2

probable hypothesis to account for the phases of contraction and relaxation (1) a production of lactic acid and of myosin during the former, (2) a production of CO, and a resolution of myosin during the latter. In pursuing our examination beyond this stage, it is to be remembered that the chemical differences between active and inactive muscle relate in each case to muscle that is dead, having necessarily been killed in the process of analysis. In correspondence with the statement just made, that muscular activity is attended with an evolution of CO2, it is found that the CO, collected by means of the gas-pump (a) from rigid previously active muscle, (b) from rigid previously resting muscle, is of greater amount in the second case than in the first; i.e. previously active muscle contains less CO2-yielding substance than previously resting muscle. Further than this only some isolated statements have been made by various observers, to the following effect:

1. Previously active muscle yields a smaller amount of waterextractives but a larger amount of alcohol-extractives than previously resting muscle (Helmholtz).

2. Pyrogallic acid is more readily oxidised by an aqueous extract of resting muscle than of tetanised muscle. Sulphate of indigo is more readily reduced by an aqueous extract of tetanised muscle than of resting muscle (Grützner).

3. Resting muscle contains more glycogen; tetanised muscle contains more sugar (Nasse).

The properties of muscle.-The physiology of muscle is almost entirely derived from the study of voluntary muscle, and the considerations upon which we are now entering relate almost exclusively to voluntary muscle.

Living muscle possesses two chief properties: (1) It is extensible and elastic; (2) it is excitable and contractile.

Extensibility and elasticity; viscosity.- Muscle can be stretched beyond its normal length, and can recover its original length when the extending force is removed, unless the extension has been excessive. Extensibility is demonstrated by the elongation of a muscle fixed at one end and weighted at the other; elasticity is demonstrated by the shortening when the weight is removed. The magnitude of these alterations is conveniently studied by means of a long light lever, which is attached to the

'Considerable confusion still prevails in the use of the term 'elasticity.' Sometimes it is improperly used to denote the yieldingness' of a body, which after it has been stretched beyond its length returns to its original length. This use of

lower end of the muscle, and records the changes of length upon. a smoked surface. A frog's gastrocnemius so disposed, and extended successively by weights of 10, 20, 30, 40, &c. grammes, will show for each successive equal increment of 10 grammes a diminishing increase of length; i.e. the elongation increases with the increased weight, but in a diminishing ratio; for instance, the elongation when the weight is increased from 20 to 30 is less than when the weight was increased from 10 to 20; it is less still when the weight is increased from 30 to 40, and so on. In this

respect the extensibility of muscle resembles that of arteries, and differs from that of substances such as caoutchouc or indiarubber, which give a series of elongations nearly proportional to the weights used. The recovery of length dependent on elasticity is observed in either case by the successive removal of the weights. It will be noticed in the case of muscle that each recovery of length as each weight is removed is greater than the preceding one, and that the recovery is not quite perfect. It is, however, usual, and no doubt correct, to say that the elasticity of muscle in its normal relations is perfect; though, out of the body and abnormally stretched, the muscle does not exhibit this perfect elasticity. Active muscle is more extensible than resting muscle;

the term is incorrect; extensibility is the correct word to use. Sometimes it is used to denote resistance to stretching; but this is inconvenient, for in this sense of the term an inextensible body has great elasticity. It is preferable to limit the term to express the force with which a stretched or compressed body tends to return to its original dimension.

rigid muscle is less extensible than normal muscle, and its elasticity is very imperfect.

If a muscle be weighted and left so, it will be noticed that the weight causes an immediate elongation, followed by a gradual

In rigor

In tetanus

Normal

Fatigued

elongation, which continues in a diminishing degree for an indefinite time. If a weighted muscle be relieved of weight, it will be noticed that the immediate elastic shortening is followed by a gradual indefinite shortening. These after-effects (viscosity) are not characteristic of muscle, but common to all extensible and elastic substances; they are best demonstrated by causing the recording lever to mark its excursion upon a slowly-travelling surface, e.g. a smoked cylinder fixed to the hour-axis of an ordinary clock.

Muscular extensibility and elasticity are useful in modifying sudden muscular contractions in the living body, rendering the application of their force more gradual, and thus obviating Tested by 50 grammes applied for short sudden jerks or rupture of tis

periods.

sue; moreover, the muscles are normally slightly stretched between their points of attachment to the bones, and are thus favourably disposed for prompt commencement and smooth execution of movements.

