PRACTICAL EXERCISES 813 



6. Influence of Temperature on the Muscle-Curve. Pith a frog (brain 

 and cord), make a muscle-nerve preparation, and arrange it on a 

 myograph. Lay the nerve on electrodes connected through a short- 

 circuiting key with the secondary coil of an induction machine, or 

 connect the muscle directly with the key by thin copper wires. Take 

 a Daniell cell, connect one pole through a simple key with one of the 

 upper binding-screws of the primary coil, and the other pole with the 

 metal of the drum. A wire, insulated from the drum, but clamped on 

 the vertical part of its support, and with its bare end projecting so as 

 to make contact with a strip of brass fastened on the spindle, is con- 

 nected with the other upper terminal of the primary (Fig. 28/). At 

 each revolution of the drum the primary circuit is made and broken 

 once as the strip of brass brushes the projecting end of the wire. The 

 object of this arrangement is to ensure that when the writing-point of 

 the myograph lever has been once adjusted to the drum, successive 

 stimuli will cause contractions, the curves of which all rise from the 

 same point. Close the key in the primary, set the drum off (fast speed), 

 open the short-circuiting key, and as soon as the muscle has contracted 

 once, close it again. NOW stop the drum, mark with a pencil the 

 position of the feet of the stand carrying the myograph plate, take the 

 writing-point off the drum, and surround the muscle with pounded ice 

 or snow. After a couple of minutes brush away any ice which could 

 hinder the movement of the muscle, rapidly replace the stand in exactly 

 its original position, with the writing-point on the drum, and take 

 another tracing. Again take off the writing-point, and remove all 

 unmelted ice or snow. With a fine-pointed pipette irrigate the muscle 

 with physiological salt sohition at 30 C., and quickly take another 

 tracing. Then put on a time-tracing with the electrical tuning-fork. 

 Fig- 2 55> P- 748. shows a series of curves obtained in this way. 



7. Influence of Load on the Muscle- Curve. Arrange everything as 

 in 6. Take a tracing first with the lever alone, then with a weight oi 

 10 grammes, then with 50, 100, 200, and 500 grammes (Fig. 254, p. 748). 



8. Influence of Fatigue on the Muscle-Curve. Arrange as in 7, but 

 leave on the same weight (say 10 grammes) all the time. Place the 

 nerve on the electrodes. Leave the short-circuiting key open. The 

 nerve will be stimulated at each revolution of the drum, and the writing- 

 point will trace a series of curves, which become lower, and especially 

 longer, as the preparation is fatigued. Two or four curves can be 

 taken at the same time, if both ends of one or of two brass slips be 

 arranged so as to make contact with the projecting wire at an interval 

 of a semicircumference or quadrant of the drum (Fig. 287). (For 

 specimen curve, see Fig. 288, p. 814.) 



9. Seat of Exhaustion in Fatigue of the Muscle-Nerve Preparation for 

 Indirect Stimulation. When the nerve of a muscle-nerve preparation 

 has been stimulated until contraction no longer occurs, the muscle can, 

 under ordinary conditions, be made to contract by direct stimulation. 

 The seat of exhaustion is, therefore, not the general contractile sub- 

 stance of the muscular fibres themselves. To determine whether it is 

 the nerve-fibres or some structure or substance intermediate between 

 them and the ordinary contractile substance of the muscle, perform 

 the following experiments : 



(a) Pith a frog; make two muscle-nerve preparations; arrange them 

 both on a myograph plate, which has two levers connected with it. 

 Attach each of the muscles to a lever in the usual way, and lay both 

 nerves side by side on the same pair of electrodes. Cover with moist 

 blotting-paper. The electrodes are connected with the secondary of an 

 induction machine arranged for tetanus. With a camel's hair brush 



