A muscle is capable of changing from the passive elongated condition, the properties of which have just been described, into a state of contraction or activity.. Besides the change in form, obvious in the contracted state of the muscle, its chemical, elastic, electric, and thermic properties are altered. The capability of passing into this active condition is spoken of as the irritability of muscle. This is directly dependent upon its chemical condition, and therefore related to its nutrition and to the amount of activity recently exerted, which, as will hereafter appear, changes its chemical state.

Under ordinary circumstances, during life, the muscles change from the passive state into that of contraction in response to certain impulses communicated to them by nerves, which carry impressions from the brain or spinal cord to the skeletal muscles. The influence of the will generally excites most skeletal muscles to action. Nearly all muscular contraction depends on nervous impulses of one kind or another. But there are many other influences which, when applied to a muscle, can bring about the same change. These influences are called stimuli.

We utilize the nerve belonging to a muscle in order to throw it into the contracted state, but the great majority of stimuli can bring about the change when applied to the muscle directly. Since the nerves branch in the substance of the muscle, and are distributed to the individual fibres, it might, as has been argued, be the stimulation of the terminal nerve ramifications that brings about the contraction, even when the stimulus is applied to the muscle directly, for the terminal branches of the nerves are affected by the stimulus applied to the muscle.

That muscles can be stimulated without the intervention of nerves is satisfactorily proved by the following facts: i. Some parts of muscles, such as the lower end of the sarto-rius, and many muscular structures which have no nerve terminals in them, respond energetically to all kinds of muscle stimuli.

2. There are some substances which act as stimuli when applied directly to the muscle, but have no such effect upon nerves, viz., ammonia.

3. For some time after the nerve has ceased to react, on account of its dying after removal from the body, the attached muscle will be found quite irritable if directly stimulated.

4. The arrow poison, Curara, has the extraordinary effect of paralyzing the nerve terminals, so that the strongest stimulation of the nerve calls forth no muscle contraction. If the muscles in an animal under the influence of this poison be directly stimulated, they respond with a contraction.