Muscles, by which term I here denote only the fleshy part of the muscle, not the tendons, display, while living, three properties most important to our understanding of the physiology of muscle - extensibility, elasticity, and contractility.

By extensibility we must exclusively understand the power a muscle has of being drawn out, without tearing, to a greater length than it has when no power is at work drawing it out. We must clearly distinguish between extensibility and elasticity. There are many substances, e.g., sealing-wax at certain temperatures, which are extensible without being elastic. Extensibility is found in both living and dead muscle. During rigor mortis - which is probably, though possibly only in part, the result of the coagulation of myosin - extensibility is greatly limited. But every medical student who is familiar with dissecting knows well how extensible special muscles with parallel fibres, e.g., sterno-cleido-mastoid and sartorius, become after rigor mortis has passed off, until they fall to pieces from decay. One also finds that dead stretched muscle does not, after extension, return to its former length, but remains lengthened - i.e., it has by death lost its elasticity. In order to understand our movements it is necessary to remember that the skeletal muscles during life are extended or stretched between their origin and insertion in such a way that their extensibility to a certain extent in any position is taken into account; in this way they are in a condition to respond quickly, i.e., within a very small fraction of a second, to the cerebro-spinal command for movement. The surgeon, who often has occasion to Observe extensibility in living muscles, knows that it is very considerably greater in young than in elderly persons, or, in other words, that the resistance of a muscle to extension diminishes with years and that muscle strains are much more common in elderly people than in the young.

Elasticity is found to any considerable extent only in living muscle, and has the same characteristics there as elsewhere, i.e., the muscle returns to the form which it had lost as the result of force. This does not only apply to change of form through stretching, but also through pressure at right angles to the long axis of the muscle as well as in other directions. The elasticity of a muscle may be diminished by infiltrations, oedema, etc. (see Myositis).

In physics the elasticity of muscle is said to be small because it can be drawn out and held extended by slight force, but it is called perfect because the extended living muscle completely recovers its former size when the extending force ceases. As in all organised bodies, the length to which it may be drawn out is not proportional to the extending force, but the more the muscle is stretched the less it is finally extended by a certain definite force. When the extension ceases a muscle exhibits an elastic recoil, it does not contract at once to its pre-existing length, but only by degrees.

Contractility of muscle under the influence of stimuli is a property which represents its specific energy, and is the means by which it is able to perform work. We must also remember that contractility in response to other irritations is also found in other protoplasmic structures than muscle, and exists even in the lowest organisms of the animal and vegetable worlds. The myxomycetae, an undifferentiated, low kind of fungus, moves about by lengthening and contracting its protoplasm, and I for my part believe that it is still unproved that any cell can be found which does not in some way exhibit contraction of its protoplasm in its "continuous internal adaptation to external conditions." With regard to muscle, the non-striated muscle of internal organs as well as the striated skeletal muscles, but especially the latter, respond to stimuli by extensive contractions. But their "adequate," normal, and usual stimulus consists of a nervous impulse, and the nerves of the skeletal muscles cause them specially to contract under the influence of the will either voluntarily, or as a result of reflex action. To a single impulse the skeletal muscle responds with only one twitch, though the shortest voluntary movement always consists of at least three or four impulses. In order to contract for a longer period, i.e., to be able to bring about our movements and positions, a muscle requires a series of nervous impulses, and receives at least twenty in a second. It usually does not shorten by more than half its length; in tetanus the shortening may be 85 per cent. of the whole muscle length. How much a contracting muscle may increase in volume is not, so far as I know, very definitely known.

The obvious increased hardness to palpation of a muscle while contracting, spoken of above, is not accounted for by the contraction itself, but by the increased tension of the muscle between its origin and insertion.

It is easy to demonstrate all these conditions upon a live rabbit, which is first put under chloroform so that all voluntary nervous influences are excluded. For example, the muscles of one of the posterior extremities are exposed; one of them is chosen and its tendon cut at the insertion without touching the muscle itself, whereupon the muscle contracts slightly as a result of having been somewhat stretched between its origin and insertion and as a result of its elasticity. The free end of the tendon is grasped in pincers, and the extensibility of the muscle is further demonstrated by gently drawing it out to a greater length; when the tendon is let go the muscle shows its elasticity by contracting to the length it had after the tendon had been cut. The chloroform is now cut off and the animal is allowed to recover. As this takes place the muscle is seen to shorten and thicken still further, even if the animal does not attempt to perform voluntary movements with it. This shortening is due to the gradually returning "tone" after the anaesthetic, through the nervous impulses which it continually receives from its motor nerve centres without any special stimulus, and which seems to be of a reflex nature.