GEAPHIC REGISTRATION OF MUSCULAR CONTRACTION 57 



while at the same time the long arm of the latter is prevented from 

 moving upwards by a counter force, such as a spring (Fig. 27D), 

 the shortening of the muscle will be insignificant in comparison with 

 the tension to which it is subjected. A curve of this kind, displaying 

 almost no change in the length of the muscle and practically no me- 

 chanical energy, is characterized as isometric. In this way, a relatively 

 much larger proportion of the total energy liberated is transferred into 

 heat. While the muscles ordinarily used by us in the production 

 of work, are not arranged in a strictly isometric manner, our con- 

 tractions most generally possess an isometric 

 character for the reason that they are ex- 

 ecuted against resistances. 



Electrical Stimulation. Battery. Poten- 

 tial. Strength of Current. Resistance. A 

 muscle-nerve preparation may of course be 

 subjected to the different kinds of stimuli 

 mentioned previously, namely, mechanical, 

 such as may be produced by pricking or 

 pinching; chemical, such as result from con- 

 tact with sodium chlorid and other agents; 

 thermal, such as may be caused by a heated 

 wire, and electrical. Any one of these influ- 

 ences may be brought to bear upon the 

 muscle directly or through the intervention 

 of its nerve. Under ordinary conditions of 

 experimentation preference is given to the 

 electrical method of stimulation, because it 

 is by far the most convenient, and although 

 the electricity may be produced by a mag- 

 net or by friction, the common practice is 

 to derive it from a Voltaic cell. 



Cu 



FIG. 28. DIAGRAM OF 

 DANIELL CELL. 



Cu, copper plate ( +) ; Z, 

 zinc plate ( ). The direc- 

 tion of the current is indicated 

 by the arrows. 



The place of the generator may be taken by a 

 Daniell, Grove or Le"clanche cell. The first consists 

 of a glass jar filled with a concentrated solution of 



sulphate of copper in which is immersed a round sheet of copper. Inside the latter 

 is a porous earthen cup filled with dilute sulphuric acid in which is contained a rod of 

 zinc. If the outside poles of this cell are now connected by wires, the current leaves 

 at the copper and enters at the zinc. The former pole, therefore, is the positive 

 pole or anode, and the latter, the negative pole or cathode. Inside the cell, of 

 course, conditions are reversed, because in order to complete the circuit the current 

 must flow from the zinc to the copper. The former, therefore, must be positive 

 and the latter negative. A cell of this kind generates a constant electromotive 

 force of about 1.07 volts, but possesses the disadvantage of giving off fumes and 

 acids and requires to be renewed from time to time. These difficulties are not 

 present in the so-called dry cell which is usually a rnodified type of the Le'clanche 

 cell. The latter consists of a glass jar filled with a saturated solution of ammonium 

 chlorid and containing a plate of amalgamated zinc. The inner area of this cell 

 is occupied by a porous cup, containing pieces of carbon and dioxid of manganese. 

 The plate of carbon projecting from this mixture forms the positive pole, while the 

 negative pole is represented by the zinc. The electromotive power of this cell 



