GENERAL PHYSIOLOGY OF MUSCLE AND NERVE TISSUE 53 



thus used in nearly all work in the neuromuscular system, the 

 circulatory system, and the respiratory system. 



The instrument consists of a cylinder or drum kept in rotation by 

 clock-work. The rate of the rotation is usually governed by fans 

 of varying sizes, also by adjustments of the propelling mechanism. 



To prepare the kymograph for work, remove the cylinder, stretch 

 a sheet of prepared glazed paper tightly upon the surface and place 

 it upon such a stand as that shown in Fig. 33. Set the drum to 

 rotating and bring a gas flame or preferably a triple flame under the 

 drum. In a few moments it will be evenly covered with a film of 

 carbon, which is as sensitive to touch as a photographer's plate is 

 to light. 



To fix the carbon tracings and make the record a permanent one 

 see directions in Appendix, 8. 



3. Experiments and Observations. Pith a frog; mount it upon 

 the frog-board myograph as directed above. Prepare the kymograph 

 for receiving a tracing and adjust it for slowest rotation. Adjust 

 the myograph so that its tracing lever stands horizontal and tangent 

 to the drum with the tracing point lightly touching the side of the 

 drum. Set up the electric apparatus with one dry cell or one 

 Daniell cell so joined in the primary circuit as to avoid the action 

 of the vibrating hammer. Use a contact key in the primary circuit 

 and a short-circuiting key in the secondary circuit. 



(1) 'Determine the stimulus of liminal intensity by moving the 

 secondary coil to position of minimum strength; then, while slowly 

 "making and breaking" the current in the primary circuit, move 

 the secondary coil up until the strength of the induction shock is 

 sufficient to cause a contraction of the muscle. The weakest shock 

 which will cause a contraction is the stimulus of liminal in- 

 tensity sought. Note that this occurs on the break of the primary 

 circuit. 



(2) Determine the stimulus of optimum intensity by starting the 

 kymograph to rotating slowly; meanwhile make and break the 

 primary circuit while continuing to move the secondary coil from 

 the position of liminal intensity toward the position of maximum 

 intensity. The myograph will trace a series of myograms with the 

 rise and fall of the lever, when the muscle contracts and relaxes. 

 The tracings will present a series of sharp-pointed waves varying 

 in height, showing the varying extent of contraction. At first all the 

 contractions occur on break of primary circuit, then on both break 

 and make of the primary circuit. As the secondary coil is moved 

 toward the maximum position the myograms become higher and 

 higher, finally reaching a maximum height which is not exceeded, 

 however strong the stimulus is made. 



The stimulus of optimum intensity is the weakest stimulus which 

 will produce the maximum contraction. 



