THE MUSCLES. - .j j_ 



NERVOUS STIMULI. The most important of all the excitatory 

 forces of the muscle is innervation. In the normal state there is 

 scarcely any other than this to produce muscle contraction. Our 

 muscles, as well as those of all other animals, contract because the 

 motor nerve transmits to them the spontaneous or reflex excitation 

 of the nervous centers. The nerve impulses average about ten per 

 second. The stimulus is exactly proportioned to the effect which 

 must be obtained. 



ELECTRICAL STIMULI. Electricity is employed in preference to 

 any other external agent to bring into play the, irritability of muscle. 



THERMAL STIMULI. Thermic excitations also provoke muscular 

 movements. The stomach and intestines are viscera whose muscles 

 are very readily excited by heat and cold. They contract very ener- 

 getically when very cold drinks are taken and their temperature is 

 suddenly modified. On the contrary, striated muscles hardly react 

 to thermic excitants. If heat or cold be applied gradually, there is 

 not produced any muscle contraction. Excitants act only when they 

 are applied suddenly. 



MECHANICAL STIMULI. Mechanical excitants that are capable 

 of producing muscular contraction are rather common. Thus, the 

 surgeon, while performing an operation, notices slight fibrillary 

 tremblings following each stroke of his scalpel. 



CHEMICAL STIMULI. It can be stated as a rule that all the 

 substances which are .fatal to the life of the muscle are excitants of 

 the muscle. On this ground, distilled water is an excitant, for when 

 it is injected into the arterial system of a frog its muscles show 

 fibrillary twitchings. Not only does the water excite the muscle, but 

 it also kills rapidly. 



Chemical Constitution of Muscle-tissue. The chemical study of 

 muscle is one of the most difficult of physiological chemistry. There 

 are in the muscle proteid matters very like one another and which 

 can be distinguished only by superficial characters. This renders 

 results far from being satisfactory or reliable. 



Besides, it is necessary, in order to know chemical reactions of 

 muscles, to study only living muscle. But from previous study it 

 will be recalled that even the weakest chemical actions produce very 

 decided changes in the muscles, with consequent alteration of its 

 chemical functions. 



Then, too, muscle-fiber is mingled with many other tissues, 

 arteries, veins, nerves, connective tissues, etc.; the separation of the 



