CHAPTER X

MUSCLE

Living matter, protoplasm, is irritable. It responds to influences

impressed upon it by its environment. An effective influence, termed

a “stimulus,” produces a change in protoplasm at the spot at which

it acts. From this spot the change spreads outwards as an “impulse.”

Protoplasm is said to “conduct.” A stimulus may be likened to a

blow given to a fixed but elastic mass; an impulse to the vibration

which travels outwards from the spot struck. Unfortunately, the term

“stimulus” is used both for the stick that strikes, the stimulator,

or stimulant, and for the blow that is struck; but breaches of logic

seldom lead to confusion in an experimental science. The context

indicates the particular application of the term. The manifestation of

stimulation is a physical or chemical change—most obvious when it is

one of form. This change of form may occur at the spot stimulated, or

may be deferred to a distant part to which the impulse is conducted.

In opening the study of muscle and nerve we need to form a conception

of the nature of these three functions—irritability, conductivity, and

changeableness of form. Not that the functions are as distinct as the

ideas to which the three terms give rise. They are three aspects of a

common function; although this is a reflection which will carry more

weight when the ways in which protoplasm reacts to external forces have

been considered.

A stimulus may be mechanical, something in the nature of a blow which

displaces the particles of protoplasm; or it may be chemical or

thermal, disintegrating a portion of its substance; or electrical,

divorcing the ions of its molecules. Only the last in any way

resembles a natural stimulus; since electrical stimulation alone can

be repeated without the substance stimulated showing any evidence of

injury in the process. Mechanical, thermal, chemical stimuli destroy

a portion of the protoplasm upon which they act. Yet even the weakest

of electric currents is a gross disturbance as compared with natural

stimuli, such as touch, warmth, sound, light. The essential and most

distinguishing quality of living matter is its return to its original

state immediately after stimulation. It does not even wait until the

stimulator has ceased to act. An effective influence is a sudden change

in the environment. It is answered by a sudden response, followed by a

return of protoplasm to the state in which it was before the impact of

the external force. The change progresses through the protoplasm as a

transitory alteration of state, the particles concerned in conducting

it returning to their original condition the moment it has passed.

No non-living matter responds to force in this way. If a stone is

dropped into a pond, a wave circles outwards from the spot it strikes;

but this is a wave of displacement, not a change of state. Suppose

the pond contained a solution of sugar which the impact of the stone

changed into vinegar, and that the zone of vinegar spread outwards,

the liquor returning to the condition of sugar and water as it passed.

Here we should see some analogy to the progress of an impulse. But no

non-living matter behaves like this. A product of the laboratory may

be so unstable as to explode when shaken, passing on the slightest

provocation into a more stable state. It does not return after the

explosion to its previous strained condition. Having thrown away its

energy, it continues on a lower plane. Protoplasm parts with energy

to recover it again. It returns to instability after assuming a more

stable form.

If we are to form a conception of the cause of the irritability of

living matter, we must have a mental picture of the physical conditions

which distinguish life from death. All matter is in a state of motion.

It consists of separate molecules, each moving in its orbit with vast

rapidity. A molecule is a cluster of atoms. The dimensions of its

orbit depend upon the number and weight of the atoms in its cluster.

If we could watch the dance of the molecules of proteins and other

substances into which protoplasm breaks up on dying, we should see

each separate cluster executing the figure appropriate to its mass,

indifferent to the movements of neighbouring groups. But if living

protoplasm were of the company, the scene would be one of vastly

greater animation; for now it is the ambition of our dancers to form

a single group. To this they can never attain. There is a physical

limit to the number of dancers who can hold together while the music

carries them in wide sweeps backwards and forwards across the floor.

At every gust of wind which bursts through an open doorway a group

breaks, to clasp hands again as the wind subsides. Protoplasm is

always on the verge of instability; always snatching at additional

atoms which it draws within its ring; always shaking off other groups

of atoms because the ring is too large to hold together. Touch it,

and it falls into simpler combinations. Kill it, and it becomes a

mixture of organic and inorganic compounds which we know and can

name. But as long as it is alive—as long as it is protoplasm, that

is to say—integration and disintegration are occurring. Simultaneous

complication and simplification _is_ life. The protoplasm-molecule, if

we dare to think of it as a molecule, in the sense in which a chemist

uses the term, is always changing. It is its variability which makes

stimulation possible. Irritability is a tendency to dissociation under

the influence of an external force, with reassociation when the force

ceases to act.

The molecules which protoplasm gathers into itself may be classified

under the headings oxygen, foods, water, and inorganic salts. It is the

two latter which most affect its state, conferring upon it the capacity

of exhibiting the phenomena of life. Water and the ions of salts

dissolved in water, electrolytes, are linked to the other elements of

its groups. Striving to find room for more molecules of water and more

ions, protoplasm expands. It becomes more mobile and more irritable;

for irritability and mobility vary as the number of these extraneous

groups of atoms which protoplasm is in a position to let drop. As

an impulse travels through it they lose their hold, recovering it

as they pass the impulse on. This progress towards expansion is the

lifeward tendency; the quickening of activity which leads also to the

incorporation of additional atoms of nitrogen-containing substances,

and consequent growth.

The opposite tendency is deathwards. Protoplasm drops extraneous groups

of atoms; retires into itself; loses irritability; settles down to rest.

The molecules of proteins exhibit a property which appears to pertain

in some degree to living matter also. When their relation to the water

in which they are dissolved and the electrolytes which it contains is

disturbed, they appear to go out of solution, they coagulate. This

disturbance is brought about in all proteins by heat; in some it is

the result of altering the amount of salt in the water in which they

are dissolved. Coagulation in protoplasm is the prelude to death; but

it would appear that a step in this downward path is taken whenever

an impulse is conducted. Coagulation is due to the clustering of the

molecules of a protein. When protoplasm drops electrolytes and water

its molecules cluster in some degree, regaining their independence and

reattaching their accessory groups of atoms as the cause which drove

them to make for safety passes by.

Our conception—not of life, but of “the physical basis of life,” may

be very wide of the mark. The account given above is intended as little

more than a hint of the lines along which thought is travelling at the

present time. The reader must not regard it as a serious attempt to

present in detail the views of any of the workers who are endeavouring

to apply the results of recent discoveries in molecular physics to the

solution of problems in the chemistry of living matter. There can,

however, be little doubt but that we are on the eve of further advances

which will secure data upon which it will be legitimate to construct

hypotheses. At present it would be unreasonable to do more than

indicate the direction from which it may be hoped that light will shine.

A stimulus is a change of circumstance rather than a transient

disturbance. When an electric current is thrown into it, protoplasm

dissociates—parts with something. It instantly reassociates. The

continued passage of the electric current does not maintain it in a

dissociated condition. When the current is cut off, the sudden change

again acts as a stimulus. Within limits, the efficiency of an electric

stimulus varies as its suddenness. Similarly with all other stimuli to

which protoplasm responds: crushing, burning, chemical decomposition,

are effective at the moment of their occurrence. When they generate a

succession of responses, it is because they continue to produce changes

in the protoplasm. Their continued action does not, under ordinary

circumstances, prolong the response.

Response to stimulation travels as an _impulse_ through protoplasm.

An impulse is commonly likened to a wave, but enough has been said

already to prove that the simile is misleading. It is not of the same

nature as the wave which a stone starts on the surface of a pond, a

pulsation of sound through air or water, an undulation of light or heat

in the æther. These various kinds of waves are waves of displacement,

a swing first to one side and then to the other. An impulse traverses

protoplasm, whether it be the apparently diffuse protoplasm of a

leucocyte or the severely oriented protoplasm of a nerve or muscle, as

a change which may be described as chemical, with reservations as to

the meaning allowed to this term. We may without impropriety represent

the fall (dissociation) and subsequent rise (association) graphically

as a wave; but even then it is but a half-wave, and inverted. It is

a very different thing to the onward progression of an accession of

force, with which it is not infrequently confused.

All protoplasm is not equally susceptible of stimulation. Probably it

is safer to put this in a different form. Protoplasm is not everywhere

equally exposed to stimulation, nor is it when especially exposed

to stimulation in one way equally accessible to all other effective

forces. A sense-organ is a collection of cells in which protoplasm is

so disposed as to be susceptible to a certain kind of stimulus. It is

a “receptor” for a particular force. At the same time it is essential

to its efficiency that it should be insusceptible to other forces. The

protoplasm in certain of the sense-organs of the skin dissociates when

compressed, in others when warmed. The cells of these receptors have

a certain structure which exposes their protoplasm in such a manner

that it cannot escape dissociation when, in the one case, the cells are

squeezed, or when, in the other case, they are heated. The ear contains

sensory cells so constructed that the protoplasm which they contain

dissociates when affected by pulsations of sound. In the receptors of

the tongue and the nose protoplasm is exposed to the influence of

chemical stimuli; in the eye it is exposed to the dissociating action

of light.

Protoplasm is responsive to external force. It conducts the impulses

to which stimulation gives rise. Eventually the impulses, which

travel along strands of tissue highly specialized for the purpose

of conduction—nerves—reach collections of protoplasm which are so

disposed that when they dissociate energy is set free. A comprehensive

term is much needed for the connotation of this third essential

property of protoplasm, the capacity of liberating energy which

characterizes “effectors.” An external force, so small in intensity

as to be negligible when we are dealing with the body’s accounts,

acts upon the protoplasm of a receptor. A change in state results.

The change is conducted to an energy-liberating organ. This organ is

supplied with blood which brings it food. Food is its store of energy,

the raw material from which it manufactures its ammunition. When an

impulse reaches an energy-liberating organ its protoplasm dissociates.

But here the protoplasm is so disposed—the cells which contain it have

such a form—that when it dissociates a change in the cell follows;

it alters in shape, or it discharges into its environment heat, or

electricity, or light. The dissociation and reassociation of the

protoplasm of an effector involves chemical change. Molecules of water

and of carbonic acid are cast off. The energy sacrificed in letting

matter fall into these very stable forms is the energy made visible, as

it were, in lifting a weight or dispersing heat. It must be replaced if

the organ is to retain its power of acting when next an impulse reaches

it. To replace it, protoplasm takes up food and oxygen from the blood.

The liberation of energy which occurs when a muscle contracts is not

a special phenomenon—something which does not occur when the muscle

is at rest. It is an intensification of a process which is always

taking place. The substance of muscle, like that of nerve and every

other tissue, is always combining with oxygen and giving off water and

carbonic acid. When we are auditing the body’s accounts, we enter so

and so much food and oxygen on the debit side, we credit it with the

same weight of water and carbonic acid; or we debit it with the energy

potential in the food, and enter to its credit the mechanical work done

and the heat set free by the oxidation of this food. Food is the petrol

the combustion of which causes the movement of the car. The external

force which stimulates a receptor is too insignificant in amount to be

carried to account. Physiologists neglect it, just as engineers neglect

the energy liberated by the sparking-plug which ignites the petrol,

when they are estimating the efficiency of a motor.

