452 MANUAL OF PHYSIOLOGY. 



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 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 : 



J . Mechanical Stimulation. Any sudden blow, pinch, etc., of a 

 living muscle causes a momentary contraction, which rapidly 

 passes off when the irritation is removed. 



2. Thermic Stimulation. If a frog's muscle be warmed to over 

 30 C. it will begin to contract, and before it reaches 40 C. the 

 muscle will pass into a condition known as heat rigor, which will 

 be mentioned presently. If the temperature of a muscle be 

 reduced by C., it shortens before it becomes frozen. 



3. Chemical Stimulation. A number of chemical compounds 



