704 A MANUAL OF PHYSIOLOGY 



Fix the preparation on the cork plate of the myograph by a pin 

 passed through the cartilaginous lower end of the femur, and attach 

 the thread to the upright arm of the lever by one of the holes in it . 

 Hang not far from the axis by means of a hook a small leaden weight 

 (5 to 10 grammes) on the arm of the lever which carries the writing- 

 point, and move the myograph plate or the muscle-nerve preparation 

 until this arm is just horizontal. Fasten the electrodes from the 

 secondary coil on the cork plate with an indiarubber band ; lay the 

 nerve on them ; and cover both muscle and nerve with an arch of 

 blotting-paper moistened with physiological salt solution, taking care 

 that the blotting-paper does not touch the thread. Or put the pre- 

 paration in a moist chamber* (Fig. 294, p. 739). Adjust the writing- 

 point to the drum. Begin with such a distance between the coils 

 that a break contraction is just obtained on opening the key in the 

 primary circuit, but no make contraction. The lever will trace a 

 vertical line on the stationary drum. Read off on the scale of the 

 induction machine the distance between the coils, and mark this on 

 the drum. Now allow the drum to move a little, still keeping the 

 writing-point in contact with it ; then push up the secondary coil 

 i centimetre nearer the primary, and close the key. If there is a 

 contraction, let the drum move a little before opening the key again, 

 so that the lines corresponding to make and break may be separated 

 from each other. If there is still no contraction at make, go on 

 moving the secondary up, a centimetre (or less) at a time, till a make 

 contraction appears. When the coils are still further approximated, 

 the make may become equal in height to the break contraction, 

 both being maximal i.e., as great as the muscle can give with any 

 single shock (Fig. 259). 



(c) Attach a thin insulated copper wire to each terminal of the 

 secondary. Loop the bared end of one of the wires through the 

 tendo Achillis, and coil the other round the pin in the femur, so 

 that the shocks will pass through the whole length of the muscle. 

 Repeat the experiment of (b), with direct stimulation of the muscle. 



2. Stimulation of Nerve and Muscle by the Voltaic Current. (a) 

 Connect a Daniell cell through a key with a pair of electrodes on 

 which the nerve of a muscle-nerve preparation lies. Observe that 

 the muscle contracts when the current is closed or broken, but not 

 during its passage. 



Connect the cell with a simple rheocord, as shown in Fig. 260, so 

 that a twig of the current of any desired strength may be sent through 

 the nerve. As the strength of the current is decreased by moving 

 the slider S, it will be found that it first becomes impossible to obtain 

 a contraction at break. The current must be still further reduced 

 before the make contraction disappears, for the closing of a galvanic 

 stream is a stronger stimulus than the breaking of it. The break 

 or make contraction obtained by stimulating a nerve with an in- 



* Porter's moist chamber is found in many laboratories, and is very 

 convenient. It consists of a porcelain plate, around which runs a groove. 

 A bell-shaped glass cover, which can be lifted off at will, rests in the groove. 

 The femur of the muscle-nerve preparation is fixed in a small clamp, 

 composed of a split screw on which moves a nut. By means of the nut 

 the clamp is tightened on the femur. The gastrocnemius hangs vertically 

 down, the thread on the tendo Achillis passing through a hole in the 

 porcelain plate to a lever separately supported on the same stand as the 

 moist chamber. A piece of wet blotting-paper fixed inside the cover 

 keeps the air in the chamber saturated. 