Compared with the amount of energy actually received from the

environment when a sensory cell of the eye or ear is excited, the

energy needed to start an artificial impulse in a nerve is relatively

enormous; yet a well-known comparison of the energy conveyed to the

nerve in a certain experiment with a nerve-muscle preparation from a

frog, and the energy expended by the muscle in contracting, brings home

to our minds the fact that it is impossible to carry even this item to

account. The energy furnished to the nerve from an electric condenser

measured 0·001 erg; the energy expended by the muscle reached 100,000

ergs.

It is easy to determine the amount of mechanical work which results

from a given expenditure of energy. By alternately flexing and

extending the joints of his legs, a man lifts his own weight up a

hill of a certain height. The work can be measured in foot-pounds or

in kilogrammetres. But this by no means accounts for all the energy

potential in his food. A still larger amount is expended for the

purpose of keeping the body warm, or, not improbably, making it too

warm; in either case generating heat which is dissipated into the

atmosphere. When a machine is being planned, attention is concentrated

upon the problem of how to get the largest result in work for a given

quantity of fuel. Fuel costs money. All energy dissipated as heat

is wasted. Every ounce saved makes for economy. Engineers therefore

speak of the “efficiency” of an engine as the relation between the

work actually done and the work which would have been done if no

energy had been wasted. In the best steam-engines it stands at about

1 to 10. Since the chief function of muscle is to do mechanical work,

physiologists are apt to adopt the engineer’s point of view. But in the

case of muscle this is justifiable only in a limited degree. The body

of a warm-blooded animal is maintained at a temperature higher than

that of the surrounding air. Muscles are the chief producers of heat.

If they turned all the energy which they receive into work, they would

be inefficient as regards this very important function. Yet even from

the engineer’s point of view muscles are more efficient than the best

of engines.

It is almost impossible to determine with accuracy, in regard to

isolated muscles, the amount of food taken up from the blood, and the

return in work by the muscles of the energy potential in the food.

Calculations have to be based upon observations of food consumed, gain

or loss of body-weight, work done by a man or an animal during a period

lasting for several days. We shall consider the evidence obtained

in this way in a subsequent section (p. 149). But whether we study

isolated muscles or the body as a whole, the relation between work and

heat varies within wide limits. So wide, indeed, are the variations as

to justify the conclusion that there is no necessary relation between

the two phenomena. Muscles develop heat when they are quiescent.

Activity is accompanied with an increased evolution of heat; but, if it

be desirable, the evolution of heat is reduced until it is, relatively

to the output of work, much smaller than in the case of any engine

which has yet been made. It is sufficient in this connection to state

that, under certain conditions, the return in work may amount to about

one-half. The comparison with an artificial motor, of whatever kind,

breaks down. In an engine combustion develops heat, heat causes steam

or gas to expand, the expanding gas pushes a piston. In muscle certain

of the carbon, hydrogen, and oxygen atoms contained in protoplasm

combine to form water and carbonic acid—compounds too stable to be

reassociated with the remaining atoms of the protoplasm-molecule.

They are replaced by complex, energy-yielding substances—foods—and

by oxygen, carried in the blood. Their displacement brings about a

change in the form of the molecules which involves, owing to their

peculiar orientation, a change in shape of the muscle as a whole.

Such an explanation is, perhaps, more exact than our knowledge at

present warrants; or rather let us say, since we do not know what the

expression “the form of a molecule” means, it has an appearance of an

exactitude which does not characterize it. It is merely intended to

help the reader to realize the hopelessness of attempting to compare

muscle with any mechanical contrivance. In the boiler of a steam-engine

heat is applied to water until its molecules cannot remain in so

close a state of aggregation. Their orbits are greatly increased. The

cause of the thrust given to the piston of an engine is the increased

amplitude of movement of the molecules of steam behind it. In a

combustion engine a mixture of petrol and air is ignited. Energy is

set free by the resolution of unstable petrol into stable water and

carbonic acid. This energy heats the gases, causing them to expand.

Waste of energy as heat is inevitable in a machine which depends for

its motive-power upon the translation of molecules. The source of

muscular force (if it be not intramolecular change) is certainly not,

directly, increased amplitude of molecular swing.

But we must not conclude, as we are tempted to do, that muscle is

capable of liberating as mechanical work the whole of the energy

supplied to it in food, seeing that its activity is always accompanied

by evolution of more heat than can be attributed to friction. If

the bulb of a thermometer be inserted into a group of muscles, the

instrument shows a marked rise of temperature when the muscles

contract. Even though the temperature of the chamber in which an animal

is placed is equal to its own, the animal makes more heat if compelled

to work, notwithstanding the fact that the consequent rise of its body

temperature may prove fatal.

Nor can muscles dispense unlimited heat without doing mechanical work.

If I am too cold, the obvious means of getting warm is jumping about.

There appears to be a level of heat-production which cannot be exceeded

without movement. When more heat is called for than quiescent muscles

can produce, they exhibit flickering contractions, shivering, without

moving the limbs. The signal for increased production is given by

the skin. The skin is sensible of the amount of heat which is being

lost. Exposure to cold air makes one shiver, by suddenly withdrawing

heat. But an increase of temperature in the blood behind the skin

has an exactly similar effect. In the first stage of fever, when the

temperature of the body has risen two or three degrees, and before

the system has become accustomed to this state of affairs, the skin

announces to the muscles that heat is being rapidly lost. A severe

shiver, termed a “rigor,” is the result. At the same time loss of

heat by evaporation is checked, just as it is when the skin is cold.

The sweat-glands are rendered inactive. A phenomenon which marks the

nightly fall of temperature in consumptive patients is the sudden

return of activity in these glands.

Muscle when most highly developed has an extraordinarily definite

structure. It is minutely subdivided into units which appear, looked

at separately, simple in design. We are tempted to believe that the

explanation of the way in which each of these units works is not far to

seek. It is disappointing to be obliged to admit that, notwithstanding

all the thought which has been devoted to the problem, we are as far

as ever from a definitive solution. We understand the principles on

which steam-engines, combustion-engines, electric motors are planned.

We compare muscle with each of these mechanical contrivances in

turn, expecting to discover the principle of its construction. Many

ingenious hypotheses have been formulated; but the fact that some of

these are mutually destructive shows clearly enough that as yet no

approach to certainty has been made. Probably the fundamental error

lies in attempting to compare muscle with a mechanical contrivance. The

apparent simplicity and regularity of structure of “striped muscle”

misleads us. We ought to have commenced our investigations at the other

end of the scale of mobile tissue—to have begun with semifluid and

apparently homogeneous animal matter, working upwards to the tissue

which, being limited to the one function of movement, and movement in

one direction only, has, as it were, crystallized along the lines of

force.

All protoplasm is mobile. Its particles move one on another. Hence

follows either circulation of the living matter within the cell or

change in shape of the cell. The two phenomena are identical in nature.

Circulation is best studied in a large-celled, transparent part of a

plant. A filamentous water-weed is suitable for the purpose. If this

be examined with a microscope while still alive, its cells are seen to

contain a watery juice enclosed in spaces of denser cell-substance.

Bridges of cell-substance span the spaces. The particles of which

these bridges consist are in a state of constant streaming motion,

which has, it is needless to say, no effect upon the shape of the cell

(_cf._ p. 9).

The unicellular animal amœba, leucocytes, and certain spores of plants,

are devoid of cell-wall (_cf._ p. 28). Their soft protoplasm is not

limited by a rigid case. When it streams, the form of the cell is

changed. True, we must not think of the body-substance of an amœba as

homogeneous. It exhibits an internal structure. Yet its architecture

is not, so far as we can see, sufficiently fixed to restrict the

directions in which it can stream. Any change of shape is possible.

We cannot find in Nature an isolated clump of living protoplasm; nor

do we suppose that, if we found it, it would prove to be homogeneous.

It appears to be necessary that protoplasm and metaplasm—the terms

have no chemical significance; “primary” and “secondary,” or “chief”

and “subsidiary” would be equally distinctive—should be intermixed.

Streaming is apparently due to alterations in the surface relations of

the two substances.

In multicellular animals certain elongated cells are arranged in

groups, with their long axes all pointing in the same direction. They

can change in shape, diminishing in length, with equivalent increase in

breadth. Since all the cells of a group undergo this change of form at

the same time, the result is an alteration in the shape of the animal

of which they are a part. Applying the experience which we have gained

in studying the movements of unicellular organisms, we conclude that

these elongated cells are composed of two substances—protoplasm and

metaplasm. The restriction of their capacity for altering their shape

to one direction indicates that their protoplasms and metaplasm are not

indifferently mixed. The two substances set in lines in the direction

of the long axis of the cell. Hence, when streaming occurs—when the

force which keeps the molecules of protoplasm and of metaplasm in their

respective rows is relaxed—the lines thicken. The cell broadens, with

an equivalent diminution of length.

Muscle-fibres exhibit all degrees of specialization. The simplest,

“plain muscle-fibres,” are found in the wall of the alimentary canal,

of bloodvessels, of ducts, in the tissue of the spleen, in the skin,

and elsewhere. Each fibre is a fusiform cell. Save for its central

nucleus and a little granular protoplasm in which the nucleus is

embedded, the cell may show no architectural features. But in most

varieties of plain muscle, and especially in that of the alimentary

canal, the substance of the fibres is striated longitudinally. This is

visible evidence of the orientation of the molecules of protoplasm and

metaplasm in the direction of the long axis of the fibre. It shows that

the streaming of particles occurs along these lines. It is, as it were,

a diagram of the lines of force.

Heart-muscle has been described already (p. 224). Its striation, which

is both transverse and longitudinal, is so delicate as almost to defy

microscopical analysis. The transverse striæ are the darker and more

distinct. But close examination shows that the transverse striæ do not

indicate the direction in which the particles of cell-substance are

oriented. They are oriented longitudinally. The cell is a bundle of

rods of substance A, embedded in substance B. The transverse markings

are very thin lines which cross the bundles at right angles.

