This condition of summation of contractions is called tetanus, and is said, by some, to be the manner in which muscular motion is produced by the action of the nerves in obedience to the will.

With from fifteen a second to upwards of many hundreds of induced shocks one can produce tetanus in a frog's muscle. The lowest rate of electric stimulation at which human muscle passes into complete tetanus is about 25 per sec. The number of stim

uli required varies with the rate of contraction of the muscle employed, the quick-contracting bird's muscle requiring 70 per second, while the exceptionally slow-moving tortoise muscle only requires 3 per second. According to some, there is a limit to the number of stimuli which will cause tetanus-360 per second is named as the maximum for a certain strength of

Tetanus produced by 8 stimulations per second. The more perfect fusion of the single contractions shown toward the end of the curve depends on the altered condition of the muscle.

stimulus; with stronger stimuli, even when more frequent, tetanus occurs. It has been shown that many thousand stimuli per second can cause tetanus even with very weak currents. If tetanus be kept up for some seconds, and the stimulation be then suddenly stopped, the lever falls rapidly for a certain distance, but the muscle does not quite return to its normal length for some few seconds. This residual contraction is easily overcome by any substantial load. If kept in a state of tetanus by weak stimulation, after some time the muscle commences to relax from fatigue, at first rapidly, then more slowly. This falling off of the tetanic contraction may be prevented by increasing the stimulus.

MUSCLE TONE.

Although the tracing drawn by a lever attached to a muscle in tetanus is straight, and does not show any variation in the tension of the tetanized muscle, some variations in tension must occur, since a low humming sound is produced during contraction. A muscle tone, like the purring of a cat, can be heard by applying the ear firmly over any large muscle (biceps) while in tetanus, by throwing the muscles attached to the orbit and Eustachian tube into powerful action, or by spasm of the muscles in mastication.

The number of vibrations which has been estimated to occur in the voluntary contraction of human skeletal muscles does not produce an audible note; hence it has been supposed that the note we hear has been an overtone. When a muscle is thrown into tetanus by a current interrupted by a tuning fork, a tone is produced which corresponds with that of the fork causing the interruption in the current by definite vibrations, which regulate the number of stimulations the muscle receives. If, on the other hand, a contraction of the muscle be brought about by stimulating the spinal cord, with the same rate of breaking the current, the normal muscle tone is produced, as if the contraction were the result of a nerve impulse coming from the brain.

There is no satisfactory proof, however, that the variation in tension of the continuous contraction of voluntary muscle is strictly rhythmical. The sensation of a sound like the muscle tone is produced by any nearly periodic vibrations of less rate than 25 per second. The pitch of the muscle tone varies with the tension of the membrana tympani. Hence, it has been suggested that it corresponds with the resonant tone proper to the membrane of the drum; which may be evoked by any trembling movements of the muscle fibres due to slight variations in the force or distribution of the impulses transmitted by the motor

nerves.

IRRITABILITY AND FATIGUE.

The activity of the muscle tissue of mammalian animals is closely dependent upon a good supply of nutrition, and if its blood current be completely cut off by any means for a length

of time, it loses its power of contracting. While the muscle remains in the body, and is kept warm and moist by the juices in the tissues, it will live a very considerable time without any blood flowing through it, and it at once regains its contractility when the blood stream is again allowed to flow through its vessels. This is seen when the circulation of a limb is brought to a standstill by means of a tourniquet or a tightly applied bandage. A mammalian muscle soon ceases to be irritable and dies when removed from the body, but its functional activity may be renewed by passing an artificial stream of arterial blood through its vessels, and an isolated muscle may thus be made to contract repeatedly for a considerable time.

On the other hand, the muscle of a cold-blooded animal will remain alive for a long time-many hours--if kept cool and moist. When its functional activity is about to fade, it may be revived by means of an artificial stream of blood caused to flow through its vessels, just as in the case of the mammalian muscle.

