138 PHYSIOLOGY OF GENERAL NERVOUS SYSTEM. 



brief interval (0.002 to 0.003 sec.) after it is stimulated. During 

 this period it is not irritable to a second stimulus. The same 

 phenomenon is exhibited to a marked degree by the heart muscle 

 and likewise by many nerve cells. In the motor nerve cell which 

 shows the property of discharging a series of impulses with rhythmic 

 regularity it may be supposed that the refractory period is marked, 

 and indeed is connected probably with the rhythmic character of 

 the cell's activity. But in this as in other properties it is certain 

 that there are great differences in the many varieties of nerve cells 

 found in the central nervous system. While those that act rhyth- 

 mically have probably a relatively long refractory period, others 

 may exhibit a period of unirritability but little longer than that 

 shown by the nerve fibers. In the case of the reflex motor centers 

 in the lumbar spinal cord of the frog it is stated (Langendorff) that 

 a second stimulus falling at an interval of 0.04 sec. after the first 

 is effective. The refractory period of these cells is less, therefore, 

 than this interval. 