The third variety of muscle is the kind by which locomotion is

effected. It is present in large masses—all the red tissue to which

the term “meat” is commonly applied. It accounts for about 35 per cent.

of the body-weight. This kind of muscle is not composed of single

cells, but of compound cells, or cell-complexes, termed “fibres.” A

fibre may attain a length of upwards of 2 inches, with a breadth of

about ¹/₅₀₀ inch. In most cases the fibres are attached by one end to

a bone, by the other to a tendon; and since they are shorter than the

muscle as a whole, the tendon commences as a membrane which covers the

surface of the muscle, sloping to it from the bone to which by their

other ends the fibres are attached. A fibre is developed from a single

cell. The cell elongates, its nucleus divides, and the daughter-nuclei

divide until several hundred have been formed; but cell division

does not follow. The result is a cylindrical mass enclosed within a

delicate membranous sheath, the sarcolemma. In the early stages of its

development its nuclei are in the axis of the fibre, but subsequently

they are displaced outwards. In the most highly specialized muscle,

known as the “white” variety, they lie just beneath the sarcolemma

(_cf._ Fig. 16, B).

The feature of this type of muscle is its transverse striation, almost

mathematically regular. Commonly striated muscle is spoken of as

“voluntary,” because, for the most part, it is under the control of

the Will; but the term, in so far as it implies a connection between

structure and mode of actuation, is misleading. Transverse striation

is evidence of capacity for rapid action. The muscles which the Will

directs exhibit promptitude; but striated muscle, which is not under

the direction of the Will, is found in certain situations—_e.g._, the

upper part of the œsophagus. Conversely, many animals can voluntarily

call into action muscle which is not striped. A turkey erects its

feathers by setting in motion little groups of “plain” fibres, which

pull on elastic tendons attached to the tips of the buried ends of

their shafts. Plain muscle contracts less promptly and relaxes more

slowly than the striped variety. Cardiac muscle is quicker in acting

than plain, but does not hold the contraction so long.

All striped muscle is not equally rapid. Two varieties are

distinguishable: “white fibres,” which respond suddenly to a single

stimulus and quickly relax; “red fibres,” which respond in a more

leisurely way, but remain contracted longer. In some muscles these two

types of fibre are intermixed. Others are wholly red or wholly white.

Everyone is familiar with the contrast which the white flesh of a

turkey or of the domestic fowl presents to the red flesh of game-birds

and birds of prey. In the breast of a blackcock a sheet of white

muscle overlies a mass of red. When the bird is cooked the difference

in colour is strongly marked. Of the two muscles which, in a rabbit,

correspond to our muscles of the calf, the superficial, gastrocnemius,

is white; the deeper, soleus, red. The former acts over both knee

and ankle joints; the latter over the ankle only. The muscle which,

acting over a longer range, has to contract more quickly is white; the

shorter, more slowly acting muscle is red. Experiment shows that red

and white muscles are distinguished by a difference in the promptitude

with which they respond to an electric current. It shows, too, that

the white muscle is exhausted sooner than the red. It cannot give so

many successive responses to stimulation without a rest. We shall find,

when we are considering the minute structure of striped muscle, a

difference between its two varieties which we can correlate with their

different modes of action. All human muscles belong to the red kind.

The most efficient muscle-fibres in the animal kingdom are found in

insects. This will not surprise anyone who thinks of an insect’s power

of movement. If a man could jump as many times his own height as a flea

can, he would clear the dome of St. Paul’s. An ant can drag an object

sixty times as heavy as itself, with no wheels beneath it to diminish

friction. Under the same conditions a horse cannot drag much more

than its own weight. A dragon-fly, it is asserted—although we have

not met a man who guarantees that he has made the observation—will

support its heavy body in the air by the rapid vibration of its wings

for four-and-twenty hours without alighting. The chirp of a cricket is

produced by the rubbing together of its hind-legs. A mosquito sounds

its war-cry much in the same way. The pitch of the note proves that

the insect’s muscles are contracting and relaxing at least 300 times a

second. None of these figures must be applied without qualifications in

estimating the relative strength of insect and human muscle. Weight for

weight, the muscle of a flea is not so much stronger than ours as the

figures might lead one to infer. To ascertain the numerical relation,

it is necessary to compare the total cross-section of the two chief

segments of a flea’s leg with the cross-section of the extensor muscles

of a man’s thigh and calf, and a man’s weight with the weight of a

flea. Nevertheless, after all deductions have been made, a considerable

balance of superiority lies with the insect as regards the strength

of its muscles, their rapidity of contraction, and power of repeating

contraction without fatigue. An insect’s muscle is the most suitable

that can be obtained for microscopic examination. Its pattern is larger

and more distinct than that of other animals. That the pattern should

be larger is not quite what might have been expected. It would not have

surprised us had we found the pattern finer in the more effective type.

Nothing is easier than to mount a specimen of insect-muscle. The large

water-beetle (_Dytiscus marginalis_) is an excellent subject. It is so

easily handled. Having cut off the animal’s head, a leg is pulled out

from the thorax. It is split open with a penknife, and a little of the

muscle is dug out from within its hard case, placed on a clean slide,

and covered with a cover-slip. If the preparation has been made quickly

and cleanly, the muscle remains alive for five or ten minutes. Not only

can it be studied, with the microscope, unaltered by reagents, but

under the most favourable circumstances the progress along its fibres

of waves of contraction can be watched. The structure of the fibres is

more easily made out if a little salt-solution or white of egg is added

to the preparation.

[Illustration: FIG. 16.—A, A MINUTE PORTION OF AN INSECT’S

MUSCLE-FIBRE, HIGHLY MAGNIFIED. B, WHITE FIBRE OF MAMMALIAN MUSCLE.

A, The nuclei are in the core of the fibre. B, The

nuclei lie immediately beneath the sarcolemma. The

disc on the left of this fibre, and the fibril on

its right, show the two ways in which striated

muscle-fibres tend to cleave. The dark line, or

row of dots, is known as Dobie’s line, or Krause’s

membrane. The figures are severely diagrammatic.]

Striped muscle is crossed by bands, dim, bright, and dark. The sequence

is as follows: Starting with the very thin dark line, which often

appears as a row of dots, the next band is bright; then comes a dim

band about twice as broad as the bright one; then another bright

band. This sequence is repeated with extreme regularity from end to

end of the fibre. Usually the bands cross the whole breadth of the

fibre, although occasionally it is divided by longitudinal lines into

parts in which the stratification is shifted a little backwards or

forwards. A segment of a fibre comprises the substance between two

dark lines—_i.e._, two bright bands with a dim one between them. If

the muscle has been hardened in one of the fluids commonly used for

the purpose of preparing tissues for the microscope, with its two ends

fixed, say, by binding them to a piece of a match, so that it could

not shrink, a thin clear line appears crossing the middle of the dim

band. This seems to show that the fibre is not made up of single dim

discs between two bright discs, but of couples, comprising half a dim

disc and a bright disc. The thin dark lines indicate that the fibre

is divided into compartments by transverse septa, which are probably

reticulated. The appearance of a transverse line of dots, in place of

a continuous line, is due to the existence of very fine longitudinal

markings (it is unsafe to give them a name which connotes structure).

Where the longitudinal lines cross the transverse lines, the optical

effect is the appearance of a dot.

If pieces of muscle are placed in a solution of osmic acid, they become

hard and brittle, and their markings are accentuated. Muscle from

the claw of a crab or a lobster is very suitable for this purpose,

owing to its exceptional freedom from connective tissue. After this

hardening the fibres are easily separated with the aid of needles into

fibrils immeasurably slender. An isolated fibril shows with extreme

distinctness the alternation of dark, bright, dim, bright, dark

markings already described. The appearance of a cross-section of a

fibre also proves that it is a bundle of fibrils. The cut ends of the

fibrils appear as dots surrounded by homogeneous substance. In this

respect there is an important difference between red muscle and white.

In the red fibres the fibrils are fewer and thicker than they are in

white, and the embedding substance is more abundant. It is generally

assumed that the homogeneous substance, sarcoplasm, is the nutrient

protoplasm of the fibre, the fibrils the contractile elements. The

more complete the differentiation of the fibre into fibrils, the more

rapid is its action; the more abundant the sarcoplasm, the greater its

capacity for continued work.

If a living muscle-fibre is observed while a wave of contraction

is passing down it, the ends of the fibre being free, so that its

shortening is not prevented, it is noticed that the widening of the

fibre is accompanied by the thinning, even to obliteration, of the

bright bands. The dim discs extend laterally, without any noticeable

diminution of their thickness. It looks as if the bright discs, or

something contained in the bright discs, were absorbed into the

dim discs. The fibre is, as we have already pointed out, striated

longitudinally. The striation is more clearly visible in the dim discs

than it is in the bright ones. That the dim disc has an architectural

structure absent from the bright disc is placed beyond doubt when a

muscle-fibre is illuminated with polarized light. The dim disc is then

found to be doubly refracting; the bright disc is not. When the prism

in the tube of the microscope is placed with its axis at right angles

to the axis of the prism which intervenes between the source of light

and the stage of the microscope, a succession of bright bands is seen

corresponding to the dim bands seen with unpolarized light. The rest

of the fibre is invisible, because it has not the property of twisting

the undulations of light which the lower prism has set all in the same

plane. Various hypotheses as to the cause of contraction, or, to speak

more correctly, as to what happens during contraction, have been based

upon the thinning of the bright discs. It is assumed that the dim discs

have a definiteness of structure which the bright discs do not possess.

They are thought of as being traversed by pores, or as consisting of

short rods. Microscopists who take the latter view believe that during

contraction the more fluid substance, sarcoplasm, which occupies the

bright bands is drawn into the dim bands between the short rods, or

sarcostyles, which are consequently separated more widely.

No tissue could be more unsuitable than muscle for microscopic

examination; for none other offers the same optical difficulties.

This will be evident to anyone who considers the description already

given of the markings which it exhibits. Whatever may be the true

interpretation of these markings, it is clear that they point to

an almost infinite multiplication of minute elements adjusted with

absolute accuracy side by side and end to end. A cylinder filled with

these transparent objects has to be viewed by transmitted light. The

elements, whatever may be their nature, refract light in different

degrees. It is impossible to eliminate the effects of internal

reflection, refraction, and interference of waves of light. The most

alluring hypothesis must be accepted with a considerable amount of

reserve. Any fact which seems to militate against it must be taken

into consideration. The view set forth above, in general terms, is

very attractive to everyone who wishes to bring muscle within the

category of machines. Suppose we accept the hypothesis that the dim

band is a plate made of sarcostyles surrounded by sarcoplasm then the

impulse which reaches a fibre causes an alteration in the surface

relations of the rods to the substance in which they are embedded.