Common experience teaches us that even when well supplied with blood our muscles become fatigued after very prolonged exertion, and are incapable of further action. This occurs all the more rapidly when anything interferes with the flow of blood through them, such as using our arms in an elevated position; the simple operation of driving in a screw overhead is soon followed by pain and fatigue in the muscles of the forearm, though the same amount of force could be exerted when the arms are in a lower posture, without the least feeling of fatigue.

The difficulties of experimenting with the muscles of mammals make the frog muscle the common material for investigation, and from it we learn the following facts:

1. When removed from the body and deprived of its blood supply, the muscle of a cold-blooded animal slowly dies from want of nutrition. If it be placed under favorable circumstances, and allowed perfect rest, it may live twenty-four hours. If it be frequently excited to action, on the other hand, it rapidly loses its irritability, being worn out by fatigue.

2. From a muscle removed from a recently-killed animal, we

learn, that even without a supply of blood the muscle tissue is capable of recovering from very well-marked fatigue, if it be allowed to rest for a little time, so that the muscle has in itself the material requisite for the recuperation.

The first question then is, What causes the loss of irritability which we call fatigue? And the second is, By what means is the muscle enabled to return to a state of functional activity? We know that the mere life of a tissue must be accompanied by certain chemical changes which require (a) a supply of fresh material, and (b) the removal of certain substances which are the outcome of the tissue change.

In the case of muscle, this chemical interchange is constantly but slowly going on between the contractile substance and the blood. When the muscle contracts, much more active and probably different changes go on in the contractile substance, more new material being required, and more effete matter being produced. It is probable that the accumulation of these effete matters is the more important cause of the loss of irritability in a muscle, for a frog's muscle, when quite fatigued, may be rendered active again by washing out its blood vessels with a stream of salt solution of the same density as the serum (.6 per cent. NaCl), and thus removing the injurious "fatigue stuffs," as they have been called. It is found that a very minute quantity of lactic acid injected into the vessels of a muscle destroys its irritability, and brings it to a state resembling intense fatigue. Of the new materials required for the sustentation of muscle irritability, oxygen is among the most important, though its supply is not absolutely necessary for the recuperation of a partially exhausted, isolated frog's muscle.

The slow recovery of a bloodless muscle from fatigue may be explained by supposing time to be necessary for the reconstruction of new contractile material, and probably, also, for a secondary change to take place in the effete materials, by which they become less injurious.

When working actively the muscles require an adequate supply of good arterial blood in order to ward off exhaustion; and, as already explained in speaking of the vasomotor influences, a

muscle receives a greater supply of blood when actively contracting than when in the passive state.

The irritability of a muscle and the rate at which it becomes exhausted may be said to depend upon :

1. The adequacy of its blood supply: the better the supply of new material and the more quickly the injurious effete materials are removed, the more work a muscle can do without becoming exhausted.

2. Temperature has a marked effect on the irritability of muscles, as well as upon the form of this contraction. Low temperatures approaching 5° C.-diminish the irritability of a muscle, but do not seem to tend toward more rapid exhaustion. High temperatures-approaching 30° C.-increase the irritability, and at the same time rapidly bring about fatigue. At about 35° C. an isolated frog's muscle begins to pass into heat tetanus, and permanently loses its irritability.

3. Functional activity is accompanied by an increased blood supply, and a more perfect nutrition of the muscles, hence activity is advantageous for their growth and power; while, on the other hand, continued and prolonged inactivity causes a lowering of the nutrition and loss of irritability. Thus, when the nerves supplying the voluntary muscles are injured, there is considerable danger of atrophy and tissue degeneration of the muscles; the contractile substance becomes replaced by fat granules. This degeneration also occurs in the stump when a limb is amputated, the distal attachments of the muscles having been cut they cannot act, and after some time they become completely atrophied, so that muscle tissue can hardly be recognized in them.

DEATH RIGOR.

The death of muscle tissue is associated with a set of changes which, in some respects, resemble those observed in its active state. The most obvious phenomenon is an unyielding contraction, which causes the stiffening of the body after death. Hence, it is called rigor mortis. The muscles harden; lose their elasticity, and the tissue is torn if forcibly stretched. When isolated, the muscle is seen to be opaque, and its reaction is found to be