THE MUSCULAR SYSTEM 



393 



The muscle most usually and conveniently employed for in- 

 vestigating the phenomena of muscular contraction is the gastroc- 

 nemius of a frog, dissected out in such a way as to leave its upper 

 tendon connected with a piece of the femur and its lower tendon, the 

 tendo Achillis, intact though free. At the same time care is taken 

 not to sever the connection of the muscle with its motor nerve, 

 the sciatic, which is dissected out for some considerable distance 

 back towards its point of exit from the spinal canal, the central end 

 being, if desired, left connected to a portion of the spinal cord 

 enclosed in a piece of the lower end of the spinal column. The 

 muscle is then suspended by fixing the remains of the femur in a 

 clamp ; a small S-hook is then attached to the tendo Achillis. This 

 muscle - nerve preparation 

 and its arrangement as 

 above described is shown in 

 Fig. 112. 



For purposes of experi- 

 ment the clamp is fixed 

 inside a small chamber with 

 glass sides to prevent the 

 drying of the muscle and 

 nerve ; this is effected by 

 placing a few pieces of wet 

 filter-paper inside the cham- 

 ber. The sciatic nerve is 

 laid over a pair of electrodes 

 connected by wires to bind- 

 ing-screws outside the cham- 

 ber ; by this means any 

 desired electrical stimulus 

 may be applied to the nerve. 

 A thread attached to the 

 hook in the tendo Achillis 

 passes through a hole in the 

 floor of the moist chamber, 

 and is connected with a hori- 

 zontal lever free to move 

 in a vertical plane, a small 

 weight being hung under the 

 lever to give the muscle the 

 ' load ' necessary for its 

 efficient contraction. The 



free end of the lever is then brought to bear against the vertical 

 surface of some recording apparatus, usually a cylindrical drum, 

 covered with smoked paper, made to rotate by clockwork about a 

 vertical axis. 



An inspection of Fig. 113 shows at once that if the drum of the 

 recording instrument alone rotates, the end of the lever connected 

 to the muscle must trace out a horizontal line on the smoked surface. 

 If the drum is stationary and the muscle is made to contract, the 

 lever will trace a vertical line. If now the muscle is made to 

 contract while the drum is rotating, these two lines are compounded 

 into a curve whose ordinates at each point of the curve and whose 

 general shape give us exact information as to the details of the 

 contraction from start to finish. 



The electrical currents employed in stimulating muscles to 



:i2. — A Muscle-Nerve Preparation 

 (Foster). 



m. The gastrocnemius muscle of a frog ; n, 

 the sciatic nerve dissected out back to 

 Sp c, the lower end of the spinal canal ; 

 /, femur ; cl, clamp ; / a, tendo- A chillis with 

 S-hook attached. 