Molecules of fluid from the bright bands are drawn in amongst

them; the rods are pushed farther apart; the fibre broadens with a

corresponding diminution in length. This brings muscular contraction

into the category of the phenomena which play the most important rôle

in bringing about the varied activities of the animal mechanism.

Contraction is due to osmosis.

The separation of muscle into fibrils after hardening does not seem

to bear out either the rod or the pore hypothesis of the structure of

the dim disc. It must be remembered, however, that before the fibrils

are teased apart the substance of the fibre has been coagulated. The

fluid in the bright disc may thus have become as much a part of the

fibril as the rod in the dim disc. The longitudinal striation of plain

muscle and the appearance of continuous fibrillation in heart-muscle is

more difficult to reconcile with the hypothesis that striped muscle is

composed of interrupted rods.

Muscle transforms the energy supplied to it by the blood into

mechanical work. It is doubtful whether any hypothesis as to

structure will help us to an understanding of the way in which this

transformation is effected. Explanations are seductive, but all

attempts at explaining the connection between molecular change and

change in shape must be viewed with suspicion. It is quite clear that

muscle as a motor is not to be compared with any form of motor with

which we are acquainted. It is also clear that the theory of muscle

must be applicable to all its varieties—striped, cardiac, and plain.

It must cover the alterations in form of an amœba and the streaming

movements of protoplasm within a vegetable cell. Probably it must

extend farther, and cover the discharge of electricity by an electric

organ and the emission of light by the lamp of a firefly. We are on

ground so treacherous that we are not sure whether, in crossing it, we

may lean with confidence on the laws of thermodynamics; and doubt as to

the applicability of these laws to living tissue almost upsets one’s

mental balance. Until we have evidence to the contrary, we are bound to

exclude such a misgiving from our minds. If we allow it to influence

us at all, it is merely to the extent of causing us to hesitate to

assume that the explanation of muscular contraction can be based upon

an analogy between muscle and any known mechanical contrivances for

generating power, not even excluding apparatus designed for the purpose

of measuring osmotic force.

If living muscle is frozen, pounded with snow containing 0·6 per cent.

of sodic chloride, and placed upon a filter, a fluid plasma passes

through the filter as the mixture thaws. Like blood-plasma, it clots

spontaneously—without, however, so far as is known the intervention of

a ferment.

All muscles become rigid after death, owing to the coagulation of their

plasma. It used to be thought that contraction was a stage towards

rigidity—a stage from which muscle, so long as it is alive, recovers.

This view was based upon the fact that exhausted muscle—such, for

example, as that of a hare which has been coursed—becomes rigid

much sooner than rested muscle. But this phenomenon has a different

explanation. The setting of muscle in rigor mortis is due to the

development of lactic acid (one of the waste products of active

muscle). The more there is of this ready formed at the time of death,

the more quickly does coagulation of muscle-plasma occur. The formation

of lactic acid is due to deficiency of oxygen. So long as muscle

obtains as much oxygen as it wants, its metabolism is complete. The

oxidized products which it loses are water and carbonic acid. This is

true also of the changes which occur after death. If a strip of muscle

is hung in an atmosphere of oxygen, it forms no lactic acid, and it

does not become rigid. If, on the other hand, the supply of oxygen has

run short before death occurred, rigor mortis sets in very quickly.

A dead frog takes a long while in becoming rigid, and its rigidity

is transient. Until the moment of death the frog is taking up oxygen

through its lungs, and even after death it probably takes it, as it

does when it is alive, through the skin. A fish becomes rigid very

quickly. For some time after it is caught it continues to live; but,

being unable to breathe in air, every molecule of oxygen which was in

its body when it left the water is used up before it dies.

In the human body rigor mortis usually sets in from two to four hours

after death, and lasts about two days; but both the rapidity of

its appearance and its duration depend upon various circumstances.

As muscles become rigid they contract, moving the limbs, and the

shortening is more extensive than mere coagulation of muscle-plasma

would account for. It is evident that a process similar to functional

contraction precedes coagulation. Many a watcher in the chamber of

death has been startled by the shaking of the bed. Even a sound

resembling a sigh may be caused by contraction of the muscles of the

chest. Placing his hand over the region of the heart, the attendant

finds the body warmer than it was when life became extinct, for much

oxidation has since taken place.

What chemical changes occur in muscle when it contracts? What is the

chemical source of its power? Carbonic acid is given off. This is

the only product which we can collect and measure; but it is taken

for granted that hydrogen atoms also combine with oxygen, forming

water. There is no reason for thinking that nitrogen is removed from

the molecules of its protoplasm with any greater rapidity during the

activity of muscle than when it is quiescent (_cf._ p. 212). Is the

oxidation immediate and complete, or does it occur in stages? For

many years attention has been directed to lactic acid, partly because

this substance is found in muscle which has been made to contract

under experimental conditions, partly because, on theoretical grounds,

glycogen (animal starch) is looked upon as the most important of

muscle-foods. Lactic acid—C₃H₆O₃—has the same percentage composition

as glycogen—C₆H₁₂O₆. Its formation from glycogen merely involves

a rearrangement of atoms. It has been supposed that lactic acid is

formed in the first instance, and then, if the supply of oxygen be

sufficient, oxidized to carbonic acid and water. But this hypothesis

may be resisted on various grounds. Undoubtedly, lactic acid appears

when oxygen is deficient. Under all circumstances and in all tissues a

certain amount of it is formed. There are reasons for thinking that it

carries away the nitrogen which is wasted, as lactamide. But it does

not follow that under normal conditions, when muscle is abundantly

supplied with blood, lactic acid appears in any greater quantity during

activity than during rest. The hypothesis is due to the misconception

which we have already endeavoured to correct. It is difficult to get

away from the steam-engine analogy. A steam-engine is made of iron and

brass. These materials are subject to wear and tear; but they are not

the source of its power. Its power is due to the combustion of fuel.

Muscle, physiologists formerly said, is made of protoplasm. This wears

down when it works, setting free creatin and other nitrogenous débris.

Its fuel is glycogen. This is not the way, however, in which the matter

is now regarded. Protoplasm is not the machine only, but also the

source of power. Glycogen is not burnt in a framework of protoplasm.

When muscle contracts, protoplasm casts out CO₂ and H₂O. Glycogen is

the food readiest to restore to it the atoms which it has lost.

Another consideration opposed to the hypothesis of the conversion of

glycogen into lactic acid is the uselessness of such a transformation

from a physical point of view. The stability of the atoms of C₃H₆O₃ is

so little greater than that of the atoms of C₆H₁₂O₆ that practically no

energy is set free when the one substance changes into the other. We

cannot, however, overlook the fact that the formation of acid may be

a means of profoundly altering the state of the colloid substances

dissolved in cell-juice. The casein of milk coagulates when milk turns

sour. The neutralization of a faintly alkaline solution of a protein

(and muscle is faintly alkaline) will throw it out of solution. The

appearance of lactic acid may be intimately associated with movement of

protoplasm, and yet the change of glycogen into lactic acid not be the

source of the energy which muscle expends.

=Fatigue.=—For its continued activity muscle needs an adequate supply

of food and oxygen. If the blood which distributes food is circulating

properly, and the liver, the great depot of food, is well stored,

fresh supplies are brought to the muscles as they are needed. There

are muscles—those of the eye and of the heart, for example—which

never become exhausted. However continuous their activity, they

take food from the blood as rapidly as they waste it; a statement

which, perhaps, needs qualifying by the addition, “so long as the

work exacted of them is such as may be reasonably expected.” If, in

a picture-gallery, one keeps the eyes elevated for an hour or more a

headache follows. Our eye-muscles have taken over their duties on the

understanding that we look down or straight forwards far more often

than we look up. If a long-sighted child is required to focus his

eyes upon a printed page without the aid of spectacles, not headache

merely, but actual disease of the brain, may be the result. The

ciliary muscle within the eyeball, which effects accommodation of the

eye for near objects, is unduly strained. Even the use of our modern

type, with its vertical height greater than its breadth, which has

taken the place of square Roman letters, is probably related to the

development of astigmatism of the lens, and thus indirectly a cause of

headache. It is asserted on high authority that vertical astigmatism,

the commonest form, is not present in the eyes of children before they

learn to read. Headache is an exaggeration of the feeling of fatigue.

It may be interpreted as the brain’s expression of unwillingness to

be made to work; a protest always to be listened to, notwithstanding

that it does not necessarily follow that unwillingness to work is the

result of overwork. Constipation, irritation of the sensory nerves

of the stomach, overdosing of the brain with alcohol, and many other

causes, may, through the vaso-motor system, set up the conditions

which normally result from activity unduly prolonged. The fact that

a central disturbance, headache, results from undue muscular work

calls our attention to the double nature of the mechanism concerned

in movement. Muscles are set in motion through the intervention of

the nervous system. After they have worked to an unusual extent the

nerve-centres connected with them grow tired. This, at least, is a

legitimate inference from the fact that headache occurs when certain

muscles of the eyeball have been subjected to an improper strain. But

it must be remembered that the muscles of the eyeball never tire. They

do not, like other voluntary muscles, give notice that they are in need

of rest. It is not so clear that the central mechanism is in any way

involved in the fatigue which is produced by excessive use of arms or

legs. The muscles of the limbs (and the central nervous system) are

protected by the sensations which originate in muscles when they are

overworked. The fact that a weary man can, if a great emergency demands

activity, use his muscles with as much vigour as if he were fresh from

bed, has been cited as an argument in favour of the view that fatigue

is of central origin; but it is an argument which works both ways. A

strong emotion causes a fervent response from the nervous system. Tired

muscles contract energetically when the impulses which reach them are

sufficiently urgent.

Nothing so definitely removes muscle from the category of machines

as its liability to fatigue. To speak of a muscle as tired is, of

course, to transfer to an object a term which is applicable only to a

phenomenon of consciousness; but it is necessary, unless a cumbrous

expression is to be used, to designate thus the effect upon the muscle

of prolonged activity. The petrol may be low in the tank, but the

quantity burnt in the cylinder at each stroke is not reduced. If an

isolated muscle is repeatedly stimulated by an electric current of a

certain strength, the response which it makes improves for the first

two or three induction shocks; then it begins to weaken. At each

succeeding spasm the muscle shortens a trifle less than before. More

remarkable than the diminution in the amount of work done by a muscle

which is growing tired is the prolongation of the time taken both

in contracting and in relaxing. Further, it has been shown that the

fatigue which accompanies the contraction of an isolated muscle is

not a condition dependent upon the shrinking of the store of energy

which it possessed when it was first thrown into activity. Muscles

undisturbed as to blood-supply, and contracting under the direction

of the Will, also exhibit it. Speaking generally, it may be said that

the tiring of muscle is not so much due to the exhaustion of its store

of food as to accumulation of products of action. Vigour is restored

to a tired muscle by passing through its bloodvessels a stream of

salt-solution, which brings it no food, but washes away some of its

waste. But the problem is far more complex than this. The machinery

is not simply clogged with the products of its own activity. If the

blood of a tired animal is injected into the vessels of one that is

rested, the muscles of the latter exhibit the phenomena of fatigue.

