CRANK-MYOGRAPH. 



201 



a), so that the muscle is loaded only during its contraction. Thus 

 a muscle may be " loaded " or " after-loaded " ; in the former case, 

 the muscle is loaded with a weight, both when it is at rest and 

 when contracting, but in an " after-loaded " muscle the muscle 

 raises the weight only during contraction, and is not stretched by it 

 when at rest. The experiment is arranged in the same way as in 

 Lesson XXXIV. 8. 



(a.) Make a preparation of the gastrocneniius with the lower end 

 of the femur attached. Pin the femur firmly to the cork plate of 

 the myograph covered with blotting-paper moistened by normal 

 saline. Tie a stout ligature round the tendo Achillis, by a hook fix the 

 ligature to the short arm of the lever, add a weight of 10-20 grams 

 to the lever, and see that the lever itself is horizontal. Thrust two 

 fine wires which act as electrodes from the Du Bois key in 

 the secondary circuit through the upper and lower end of the 

 gastrocnemius muscle. 



FIG. 119. Crank-Myograph. W, W. Block of wood ; M. Muscle ; F. Femur; P. Pin to 

 fix F; L. Lever ; WT. Weight ; a. Screw for after-load ; C. Cork ; B, B. Brass box. 



(b.) Arrange the style of the lever so that it writes on the 

 cylinder, and repeat, if desired, the experiments of the previous 

 Lesson. 



(c.) Use different weights 5 20 50 grams and observe how 

 the form of the curve varies on increasing the weight attached to 

 the lever. 



2. Automatic Break, i.e., Method of Excitation. It is con- 

 venient to use a single break induction shock, i.e., the secondary 

 coil is at such a distance from the primary that only the break shocfc 

 is effective. One may, of course, break the primary circuit by the 

 hand, as in the previous experiments, but this is not convenient 

 It is better to have an " automatic break" (fig. 120) done by the 

 drum itself as it revolves, the drum being introduced into the 

 primary circuit. Two binding screws are placed on the stand, but 



