by carbonic acid speedily
disappears on exposure. On the other hand, it is possible to ex-
tract from rigid muscle a certain quantity of lactic acid, or rather
of a variety of lactic acid known as sarcolactic acid ^ ; and we may
probably regard the acid reiction of rigid muscle as due to a new
formation or to an increased formation of this sarcolactic acid.
There is reason however to think that the establishment of the

1 There are many varieHes of Inctic acid, which are isomeric, having the same
composition OgHoOs, but difier in tlieir reactions and especially in the solubility of
their sine salts. The variety present in muscle is distinguisiied as sarcolactic add.



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Chap, n.] THE CONTRACTILE TISSUES. 91

acid reaction is not a perfectly simple process but a more or less
complex one, other substances besides sarcolactic acid intervening.
Coincident with the appearance of this acid reaction, though
as we have said, not the direct cause of it, a large development of
carbonic acid takes place when muscle becomes rigid. Irritable
living muscular substance like all living substance is continually
respiring, that is to say, is continually consuming oxygen and
gixing out carbonic acid. In the body, the arterial blood going to
the muscle gives up some of its oxygen, and gains a quantity of
carbonic acid, thus becoming venous as it passes through the
muscle capillaries. Even after removal from the body, the living
muscle continues to take up from the surrounding atmosphere a
certain quantity of oxygen and to give out a certain quantity of
carbonic acid^

At the onset of rigor mortis there is a very large and sudden
increase in this production of carbonic acid, in fact an outburst as it
were of that gas. This is a phenomenon deserving special attention.
Knowing that the carbonic acid which is the outcome of the
respiration of the whole body is the result of the oxidation of car-
bon-holding substances, we might very naturally suppose that the



Online LibraryArthur Sheridan Lea Sir Michael FosterA text book of physiology → online text (page 13 of 148)