Evidently muscle is self-protective. During activity it prepares

a “fatigue-substance” which poisons its own nerve-endings, making

them worse conductors from nerve to muscle of the commands which

descend from the brain. Not only does the fatigue-substance dull the

nerve-endings in the particular muscle which has contracted, but, being

distributed by the blood to the whole body, it produces a general

effect. If the legs have been severely worked, they exhibit fatigue in

the highest degree; but after a long walk the arms also are less ready

and less capable than the state of their nutrition warrants.

The condition of stiffness experienced for a day or two after excessive

exercise is due to various causes in combination. The fact that it may

be remedied by encouraging the circulation through the muscles most

affected, as by hot baths and massage, tempts us to assign it also in

large measure to accumulation of products of action; but the means

taken to reduce stiffness favour the nutrition of the muscles both by

giving them more food and by carrying off their waste.

Equally remarkable with the self-protective disposition of muscle,

which forbids it to give, except at the instance of increasingly urgent

messages from the central nervous system, more than a part of the work

of which it is capable, is its preparation for meeting an increased

demand. It grows with use. Running increases the girth of the leg by

developing especially the muscles of the calf. Raising weights enlarges

the muscles of the shoulder and arm. Use-growth may reach inconvenient

proportions. Nothing is more noticeable during the training of young

athletes, whose nutritive responsiveness is at its height, than their

liability to pass through a stage in which they are “muscle-bound.”

Their legs grow bigger, but their pace falls off.

The development by means of exercises of a strong muscular system has

received much attention during recent years. Our ancestors cultivated

strength and agility in certain movements without paying much attention

to the muscles by which the movements were performed. It is fashionable

nowadays to lay stress upon the importance of maintaining an abundant

musculature, because of its relation to general “fitness.” The balance

between muscular activity and the organic functions which is observed

by everyone who takes an active holiday proves beyond doubt that

the nutritive condition of the various glands and of the heart and

bloodvessels is in some degree dependent upon the condition of the

muscles. Possibly they secrete into the blood other “messengers” in

addition to fatigue-substance—messengers whose call wakes up the

organs of digestion. The man who is so fortunate as to be able to use

his muscles in the open air has no need of exercises in his bathroom.

Failing out-of-door opportunities, much can be done by the systematic

use of the various muscles working against resistance. It is alleged,

and we are not disposed to dispute the justice of the contention, that

movements made with the fullest degree of mental concurrence have

a more rapid effect upon the growth of muscle than actions more or

less unconscious. Muscle and nerve are parts of a single mechanism.

It may be that fixing the attention on an exercise, and watching its

performance in a looking-glass, aids the nutrition of muscles by

increasing the influence of their nerves, possibly by improving the

nutrition of their nerve-centre. Unfortunately, this is one of many

theories which hardly come within the reach of a control experiment.

Could one concentrate attention on the movements of the right arm,

then absent-mindedly repeat them with equal vigour with the left,

it might be possible to ascertain whether there is anything in this

idea. Two other contentions with regard to the best way of performing

movements, with a view to the promotion of muscular growth, appear to

be justified by their results. Working against a moderate or light load

is said to be more effective than putting muscles to a severe strain.

A small number of maximal contractions, it is said, induce more rapid

growth than many partial shortenings. According to this scheme, when a

particular muscle needs strengthening, because in a certain action it

is to be the chief performer, it is made to bring its two ends as near

together as the plan of its attachments allows. Maximal shortening is

apparently favourable to blood-supply and otherwise promotes nutrition.

=Tone.=—Hitherto we have spoken of quiescence and activity, as if

muscle were doing nothing when not visibly contracting. A wrong

impression may be engendered by these terms. Muscle is never idle.

During sleep, and still more when a person is under the influence

of anæsthetics, the muscles approach the condition of machines at

rest. But again the language of the workshop is inapplicable. When a

headless frog is hanging from a hook its legs are slightly bent. All

its muscles are weakly contracted, if we understand by contraction a

condition in which the length of muscle is less than it would be were

it not alive. But the flexors are tenser than the extensors, hence the

crooking of hip, knee, and ankle. If the sensory roots of the sciatic

nerve are cut, the leg straightens out. So long as the nerve was intact

the weight of the limbs acted as a stimulus to sensory nerve-endings,

causing a reflex “tone” of the flexor muscles via the spinal cord. The

tone of the extensor muscles was less because they were not stretched

by the weight of the limbs. Every joint is under the influence of

antagonistic muscles which are perpetually watching one another. When

the limb is extended the flexors become anxious. When it is flexed the

extensors get ready for a spring. Only when it is half flexed is there

anything approaching to a truce. And this in most cases is the position

of greatest comfort. But even when most at rest, muscles still possess

a certain degree of tone. The tendency to shortening in one set causes

it to pull against, and thereby increases the tone of, its opponents.

When a muscle contracts it does not lift a loose bone. It has to

overcome the tone of the muscles which would cause a movement in the

opposite direction. And here another adjustment comes into play. The

same gross stimulus which leads to the contraction of A starts impulses

of a finer kind for B, directing it to relax its tone. We have seen

how the heart and bloodvessels are under the influence of two sets of

nerves of opposite sign—anabolic, diminishing irritability; katabolic,

increasing it. All muscles are under similar management; but we can

rarely detect the influence of the anabolic, inhibitory nerves, the

brakes, because the katabolic display is overwhelmingly conspicuous.

We must be content with two experimental demonstrations. An animal’s

hamstrings have been cut; the flexor muscles of its thigh are therefore

severed from their attachments below the knee. The tone of the extensor

muscle keeps this joint extended. If now the pad of the foot be

tickled, the flexor muscles contract, just as they would do if they

were still able to carry out the reflex action of raising the foot.

They cannot do this, because their tendons are divided; nevertheless,

the knee bends owing to reflex relaxation of the extensor muscles.

Still more striking evidence of reciprocal contraction and relaxation

is afforded by the claw-muscles of a crayfish. A weak stimulus to its

nerve causes the claw to set open; a stronger stimulus causes it to

close. Both these movements are due, not to “contraction,” but to

change of tone. Under certain conditions, a current passed through

its abductor muscle, the claw being open at the time, causes closure

by inhibiting the tone of this muscle. In this case the stimulus acts

directly on the muscle, producing an effect which is opposite to the

one we are accustomed to associate with stimulation; in place of

contraction, relaxation.

Contraction of muscle, moving something, impresses one as a positive

phenomenon. Relaxation seems negative—the undoing of contraction—and

to a very large extent this attitude of mind is justified. Return of a

muscle to its full length is due either to stretching by the weight it

has lifted, or to the antagonism of other muscles. An isolated muscle

lying on a pool of quicksilver does not return to its full length

after it has contracted. But it is necessary to banish the machine

idea. A machine gives out all the energy it has in store. Muscle is

extremely parsimonious. No stimulus can induce it to part with more

than a fraction of its energy. Recovery is as definite a function

as disturbance. A machine starts when a crank is moved, stops when

it is replaced. Muscle has a certain degree of automatism, although

its tendency to act on its own account has been almost completely

transferred to the governing nervous system. Muscle and nerve work

together, and the efficiency of muscle depends upon the maintenance

of its relations with its nerve. If the nerve is cut, the muscle

atrophies. We will not stop to consider whether wasting may be properly

attributed to disuse, or to vaso-motor changes. In its lowest form

nervous influence shows itself in the regulation of the nutrition of

muscle. A somewhat more forcible exhibition of control is seen in the

regulation of tone. The maximum is reached when a wave of undoing which

has passed down a nerve infects the protoplasm of muscle with the same

tendency to disintegration. The muscle-substance explodes. The muscle

shortens.

Remarkable evidence of the existence of muscle-tone is afforded by

the =knee-jerk=. Place a person on an upright chair, with his legs

crossed, muscles lax, foot hanging free. With a paper-knife or the end

of a stethoscope, or even the hand used edgewise, tap the ligament

which connects his knee-cap with his shin. The tap is instantly

followed by a jerking forward of the foot. The deep muscles of the

thigh, vastus, and crureus, have contracted. This phenomenon is easy

to account for. When we are standing upright, the trunk is supported

on three joints, of which one—the hip—is a perfect ball and socket,

and the other two—knee and ankle—are of the same order so far as

the absence of any provision for locking them is concerned. If the

muscles on the front and the back of the leg did not constantly adjust

our balance, by swaying the trunk forward when it falls back, and

pulling it back when it sways forward, the joints of the leg would

double up beneath us. A photographer knows how little confidence is to

be placed in a man’s assertion that he is able to stand still. This

see-saw of alternate contraction and relaxation is kept up by means

of nerve-impulses which ascend from the nerve-endings surrounding the

separate bundles of tendons, or from the Pacinian bodies which are

found in abundance in the neighbourhood of tendons and ligaments, or

from the elaborately twisted nerve-fibres found in muscle-spindles, or

possibly from all three classes. Muscle and tendon are richly supplied

with sense-organs susceptible to pressure and stretching. There is an

abundance of nerve-endings to choose from. The slightest change in

their tension, whether due to the muscle’s own contraction or to the

action upon it of other muscles or weights, is recorded not only in

the spinal cord, but also in the cortex of the cerebellum, and, if the

contraction is an act of volition, in the cortex of the great brain.

Although it was skin which was tapped, skin-nerves have nothing to

do with the jerk. It was the result of the slight sudden stretching.

In short, the tone-mechanism has been fooled. Notice the position of

the leg. The knee is semiflexed; the foot is hanging free. There is

nothing for the extensor muscles of the thigh to do. Now, if ever, they

are justified in dozing. It is not to be wondered at that the sudden

stretching of the ligament takes them off their guard, or that on

waking they give a quite unreasonable start. The phenomenon is, as we

asserted, easy to account for. It would also be easy to explain, if it

were not for the extreme rapidity with which the jerk follows the tap.