Contractility. In common with all living tissues, muscle possesses excitability, which property manifests itself as shortening or contraction; we say, therefore, that living muscle is contractile, i.e. able to contract, and that its characteristic vital property is 'contractility,' i.e. ability to contract. Any agent that excites a muscle to contract is called a stimulus'; it may be mechanical, thermic, chemical, electrical, or physiological. Of these various kinds of stimuli, that which is usually employed experimentally is the electrical stimulus, and the electrical stimuli generally used are (1) the constant current and (2) induced currents. Normal

muscular movement in the body is excited by the physiological stimulus which arises in the brain and is conveyed by motor nerves to muscle. Mechanical, thermic, and chemical stimuli are but little employed to stimulate muscle directly. The application of an experimental stimulus directly to muscle constitutes direct stimulation; the application of a stimulus to its motor nerve constitutes indirect stimulation, these expressions being used with

Sketch to show a myograph (the lever is cut off) and commutator without crosswires arranged for direct and indirect excitation. Wires from secondary coil to middle pools of commutator; fine wires from left-hand pair of pools to tendon of muscle and to pin through femur (circuit м), and from right-hand pair of pools to a pair of electrodes across which the nerve is laid (circuit N); moving the cradle to left or right connects the secondary coil with muscle or with nerve, and current is sent through either by opening and shutting a key к in the primary circuit.

especial reference to nerve-muscle preparations,' which are usually composed of a sciatic nerve in connection with a gastrocnemius muscle removed from a recently killed frog.

Constant current.-Stimulation with the constant current is effected when the current of a battery begins to pass and when it ceases to pass through a muscle, i.e. when it is made, and when it is broken, by closing and opening a key in the circuit of which the nerve or a muscle forms part. The muscle will contract each time the key is closed (make or closure contraction) or opened (break or opening contraction); it will remain quiescent while the current passes, i.e. while the key is left closed.

This is the rule, to which, however, there are exceptions; what is known as Wundt's tetanus' is an enduring contraction that is apt to occur in a frog's muscle after injury or during the passage of a strong current; and on human muscle the effect termed 'galvano-tonus'-which occurs on normal muscle during the passage of a strong current, on degenerating muscle during that of a comparatively weak current is of a similar character.

If the stimulus is applied at one end of a muscle, the proximal portion begins to contract a little earlier than the distal portion, i.e. the contraction spreads as a wave from the point of excitation, its rate of propagation (in frog's muscle) being between 1 and 3 meters per second (Aeby).

In perfectly normal fresh muscle the contraction sweeps undiminished from end to end; in moribund muscle it is stronger at beginning than at end, or it may actually remain localised to the point excited; this is the idio-muscular contraction'; on weakly subjects it may be easily elicited by mechanical excitation.

If a strong galvanic current is passed longitudinally through a freshly-excised and gently-extended sartorius muscle of a frog, a series of undulations may be seen to sweep along the muscle in the direction of the current (i.e. from anode to kathode) with a velocity of 3 to 5 mm. per second (Kühne, Hermann). The effect, which is not to be confused with the normal wave of contraction, is an idio-muscular phenomenon, probably caused by intrapolar irregularities of current-density; moderate warmth is favourable to its production.

Comparing the effects at make and break of the constant current, it will be found that the contraction at make is stronger than the contraction at break; to determine this relation it is necessary to graduate the strength of current and consequent strength of stimulation by means of the rheochord (p. 304). It will be found by further experiments that the make contraction starts from the kathode, that the break contraction starts from the anode; in other words, that the make stimulus is kathodic, the break stimulus anodic. The experiments demonstrating this statement are as follows:

(Exp. I.) Two levers rest upon a curarised muscle near its two ends, to which the kathode and anode respectively are connected, and are arranged against a recording surface; on making the current both levers rise, the kathodic a little sooner than the anodic; on breaking the current both levers rise, the anodic a little sooner than the kathodic.

(Exp. II.) A muscle is injured at one end and stimulated by make and break of a constant current, first in one, then in the opposite direction. It is found that when the kathode is at the injured end the make stimulus is less effectual than on the uninjured muscle; when the anode is at the injured end, the break stimulus is less effectual than on the uninjured muscle.