ACTIVE STATE OF MUSCLE. 449 



The influence of the will is, then, the common stimulus which 

 excites most skeletal muscles to action. But we find that there 

 are many other influences which, when applied to a muscle, can 

 also bring about the same change. These influences are called 

 stimuli. 



We commonly utilize a 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 ap- 

 plied to the muscle directly, for the nerves, of course, would be 

 affected by the stimulus applied to the muscle. That muscles 

 can be stimulated without the intervention of nerves is satisfac- 

 torily proved by the following facts: 1. Some parts of muscles, 

 such as the lower end of the sartorius, and many muscular struc- 

 tures which have no nerve terminals in them, respond energeti- 

 cally to all kinds of muscle-stimuli. 2. There are some substances 

 which act as stimuli when applied directly to muscle, but have 

 no such effect when applied to 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 ter- 

 minals, 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 stimulated directly, they respond with 

 a contraction. 



This separation of muscles from nerves would appear rather 

 artificial, and antagonistic to the teachings of the development 

 of these tissues, both in the ascending scale of the animal king- 

 dom and in the individual. 



MUSCLE-STIMULI. 



The circumstances which call forth muscle contraction may be 

 enumerated thus : 



33 