The interval is about one-hundredth of a second. This is thought to be

too short to allow an impulse to ascend a sensory nerve, pass through

the cord, and descend a motor nerve. It is true that these reflexes

of adjustment must stand on a different level to other reflexes. The

tone-impulses which cause them are incessantly patrolling to and fro

from sense-organs to nerve-endings. The paths they follow must be the

most open in the nervous system. Receptors and effectors must, in an

electrician’s phrase, be incessantly switched on; or, to express the

analogy more accurately, the flexor and extensor tone mechanisms are

incessantly and reciprocally switching each other on and off. It must

be confessed that it is very difficult to explain the knee-jerk if it

be not a reflex action, but, as has been supposed, a direct response of

the thigh muscles to their own stretching. The latter hypothesis does

not appear to be reconcilable with its dependence upon the maintenance

of the nervous connection of the muscles with the spinal cord. It

cannot be elicited unless the “spinal arc” is intact. It ceases after

the severance of either sensory or motor roots. Nor will it occur if

the supply of blood to the lower end of the spinal cord has been cut

off. Still more difficult is it to explain its extraordinary sympathy

with everything that happens in the whole nervous system, if the

impulses which cause it do not pass through the spinal cord. By a very

simple mechanical arrangement it is possible to record the amplitude

of the knee-jerk. The foot moves a lever which writes on a travelling

surface. The jerk is elicited by the hammer of a clock strapped to the

shin. In this way it is possible to extend the period of observation

over several consecutive hours, the subject becoming completely

oblivious of the movement his foot is making once a second, if it be

screened from his view. In deep sleep the jerks stop; but the subject

may doze, and still jerk follows tap. And the record made by his foot

mirrors all the changes in his nervous system. If he clench his fist,

the movement is reinforced, as it is when a child cries, a lamp is

lighted, his ear itches. There is music in an adjoining room. His foot

is the baton which beats _fortissimo_ to Wagner, and is lulled to

_piano_ by the “Lieder ohne Wörte.” On a bright day this spinal pulse

throbs gaily. It is indolent in dull, depressing weather. The knee-jerk

is the physician’s guide to the condition of the nervous system.

=Elasticity of Muscles.=—Muscles are very extensible, and after

stretching return to their original length. Their elasticity is a

quality of great practical importance. It enables them to meet sudden

resistance without rupture, as when a man alights from a height. At

the moment when the feet touch ground elasticity dissipates the shock.

The stretching of the muscles then leads reflexly to the increase of

their tone. Here we see an advantage in the short reaction-time of

the knee-jerk. Tone comes into play long before impulses generated by

contact of the sole of the foot with the ground have had time to reach

the brain, or even to induce reflex contraction through the spinal

cord. The elasticity of the muscles is also of use in the performance

of certain sudden actions. A pea is flicked across the room by pressing

the thumbnail against the pad of a finger, or a finger against the

thumb, and releasing it with a jerk.

An electrical change accompanies an impulse in its passage down a

nerve, and a wave of contraction in its passage along a muscle. In

1788 Galvani observed that the hind-limbs of a frog, suspended by

a metal hook to metal railings, twitched when the wind blew them

against the bars. The hook passed through the lumbar plexus of

nerves. He recognized that the cause of the twitch was the closing of

a circuit. The birth of dynamic or galvanic electricity dates from

this observation; and ever since this phenomenon was first observed

the electric changes in nerve-muscle preparations made from frogs’

legs have been favourite subjects of research. Many observations with

regard to nerve-conduction and muscle-contraction may be made, and many

experiments performed, without special apparatus. A frog having been

killed by cutting off its head, or by placing it beneath a tumbler

with a wad of cotton-wool soaked in chloroform, the skin of the leg

is removed, displaying the khaki-coloured muscles, bluish tendons,

and bright white threads of nerve. A stretch of the largest nerve of

the back of the thigh, the sciatic, is isolated. All the muscles of

the thigh are then cut away and the bone nipped across just above the

knee. The bones below the knee are removed, the superficial muscle of

the calf, the gastrocnemius, being allowed to hang free, its bifid end

attached to the fragment of thigh-bone. Its lower end terminating in

the tendo Achillis, with its insertion into the prominence of the heel,

is left intact. The bone is fixed in a clamp. A light lever made from

a wooden spill is suspended from the tendo Achillis. The nerve may

then be stimulated in various ways: by crushing in a pair of forceps,

burning with a heated needle, touching with a drop of glycerin or

a strong solution of salt. But of all methods of stimulation, the

best is the current from an induction coil. Since it does not injure

the nerve, it can be applied as often as may be desired. The amateur

provided with an induction coil is in a position to study the relation

between stimulus and response. He can vary the strength of the stimulus

and vary the weight which the muscle has to lift. He can observe the

progressive onset of fatigue, and otherwise gain much information

regarding the behaviour of muscle as an isolated piece of apparatus.

It is the ambition of the expert to obtain absolutely correct records

of the time-phases and of the changes in electric potential of nerve

and muscle under varied experimental conditions. For this purpose he

needs the finest apparatus which instrument-makers can furnish, and the

knowledge and dexterity requisite for its employment. Consider, for

example, the record of the change of form. A nerve-muscle preparation,

obtained by the method already described, is arranged so that the point

of the lever scratches on a rapidly travelling blackened surface. As

the muscle contracts it makes a “tracing.” A tuning-fork vibrating

at the rate of, say, 400 times a second also scratches a tracing on

the same travelling-plate. It is easy to time the several phases of

contraction and relaxation by comparing them with the undulations made

by the tuning-fork. By means of an induction shock a single impulse

is generated in the nerve and a single spasm evoked in the muscle.

Our tracing shows that the spasm lasts about one-tenth of a second,

and that about half this time is occupied by contraction, and half by

relaxation. But the ascending curve is usually a little steeper than

the descending curve, and the apex a little nearer to the commencement

of ascent than to the termination of descent. An electric signal marked

the instant at which the current was sent into the nerve. The time

taken by the impulse in travelling from the spot where the electric

current entered the nerve to its junction with the muscle can therefore

be estimated. The contraction begins so much sooner or later, according

as the shock is delivered nearer to, or farther from, the muscle. By

shifting the electrodes up and down the nerve, the rate at which the

impulse travels is directly measured. After the time that the impulse

took in reaching the muscle has been allowed for, there still seems to

be an interval before the muscle begins to shorten. This was termed

the “latent period,” under the impression that some time is actually

lost in turning the nerve-impulse into a muscle impulse. The impulse

was supposed to be latent in the end-plates of the nerve. Various

hypotheses were formulated as to the nature of the transformation.

The progressive improvements in apparatus and methods is testified by

the diminution in this latent period as given in the text-books of

successive decades. It is now put at ¹/₄₀₀ second, and is regarded by

most physiologists as a delay due to the inertia of the muscle. Owing

to its elasticity, the molecular change in muscle does not immediately

affect its shape. When the latent period appears to be longer—say

¹/₁₀₀ second—the balance is due to the inertia of the recording

apparatus. Usually the curve shows a rise lasting ⁴/₁₀₀ second and

a fall occupying ⁵/₁₀₀, due to the fact that inertia of muscle and

apparatus delays the commencement of the rise, but does not hasten the

termination of the fall.

When an impulse is generated artificially by an induction shock, a

single spasm or twitch is the result; but in Nature contraction is

never limited to a single twitch. Impulses descending from a motor

nerve-cell to a muscle are always rhythmic. They follow at the rate

of eight or ten a second in human nerves; and since in our muscles

contraction and relaxation take longer than in a frog, a second impulse

reaches the muscle before the effect of the first has passed away.

The muscle has not had time to relax, when it is again called upon to

contract. Hence a summation of contractions. The muscle continues to

shorten until the maximum of contraction is reached. This condition is

termed “tetanus,” to distinguish it from a single spasm. In fullest

contraction the length of a muscle may be diminished by one-half, or

even by two-thirds.

It would be impossible to treat of the =electrical phenomena= displayed

by nerves and muscles without presupposing some acquaintance with the

methods and laws of physics. As this is contrary to our understanding

with our readers, we must be content with the statement of a few

salient facts. At the moment when an impulse is passing along a nerve,

or a wave of contraction along a muscle, the electric potential of the

active part of the structure, whether nerve or muscle, is different

from that of the not-acting parts on either side of it. A battery in

its commonest form is a glass vessel containing sulphuric acid in

which a plate of zinc and a plate of copper are immersed. The zinc is

electro-positive as regards the copper. In a muscle the contracted

portion is electro-positive as regards the parts uncontracted. The

degree of positivity can be measured by connecting the muscle at two

spots with the two wires of a galvanometer. When one wire makes contact

with the contracted portion, and the other with a part which is not

contracted, a current passes through the galvanometer, causing its

needle to swing; and since the wave of contraction is not stationary,

but passes down the muscle, the current is subsequently reversed.

The wave, as it were, first tilts up one end, and then, passing on,

tilts up the other, letting down the first. The contracted spot is

electro-positive to the spot not contracted, and then the latter,

contracting, becomes electro-positive to the former, which has relaxed.

The needle of the galvanometer swings first to the left, then to the

right. The importance of this method of investigation lies in the fact

that the electric variation exactly represents, both in time and in

intensity, the change which is occurring in nerve and in muscle. By

following it, we can ascertain the rate at which an impulse travels

down a nerve. We can determine its length and its “form.” Represented

on paper, it is a wave. This wave travels in warm-blooded animals

with the rapidity of 35 metres in a second. When it reaches a muscle,

its rate—that is to say, the rate at which the wave of contraction

invades the muscle—is 6 metres in a second. The time during which any

particular level in the muscle remains contracted in a single spasm,

under the influence of an artificial stimulus, is about 0·05 second.

The length of the wave is 300 to 400 millimetres. These measurements

give us a very clear idea of the events which occur in a nerve-muscle.

An impulse picked up by a motor cell in the spinal cord runs down its

axon—termed later a nerve-fibre—with great rapidity. Even the most

distant muscle is reached in less than one-thirtieth of a second.

From the end-plate of the nerve it travels in both directions along

the muscle-fibre—or group of fibres, since each nerve divides into

branchets for thirty to forty muscle-fibres—with reduced velocity.

Every particle of each fibre rises and falls; but, seeing that the wave

of contraction is much longer than the fibre, the whole fibre is in a

state of contraction at the same time, although not with equal vigour

throughout its whole length.

We cannot dismiss the further consideration of the electric phenomena

of nerves and muscles without some inquiry into their meaning. It is

evident that they are intimately related to the molecular changes which

constitute an impulse. But at present the physics of the phenomena are

beyond our grasp. We may speak in a general way of dissociation of

ions; but we do not really know what is happening at the spot which is

in a state of impulse. We cannot bring the transformation which it is

undergoing into line with chemical and physical transformations which

we understand. Probably the electrical phenomena which mark it are not

peculiar to muscle and nerve. All living changes of state are of the

same nature. Cellular activity, or protoplasmic activity, to use a

better term, wherever it occurs, is accompanied by electrical change.

