ELEMENTAEY EXPERIMENTAL PHYSIOLOGY 43 



vibrations per second can be changed. The spring has numbers stamped 

 on its upper surface, corresponding to the position at which it must be 

 clamped to give that number of complete vibrations per second. The free 

 end of the spring carries a platinum point which makes and breaks 

 contact with a mercury cup in connection with the primary circuit 

 {Fig. 47). In order to maintain the vibrations of the spring it is usual 

 to place above it, and in the same circuit, an electro-magnet, so that, when 

 the spring makes contact with the mercury, it is attracted out of the 

 cup again by the magnet. In performing a complete vibration, the 

 spring will both make and break the primary circuit and, in order that 

 the two stimuli may not cause contractions of unequal height, the 

 secondary coil must be so placed that either the make shock is just 



Fio. 47. Diagram of the vibrating reed in circuit. 



ineffective, in which case the number of effective stimuli per sec. will be 

 the same as the number of complete vibrations of the spring, or the 

 make and break shocks are made equal and maximal, in which case the 

 number of contractions per sec. will be double that of the complete 

 vibrations of the spring. 



Place the vibrating reed in the primary circuit so as to give 10 effective 

 stimuli per sec. Make a gastrocnemius and sciatic preparation, with the 

 Du Bois key closed, set the spring vibrating and bring the writing 

 point of the myograph on to the surface of the drurn, rotating at a slow 

 rate, about 3 to 4 cm. per sec. ; open the Du Bois key and record 

 the contractions for about 1 sec. Stop the drum, adjust the spring to 

 give 20 effective stimuli per sec., and record the contractions as before. 

 Eepeat again with 30 stimuli per sec. Then remove the vibrating reed 

 from the primary circuit, connect the battery with the coil so as to set 

 the Wagner's hammer vibrating, and record the contraction of the muscle 

 for a few seconds. 



Since each twitch of a gastrocnemius at 20 C. lasts about T ^th sec., a 

 muscle at that temperature could just respond without any summation 

 to 10 stimuli per sec. If, however, the muscle is colder or fatigued, and 

 each contraction therefore lasts longer, with 10 stimuli per sec., some slight 

 summation may be seen, i.e. relaxation is not complete before the next 



