206 INTRODUCTION TO GENERAL PHYSIOLOGY 



Effect of Length of Fibres 



The simplest way to see the fact that more work is done when 

 the fibres are longer is to load the muscle with increasing weights 

 and to determine the height of the contraction produced by a single 

 induction shock in each case. The product of the weight by the 

 height gives the work. If the actual value in gram -centimetres is 

 wanted, it is of course necessary to measure the relative distances, 

 from the axis of rotation of the lever, of the attachment of the 

 muscle, of the -weight and of the tracing point of the lever. For 

 the present purpose we only require to compare the different values 

 of work with each other. The point may write on a vertical smoked 

 paper. Move the surface so as to produce a short horizontal line 

 after each weight has been added and then stimulate the nerve. 

 An arc of a circle is drawn. The addition of each weight, say 

 5 gm. at a time, stretches the muscle a little more, so that the curve 

 starts from a lower level. The lever may be brought to the horizontal 

 position again by moving it in the slot of the myograph. The 

 tracing should be fixed, as described above (p. 187), and when dry 

 can be measured with compasses. It will be found that the work 

 done increases with the length of the muscle up to a certain point, 

 and then begins to decrease as the muscle becomes abnormally 

 stretched. 



The Structure of Voluntary Muscle 



This has been studied previously (p. 195), but should be brought 

 into relation with the fact of the preceding experiment. If only 

 insect muscle has been examined, it would be well to make a 

 preparation of the voluntary muscle of the rat or mouse. Mount 

 it in a little serum from the animal itself. Observe under the 

 highest power available, focussing the surface carefully. 



Effect of Temperature 



This has also been observed on the tonic contraction of the 

 frog's stomach. A further case is described under the heart (p. 225). 

 To study the action on the voluntary muscle requires special 

 methods, and is not easy to observe correctly. Note its significance 

 in relation to changes of surface tension between sarcostyle and 

 sarcoplasm. 



Production of Tetanus 



On a nerve-muscle preparation observe the height of the con- 

 traction produced by a single shock, and that in which the shocks 

 are given as quickly as possible after one another by opening and 



