PRACTICAL EXERCISES 



809 



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 preparation in a moist 

 chamber* (Fig. 322, p. 843). Muscle troughs of various kinds may also 



Fig. 284. Lucas's Muscle Trough. A, trough made of hard rubbsr; B, a hard rubber 

 boss with a hole drilled in it to receive the pin which fastens the gastrocnemius 

 preparation; H, H, electrodes cased in hard rubber except at the ends, which 

 in the trough carry platinum wires; C, a brass plate mounted on one side of the 

 trough, carrying a lever with a vertical arm F ending in a hook, which is attached 

 by a loop of thread to the tendon of the preparation; G, the writing arm of the 

 lever; K, M, holes in G for loading the muscle. C can be slid horizontally by 

 means of the slots in it, and clamped by the screw E. I, tube for running off 

 the solution. 



be used, which permit immersion of a muscle (or nerve) in Ringer's 

 solution. A convenient form is shown in Fig, 284, but a trough suffi- 

 cient for the purposes of the student can be easily improvised in any 

 laboratory. 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 <irum. 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 



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

 venient. 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. 



