92 THE CONTRACTILE TISSUES. 



is not confined to the occasions when it is actually contracting. When, at a 

 later period, we treat of the heat of the body generally, evidence will be 

 brought forward that the muscles, even when at rest, are giving rise to 

 heat, so that the heat given out at a contraction is not some wholly new 

 phenomenon, but a temporary exaggeration of what is continually going 

 on at a more feeble rate. 



Electrical Changes. 



66. Besides chemical and thermal changes, a remarkable electric 

 change takes place whenever a muscle contracts. 



Muscle-currents. If a muscle be removed in an ordinary manner from 

 the body, and two non-polarizable electrodes, 1 connected with a delicate 

 galvanometer and many convolutions and high resistance, be placed on two 



FIG. 29. 



ch.c 



Non-polarizable Electrodes : a, the glass tube ; z, the amalgamated zinc slips connected with 

 their respective wires ; 2. s., the zinc sulphate solution ; ch. c. , the plug of china-clay ; c', the por- 

 tion of the china-clay plug projecting from the end of the tube ; this can be moulded into any 

 required form. 



points of the surface of the muscle, a deflection of the galvanometer will 

 take place, indicating the existence of a current passing through the gal- 

 vanometer from the one point of the muscle to the other, the direction and 

 amount of the deflection varying according to the position of the points. 

 The "muscle-currents" thus revealed are seen to the best advantage when 

 the muscle chosen is a cylindrical or prismatic one with parallel fibres, and 

 when the two tendinous ends are cut off by clean incisions at right angles to 

 the long axis of the muscle. The muscle then presents a transverse section 

 (artificial) at each end and a longitudinal surface. We may speak of the 

 latter as being divided into two equal parts by an imaginary transverse line 

 on its surface called the " equator," containing all the points of the surface 

 midway between the two ends. Fig. 30 is a diagrammatic representation of 

 such a muscle, the line ab being the equator. In such a muscle the develop- 

 ment of the muscle-currents is found to be as follows : 



The greatest deflection is observed when one electrode is placed at the 



1 These (Fig. 29) consist essentially of a slip of thoroughly amalgamated zinc, dipping 

 into a saturated solution of zinc sulphate, which in turn is brought into connection with 

 the nerve or muscle by means of a plug or bridge of china-clay moistened with normal 

 sodium chloride solution ; it is important that the zinc should be thoroughly amalga- 

 mated. This form of electrode gives rise to less polarization than do simple platinum or 

 copper electrodes. The clay affords a connection between the zinc and the tissue, which 

 neither acts on the tissue nor is acted on by the tissue. Contact of any tissue with copper 

 or platinum is in itself sufficient to develop a current. 