But it so happens that nerve-substance and muscle-substance have a

definite orientation which gives to the electric force a cumulative

effect. In a liver-cell it is dispersed in all directions. In a muscle

the change of potential at one particle is added to the change at

the next, until the sum of all these changes, transmitted along the

length of the fibre, is sufficiently large to deflect the needle of a

galvanometer. Owing to its summation it attracts our attention.

Although they cannot tell the true significance of the electromotive

change which marks the passage of an impulse, physiologists are in a

much better position now than formerly to controvert certain popular

misconceptions. There is no such thing as “nerve-force” in the vulgar

sense. A nerve does not transmit energy to a muscle. The muscle

obtains the energy which it dispenses when contracting from the foods

with which the blood supplies it. The nerve transmits an excitation.

Over-excitability is not a sign of strength, but of weakness. Nor is

an impulse in a nerve an electric current. It may be generated by an

electric shock, but a chemical stimulus is equally as effective. The

slow rate at which it travels, as compared with electricity, puts it

altogether out of comparison with an electric current. Its relatively

rapid progress, on the other hand, equally excludes the hypothesis that

it is a movement of ions, as that phenomenon is observed in solutions

of salts.

What is the nature of the process by which energy is conveyed along

a nerve? When speaking of the passage of impulses from receptors to

the central nervous system, and through this to effectors, we have

used the vague expression “molecular change,” to avoid the necessity

of being more precise. But the problem is of such profound interest

that we look with eagerness for any hint of the direction from which

light will eventually be thrown upon it. Recent discoveries regarding

the nature of electricity, combined with investigations at present in

progress as to the physical constitution of proteid substances, give

more than a hint. Hitherto the choice has lain between a chemical

and a physical explanation; now the border-line between chemistry

and physics, always wavering, has disappeared. The hypothesis that

an impulse is a progression of chemical change has meant in the past

that the “wave” was due to the oxidation of substances contained in

nerve, with liberation of CO₂ and H₂O. Various considerations render

such metabolism of the substance of which nerve-fibres are composed

improbable. In the first place, nerve-cell bodies contain a store of

material, tigroids (p. 320), which is recognizably drawn upon during

nervous activity. It would appear, therefore, to be the tigroids,

and not the substance of the nerve-fibre, which supply the energy

transmitted along a nerve. Then, again, the axon of a nerve-fibre,

enclosed as it is in a tube of fat, is peculiarly ill-placed for the

reception of the nourishment which would be needed to make up for

waste, if its metabolism be fluctuating and at times excessive. Nor

have nerves more than a very meagre blood-supply. Secondly, observation

does not give any support to the hypothesis of fluctuating metabolism.

A nerve does not give off more CO₂ when active than when passive. Nor

does it become acid. Thirdly, nerves, or, to be quite accurate,

medullated nerves, are indefatigable. Their capacity for conduction

is not diminished by previous use, as it would be were it dependent

upon their reserve of nutriment. These various considerations rule out

a “chemical” explanation of the old-fashioned type. It is premature

to do more than outline the “physical” theory which seems destined

to take its place; and the reader will perhaps forgive if, for the

sake of clearness, the case is put with unjustifiable definiteness

and simplicity. Proteid substances are constituted of clusters of

molecules. The form of the clusters depends upon the salts (or,

more precisely, the ions) with which they are associated, and the

associations depend upon the electric charges which the ions carry.

In resting nerve-protoplasm the clusters are small, and, since the

total surface-area of a number of small spheres is greater than the

surface-area of the same weight of matter when condensed into large

spheres, there is, so to speak, more surface for the ions to cling to.

Conversely, when the ions leave the small clusters, the latter are

not protected from the influence of mutual attraction. They fuse into

larger clusters. Fusion is carried to its extreme limits when a protein

coagulates. A nerve-impulse is a “wave” of partial coagulation. The

positive electricity generated in a cell-body by the metabolism of its

tigroids repels the positively charged ions which cling to the nearest

protoplasm-clusters in the axon. Like acrobats swinging from trapeze to

trapeze, each flight of ions dispossesses the ions from the clusters in

front of it; and in this way the disturbance progresses down the axon

as an electric wave.

Thus we interpret the shadow cast by a theory of which either of

several pioneers who are diligently climbing may at any time obtain

a view. The conductivity of protoplasm (and what is true of its

conductivity will be found to hold good equally for its irritability

and changeableness of form) is due to the readiness with which its

molecules enter into unstable associations with electrolytes. The

instability of these associations is related to the tendency of

the molecules to cluster. An impulse is passed along a nerve as a

displacement of ions; the ions being transferred from one molecule,

or group of molecules, to the next. Such an explanation of an impulse

involves no chemical breakdown of nerve-substance during its passage

along a nerve. It transfers the metabolism which liberates energy

(reinforcing the impulses which have originated in sense-organs) to the

nerve-cell bodies. It is based upon certain experimental data which

appear to have been established; but, like all other hypotheses which

are intended to account for physiological phenomena, this one must be

brought to the test by varying the conditions under which impulses pass

along nerves, and ascertaining whether the consequent alteration in the

force, rate, and other attributes of the phenomena are in accordance

with physical laws. In applying these tests to the activities of

protoplasm, we are, however, met by an insuperable difficulty. The

matter which transmits nerve-impulses is alive. We have no laboratory

standards by which to judge whether the changes in conduction which

are produced by changes in the conditions of the conductor are, or

are not, consonant with physical theory. It is with protoplasm that

we are dealing, and not with a mixture of proteins in solution. If

we surround a nerve with nitrogen, it loses its conductivity in five

hours, to recover it when oxygen replaces the neutral gas. This has

been regarded as proving that metabolism of the nerve is necessary for

the transmission of impulses. But conductivity is a phenomenon of life.

Deprivation of oxygen for five hours must bring the nerve-substance

to the verge of death. It might be argued that the retention by the

nerve for so long a time of its power of conducting impulses shows

that its metabolism is not a cause of the phenomenon. Again, it has

been shown that warming the nerves of cold-blooded animals greatly

increases the rapidity of conduction. It is more than doubled in the

nerves of the “foot” of a slug-and a similar increase has been proved

for the nerves of a frog-by a rise of temperature of 10° C. Reflecting

on the results of this experiment, a physicist would exclaim: “Then an

impulse is a wave of chemical change. A rise of 10° C. increases the

rate of chemical processes from two to three times; whereas no known

physical process is accelerated by more than 5 to 15 per cent.” But the

physiologist remembers that a rise of temperature of 10° C. increases

all the activities of a frog. He is hardly prepared to say that its

greater vivacity may not be the expression of more rapid oxidation;

but he sees no fore-ordained balance of vital enterprise and chemical

change. He is, or ought to be, extremely suspicious of any explanation

which appears to over-ride physical laws; yet, at the same time, he

is aware that until he has more accurate knowledge regarding the

constitution of protoplasm he will not be in a position to understand

how physical laws apply. The protoplasmicity of protoplasm is increased

by warmth. What change of molecular constitution does this imply?

The view that in a muscle molecular change gives rise to an electrical

change, which in turn produces the change in form, has been very

widely held. The hypothesis was based on observations which seemed

to show that the electric variation travels a little ahead of the

wave of contraction; but every improvement in recording apparatus has

diminished this apparent want of synchronism. There can be little doubt

but that the lagging behind of the wave of contraction is due to the

inertia of the muscle and of the recording apparatus. Molecular change

and electric variation are simultaneous. If this be true, the electric

change cannot be regarded as the cause of the molecular change, in the

sense, at any rate, in which they used to be considered as cause and

effect.

The =power of muscle= varies as its cross-section. For human muscles

the maximum lift amounts to from 7 to 10 kilogrammes for each square

centimetre. This is a large figure, but it must be remembered that,

owing to the arrangement of the bones as levers, most muscles act

at a great mechanical disadvantage. The greater the difference in

distance from the fulcrum between the point of application of the

force and the point of incidence of the weight, when the force acts

nearer to the fulcrum than the weight, the greater is the mechanical

disadvantage. The greater also is the rapidity with which the weight

is lifted. What is lost in strength is gained in swiftness. Contrast

the slow steps of a negro, whose long heel separates the point of

application of the power (tendo Achillis) from the fulcrum (the

ankle-joint), with the springy movements of a European. A European

needs, and as a rule has, a better developed calf, which allows him

his more sprightly gait, without sacrificing his carrying power. Our

preference for slender wrists and ankles is not purely æsthetic, unless

we admit, as may be maintained, that all natural canons of taste rest

upon utility. Slimness of joints means nimbleness. A few muscles act

directly, without loss of power—as, for example, the masseter, which

lifts the lower jaw (hence a grand capacity for cracking nuts)—but

most muscles move levers of considerable length. Compare with the

masseter the biceps and brachialis which lift the forearm. Their

tendons are inserted into the radius and the ulna at a distance from

the elbow-joint which is about one-tenth as great as the distance from

it of a weight held in the hand. Their united cross-section is about

16 square centimetres: (16 × 10) / 10 = 16. One cannot hold out in the

hand, the elbow being pressed against the side, so that these muscles

alone are acting, a greater weight than 16 kilogrammes (34 pounds),

although the muscles are exerting a traction ten times as great as

this. The strength of muscle when pulling straight is well illustrated

by the thick white mass in the centre of an oyster. It keeps the shell

closed until a force equal to 1,300 times the animal’s weight has been

applied. This muscle also affords a good illustration of the part

played by reflex contraction in opposing stretching—the reaction by

which tone is maintained. Anyone who inserts an instrument, such as the

end of a screwdriver, between the slightly open valves of an oyster

lying under water will find that he needs to give it an exceedingly

smart twist if he would catch the muscle asleep. Stretching it causes a

reaction proportional to the stretching force.

[Illustration: FIG. 17.—BICEPS MUSCLE IN ACTION.]

The fact that the output of energy by muscle is proportional, within

certain limits, to the work to be done, is brought out even in

laboratory experiments. A nerve-muscle preparation teaches that the

amount of work is not a function of the stimulus. Within certain limits

a stronger stimulus evokes a higher and stronger lift; but the stimulus

remaining the same, the work done by muscle (_i.e._, the product of

weight multiplied by height) is, up to a certain optimum, increased

by increasing the weight. Often a very light load is not lifted as

high by a nerve-muscle preparation as a slightly heavier one. No

satisfactory theory of this reaction to load has yet been formulated.

Explanations have been put forward, but they merely substitute one

unknown for another, a not uncommon drawback to explanations.

