GENERAL MUSCLE PHYSIOLOGY 195 



They are investigated with the same apparatus as the action- 

 currents of nerves (see page 217). 



Secondary contraction, secondary tetanus. If the nerve of 

 a muscle-nerve preparation is placed upon the surface of 

 another muscle, and if the latter or its nerve be stimulated, 

 both muscles are thrown into simple contraction or tetanus. 

 The action-current of the stimulated muscle passes over into 

 the nerve of the other muscle-nerve preparation and stimu- 

 lates it. Lasting contractions, not of a tetanic nature, do 

 not produce secondary tetanus. 



If a muscle is cut across and one of the electrodes of a 

 galvanometer is connected with the transverse section, while 

 the other electrode is connected with the longitudinal sur- 

 face, a current passes through the galvanometer from the 

 longitudinal to the cut surface. This is called the current 

 of rest. At the cut surface the muscle dies and this is con- 

 nected with processes which render the dying part negative 

 to the part intact. If now the longitudinal surface is stimu- 

 lated, the intensity of the current of rest is decreased; this 

 is called the negative variation. 



The electromotor force of the current of rest of a muscle 

 is about 0.07 volt. 



The cause and the significance of these electrical phe- 

 nomena in stimulated and dying muscles is not well under- 

 stood. 



3. THE STIMULATION AND THE IRRITABILITY OF 

 THE MUSCLE 



The stimulations which call forth the activity of the 

 muscle may be divided into : 



A. Indirect, i.e. stimulations which act upon the motor 

 nerves and thus upon the muscle. In this class belong the 

 normal physiological stimulations which are carried from the 

 central nervous system through the motor nerves to the 

 muscles. 



B. Direct, i.e. stimulations which affect the muscles 



