56 EXPERIMENTAL PHYSIOLOGY 



CHAPTEE IV 



SUMMATION OF MUSCULAR CONTRACTION. TETANUS 



In order to study the nature and the production of a prolonged tetanic 

 muscular contraction it is necessary to first examine the effect of 

 applying a second stimulus to a muscle before the contraction caused 

 by the first has ceased. This may be done by using some form of 

 apparatus of the general principle of that described in the following 

 experiment. 



Experiment 1. — To the upper end of the shaft of the drum are fixed two 

 brass collars, s 1 , s 2 , which can be clamped in any position by means of screws. 

 Held in the collars are two brass rods, R 1 , R 2 , terminating in flexible strips of 

 brass, m 1 , m 2 . To the upright of the drum is clamped an insulating vulcanite 

 base, on which is fixed a stiff brass rod, x, to which a binding screw, b 1 , is 

 connected. A second binding screw, b 2 , is attached at any convenient position of 

 the metal stand of the drum. The arm r 2 is now adjusted in the collar, so that 

 the spring m 2 just touches the tongue x, and the collar is rotated until a smooth 

 piece of the blackened paper is opposite the writing point when the two are 

 in contact. The second arm r' is similarly adjusted. The primary circuit 

 is now arranged so that the current passes from the battery to one terminal 

 of the primary coil, from the second terminal of this to the binding screw b 1 , 

 and from the binding screw B 2 back to the battery. Whenever either arm 

 R-, R 1 touches the metal x the circuit is then closed and travels through 

 the coil to b\ through the metal x to the arm r 2 , and so up the shaft of the 

 drum to its stand, and then through the binding screw b 2 back to the battery. 

 As the drum is rotated a little further the spring m- is bent until it slips off 

 the metal-piece x and the circuit is broken. 



Make a nerve-muscle preparation (the muscle may be stimulated directly, 

 in which case the nerve need not be prepared) and fit it to the myograph. 

 Place the electrodes under the nerve (or so that the stimulus is sent through 

 the whole length of the muscle) and move the secondary coil until the break 

 shock will give a maximal contraction while the make shock is still ineffec- 

 tive. Bring the writing point to the surface and mark the two points of 

 stimulation by very slowly rotating the drum by hand. As the first spring 

 touches the metal-piece an induced shock is produced in the secondary circuit, 

 which, however, is not sufficient to stimulate the nerve or muscle. When 

 rotated a little further the spring leaves the metal-piece x, the cmrent is 

 broken, and the induced shock stimulates the nerve or muscle, the muscle 

 contracts, and so records the point of stimulation. In a similar way the 

 second point of stimulation is also recorded. 



At first the second contact m 1 should be fixed at some little angular distance 



