40 PRACTICAL PHYSIOLOGY 



The rate at which fatigue comes on in a muscle under the above 

 conditions is increased by raising the temperature and the load. 



Another method of studying the effects of fatigue on a hyoglossus 

 or gastrocnemius muscle is as follows. In this case the primary circuit 

 is made and broken by hand, and the contractions are recorded as 

 nearly straight lines on a drum revolving at the slowest possible speed. 

 The secondary coil is moved up to the primary until both make and 

 break shocks are maximal, and the muscle receives a stimulus once 

 every 5 sees. In this way Fig. 45 was produced. It will be seen 

 that the height of the contractions, after remaining fairly constant at 

 the beginning, gradually decreases until, at the end of 15 minutes, the 

 muscle was incapable of lifting the load. Further, it is seen that in 

 the last two-thirds of the tracing the basal points of the twitches 

 gradually fail to reach the base line, thus showing a 'contraction 

 remainder.' If the muscle had been stimulated at shorter intervals, 

 this appearance would have come on earlier ; for, as soon as the period 

 of relaxation began to increase, the next stimulus would have reached 

 the muscle before there had been time for relaxation to be completed. 



If the muscle be allowed to rest for a few minutes and then the 

 stimulation is continued, it will be found that even excised muscle is 

 capable of slight recovery from fatigue (Fig. 45). 



One other point shown by Fig. 45 must be referred to ; the -height 

 of the first twenty twitches increases, showing a 'stair-case* effect. 

 This short and temporary improvement in the condition of muscle, 

 brought about by the repetition of a stimulus of constant strength, was 

 at one time thought to be peculiar to cardiac muscle (see Heart) ; but 

 although shown best perhaps by the heart, it is also shown by all forms 

 of muscular tissue. 



CHAPTER VII. 

 TWO SUCCESSIVE STIMULI. GENESIS OF TETANUS. TETANUS. 



WHEN a second stimulus reaches a muscle after the contraction caused 

 by the first is over, the muscle responds with a second contraction 

 similar to or perhaps slightly higher than the first (see Fig. 45). When, 

 however, the second stimulus reaches the muscle before the contraction 

 caused by the first is completed, the response given by the muscle to 

 the second stimulus depends upon the exact phase of its twitch, in 

 which it happens to be when the second stimulus reaches it. 



In order to investigate this point, arrange the drum and circuits as 



