[This tone is not only recognised by the obviously stronger tension of the muscle between its origin and insertion when waking, for tone during sleep is weakened, and under a strong narcotic is done away with. It also shows itself by a more lively metabolism in a muscle with tone ("chemical tone ") than in the entirely, or almost entirely, uninnervated muscle.]

The nerve leading to the muscle is now laid bare and stimulated by an induction current. While the nerve is being prepared it often happens that the animal innervates the muscle in the attempt to move; the contraction of the muscle in voluntary innervation is then seen, the muscle usually shortening to about half its normal length. When the nerve is afterwards stimulated by a faradic current the muscle further shortens to over 80 per cent. of its length, takes up a condition of strong tonic contraction which is called tetanus, and hangs like a short, thick lump from its origin on the bone. One has now the opportunity of convincing oneself that the hardness of contraction in the natural position of a muscle is not a consequence of the contraction itself, but of the increased tension; the muscle, contracted to the uttermost in tetanus, feels quite soft when loosened from its insertion.

Let us now consider how the three essential physical properties of a muscle - extensibility, elasticity, and contractility - are shown by its functions. We have to notice first that all three properties exist, even in a muscle at rest. The muscle is, as already mentioned, stretched between its origin and insertion to an extent varying according to the part of the body. Its elasticity therefore continually asserts itself in an attempt to contract, an attempt which, on account of gravity and other smaller forces (friction between different tissues), and of a similar attempt on the part of the antagonists, does not affect the movement in any way. But contractility also asserts itself, like extensibility and elasticity, in the muscle at rest, though to a varying extent (e.g., waking or asleep). Even during sleep the muscle receives from its motor centre an unbroken series of weaker impulses amounting to a considerable number in a second. This is the cause of the tone already spoken of in certain muscles while waking, which is perceptible to pal-pation.

In active movements, e.g., flexion of the elbow joint, the muscles which perform the movement are innervated, i.e., in this case brachialis and biceps. During shortening their contraction, which depends essentially upon contractility, is helped to some extent by their elasticity. Their antagonist triceps in many cases receives an inhibiting impulse which diminishes the resistance to the exercise which its tone would give. In other cases, especially in all cases of minutely calculated movements, it is very definitely innervated and resists the flexion. Triceps thus carries out eccentric work in the actual performance of flexion. In any case, its extensibility during flexion is made use of.

So far as I know it is not yet certain what part, if any, contractility plays in passive movements. In passive flexion, e.g., in the elbow joint, the extensor (triceps) is stretched, while the flexors biceps and brachialis shorten. It is obvious that, particularly in slow passive flexion, the elasticity of the flexors must play some part; they shorten as a result of this property, partly because they have previously been stretched by extension, partly because in any position of the joint they are slightly stretched between their origin and insertion. But other reasons for their shortening must also assert themselves, especially in quick flexion. Pressure transmitted from the bones moving against one another by means of the tendons in their sheaths must be considered. But one may also assume - though this has not, to my knowledge, been scientifically explained - a reflex innervation of the flexors which are about to shorten. Contractility of a muscle, when innervated, will, if this hypothesis is correct, be made use of also in passive movement. All these factors work in very various degrees according to the speed of the passive movements.

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In a work of this kind there can be no question of a detailed account of such an extensive branch of knowledge as that of the mechanics and dynamics of movement. We must content ourselves with essentials, and must especially point out various changes in our conceptions which the last few years have produced, as a result partly of the facilities for observation which instantaneous and Rontgen photographs have given us.

Professor Otto Fischer of Leipzig has made important contributions to the question of the effect of muscle power on movements in the joints. In particular he has shown the fallacy of our tendency, in thinking of muscular contraction, to imagine either the origin or the insertion of the muscle to be fixed, and movement only of one bone to take place. This is true of some small muscles, e.g., of the ear or eye. But with regard to most muscles, unless one of their bones is fixed by some force, both the bones move towards each other as the muscle contracts while the angle between them diminishes.

O. Fischer points out that if there existed a creature composed only of two parts forming levers with equal weight, which were united by means of a joint and stretched by a muscle between two points on these levers at equal distances from the joint, both parts would move similarly towards each other when the muscle contracted. If one part had less weight than the other, then the part of smaller weight would perform larger movements than the other part of greater weight.

We must further notice that when the muscle with one of its ends not fixed works no longer in one direction, but in two, it does not only produce movement in the joint over which it passes, but also in the neighbouring joints outside this area. Thus a muscle passing over one joint works in such a manner that when it ilexes the joint over which it passes in one direction it moves the neighbouring joint in the opposite direction. Thus brachialis anticus flexes the elbow joint and at the same time extends the shoulder joint. The short head of biceps femoris (passing over one joint) does not only perform flexion (backward) in the knee joint, but at the same time flexion (forward) in the hip joint. The three vasti extend the knee joint (forward) and at the same time extend the hip joint (backward).