Muscles are strongest when at their full physiological length. As he

dips an oar into the water a man exerts the greatest force of which he

is capable, provided that he is not guilty of “missing the beginning.”

Hands over the stretcher, body between the knees, ankle, knee, hip,

fully flexed, arms straight—all his strongest muscles are at their

greatest physiological length. Rowing is an exercise which has no

rival. Every muscle in the body, from little toe to little finger,

comes into play under the conditions which suit it best. And not less

admirable is the effect upon the abdominal muscles during recovery at

the end of the stroke; and the rhythmic movement which encourages deep

and measured respiration.

The greatest output of work is obtained when muscles contract against

a progressively diminishing load. Towards the end of the lift the

load must be small, if contraction is to be carried to its extreme

limit. The provision for this is well seen in the case of the muscles

of the arm when lifting a weight up to a position above the head. A

portmanteau is held in the hand. Its handle is gripped by flexing the

fingers. And here it may be noted that, since the range of movement of

a muscle varies as its length, the thumb and fingers are not worked

only by muscles contained in the palm of the hand. Fingers are bent

and wrist flexed by muscles of which the origin is carried up even to

the lower end of the humerus. As the portmanteau hangs by the side,

biceps and brachialis are at their fullest length. Suppose it to be

necessary to place it on a cab. These muscles begin the work under the

best conditions. They could not, however, lift the portmanteau far did

not the muscles of the shoulder displace the elbow from the side, so

that at the end of their pull, the forearm being almost vertical, the

muscles of the arm have little more to do than to move the hand inwards

towards the head, in preparation for the extensor thrust. The secret of

getting the greatest amount of work out of any particular muscle lies

in securing for it the due co-operation of other muscles.

ELECTRIC ORGANS.

Muscle disperses energy in the forms of mechanical work, heat and

electricity. Its structure, as already pointed out, is peculiarly

favourable for the display of electromotive force. In certain fishes

muscle is so modified as to give an electric discharge without

developing mechanical work. The production of an electric change is a

by-phenomenon of muscular activity. It becomes the sole function of

an electric organ. If the skin be removed from the tail of a skate, a

cylindrical column of brawny tissue about the size of a finger will

be found embedded amongst the muscles near its root on either side.

These are electric organs, although so weak that it is barely possible

to feel the shock which they give in a live fish. The nearly allied

Torpedo of the Mediterranean has far more powerful batteries. They are

situate near its gills, occupying the whole thickness of the fish from

skin to skin. When the back of a torpedo is pressed, it discharges a

current of 30 volts, or even more. Still more violent are the shocks

given by an eel—Gymnotus—which haunts the tributaries of the Amazon,

a terror to all who have to cross their fords on foot; or the African

fish, Malapterurus. The current which these animals develop attains an

intensity of 200 volts. With the exception of those of Malapterurus,

all electric organs are modified muscle, and closely similar in

structure. The organs of Malapterurus appear to be modified glands.

The skate’s electric organ may be taken as typical of the rest. When

sliced with a knife, it is seen to be divided by firm connective tissue

into minute chambers. These chambers are piled into hexagonal columns,

which lie lengthwise in the organ (they are set dorso-ventrally in

Torpedo). Each chamber contains a jelly-like substance which embeds an

electric disc. The disc divides the chamber into a smaller anterior and

a larger posterior compartment. Each chamber is supplied with several

nerves which ramify into innumerable twigs on the front surface of

the disc. The development of the disc must be considered for a moment

if its structure is to be understood. It starts life looking as if it

would grow into a voluntary muscle-fibre. A nerve joins it, forming an

end-organ in the usual way. Then the end-organ increases its spread

unduly, while the rest of the fibre fails to grow. The structure

becomes toadstool-shaped, with the nerve arborizing on the seat of

the stool. The front aspect of the disc, therefore, corresponds to

a nerve-ending in a muscle. Its middle layer indicates clearly that

the fibre makes an abortive attempt to develop cross-striation. It is

laminated, the laminæ strangely contorted; in section they appear,

not as plain lines, but as rows of dots, evidently a suggestion of

longitudinal striation. The posterior layer of the disc consists

of granular protoplasm drawn out as a number of short backwardly

directed tongues, and one long process, the stem of the stool. No

structure could be more suggestive of the function of the organ; but

no one has as yet succeeded in catching the suggestion and pressing

it into a definite explanation of the way in which it works. Certain

physiologists, laying great stress on the fact that the functional

connections between an electric organ and its nerves are not easily

interrupted by the administration of curari, atropin, and other drugs,

which block the passage of impulses from nerves to muscles, look upon

the nerve-layer of the disc as the generator of electricity, and the

rest as an accumulator or resonator, which stores, or exaggerates,

the electric charge. Others consider that the portion of the disc

which is altered muscle-fibre—the middle, or middle and posterior

layers—generates the electromotive force, the nerve simply calling it

into activity. All agree that a brief interval (about 0·003 second)

elapses between the arrival of the nerve-impulse and the discharge of

an electric shock. This “latent period” may be used as an argument in

favour of either view. It would be in harmony with the general account

which we have already given of protoplasm as a liberator of energy

to suppose that a nerve-impulse, having reached a disc, immediately

infects the protoplasm of the disc, inducing molecular commotion,

and that the ions move in such directions as to disturb the electric

equilibrium of the disc, its front surface becoming in relation to the

back as zinc to copper in a battery. The current generated in the fish

is in the direction from head to tail. It is certain that the change

does not occur until an impulse reaches the organ. The organ is not

charged by the nervous system during a period of inactivity, and then

discharged by a releasing impulse. This is sufficiently evident from

the fact that when a piece of the organ, with its nerve, is removed

from the fish, although much sooner exhausted, it responds like a

nerve-muscle preparation to repeated stimulation.

[Illustration: FIG. 18.—ELECTRIC ORGAN OF A SKATE IN LONGITUDINAL

SECTION—A, SLIGHTLY, B, HIGHLY MAGNIFIED.

A shows the compartments into which septa of fibrous

tissue divide the organ. In the centre of each

compartment is a disc formed from a modified

muscle-fibre. Nerves ramify in abundance on its

anterior surface. B, a minute portion of a disc. At

the top are seen nerve-fibres in delicate nucleated

sheaths; then follow the nucleated layer with which

they come in contact, the contorted laminæ which

represent the striations of the muscle-fibre, the

granular nucleated substance of its posterior

layer, some connective tissue, a capillary

bloodvessel containing oval nucleated corpuscles.

In a tissue space, a single coarsely granular

leucocyte is to be seen.]

The usefulness of a torpedo’s electric organs is unmistakable. They are

powerful enough to paralyse every animal that touches its back, whether

foe or little fish suitable for food. But of what service is its feeble

battery to a skate? This and the allied question as to the advantages

which can have accrued to the ancestors of the torpedo who first

began to change innocent muscle into a weapon of offence are usually

answered by pointing to the liability of flat fish lying on the bottom

of the sea to become resting-places of parasites, corallines, and other

fixed growths. Very mild shocks would suffice to disturb the peace of

would-be settlers. In the same way, the electric organs of fresh-water

fish may, when rudimentary, have protected the skin from invasion by

moulds.

LUMINOUS GLANDS.

If it be difficult, when considering the dispersal of energy as

mechanical work, heat, or electricity, by living tissues, to bring the

phenomena into line with those of which physics takes experimental

cognizance, how are we to approach the problems involved in the

generation of light? Yet the photogenic property of protoplasm is

widely distributed. Protozoans and various other invertebrate animals

cause the so-called phosphorescence of the sea. The abysmal depths

of ocean are lighted by forests of luminous polyps, and traversed by

fishes whose heads are furnished with lamps. By her own light the

female glow-worm enables her winged mate to keep his tryst. Fireflies

(Lampyrus) flash amongst the orange-trees of Italy, and blaze

(Pyrophorus) beneath the mangoes of Ceylon.

Luminous organs vary too widely in structure to allow us to pick out,

as in the case of electric organs, the features which are common to

them all. In Pyrophorus the organ is a double mass of cylindrical

cells near the tip of the abdomen. The cells are set vertically to

the surface, and are supported by a tubular membrane. Their substance

contains a kind of fat. Beneath them there is a layer of cells, not

luminous, but evidently a part of the photogenic apparatus, containing

chalky granules. The organ is well supplied with nerves and with

respiratory tubes (tracheæ).

More interesting than its structure is the study of the peculiar

character of the light which the organ emits. It gives a spectrum which

extends from the red (beyond Fraunhofer’s line B of the solar spectrum)

to the first blue rays (F). It shows no lines. Green rays appear only

when the light is bright, and then they are the brightest of all the

rays. The light is practically destitute of actinic or chemical rays.

A photographic plate may be exposed for several minutes, almost

without changing, to the light of a firefly bright enough to enable one

to read with ease in a dark room; whereas light of equal brilliance

from any other source would change it in the fraction of a second. Nor

are heat-rays mixed with the light. Measurements show that the activity

of the photogenic organs does not give rise to any greater rise of

temperature than would occur in the case of any other gland.

The contrast between the emission of light by an animal and its

production in any other manner is very striking when the physical

evidence, or want of evidence, of what happens in the protoplasm which

produces it is considered. The fact that no heat accompanies the light

precludes us from attributing it to oxidation. If a firefly is enclosed

in a vessel of oxygen, its lamp burns no brighter—clear evidence that

its luminosity has nothing in common with the burning of a match or

the glowing of a stick of phosphorus. Nor is the lamp put out when

the insect is suddenly exposed to great cold (-100° C.). It continues

to shine until the cold kills it. There is no relation between the

luminosity of a firefly and the phenomenon termed “phosphorescence”

by physicists. Sulphide of calcium—the substance used for rendering

matchboxes visible in the dark—returns light which it has absorbed. A

firefly’s power of emitting light is in no wise affected by keeping it

for a long while in the dark.

Like all other events in vital chemistry, the generation of light by

protoplasm is due to a process of fermentation. The luminous organs

may be crushed, and the mixture of fermentable substance and ferment

extracted with water. The extract is luminous. If an extract is

prepared rapidly, and evaporated to dryness _in vacuo_, the residue

glows when moistened with water. That two substances are present in the

extract, one (luciferin) fermentable, the other (luciferase) a ferment,

is proved by the following experiment: A certain quantity of extract is

divided into two portions. One part (A) is allowed to glow until its

capacity for emitting light is exhausted. The other portion (B), as

soon as it is separated, is heated to 55° to kill the ferment. B still

contains luciferin; A contains luciferase, although all its luciferin

has been used up. Recombined, the extract is luminiferous.