MUSCULAR CONTRACTION. 



137 



electrical currents can be detected, from their causing the deflection of a deli- 

 cate and lightly-suspended magnetized needle. Such a needle, however, sus- 

 pended singly, is subject to the magnetism of the earth, which would derange 

 or arrest the operation of very feeble currents. Hence, to prevent this, another 

 needle, of equal magnetic power, is suspended below the upper one, and attached 

 to it by a rigid axis, having, however, its poles turned in the opposite direc- 

 tions, the north pole of one being under the south pole of the other, and vice 

 versa. In this way the effects of terrestrial magnetism are neutralized and 

 the needle is made astatic and ready to be impressed solely by such currents as 

 may pass through the coil of wire within which it is suspended. Such an in- 

 strument is influenced by electrical currents of every kind, whether developed 

 by friction, thermal influences, chemical, or vito-chemical action ; the force of 

 the current is always measurable in degrees upon the scale. 



In order to apply the thermo-electric test to the measurement of heat devel- 

 oped in a living animal or man, a U-shaped piece of wire, composed half of iron 

 and half of copper, joined together at the bend, is immersed in water of a known 

 temperature. A needle, also half of iron and half of copper, is thrust through 

 the tissues, and so adjusted that the point of junction lies in the part, the rela- 

 tive temperature of which has to be determined. The iron shank of the U-- 

 shaped wire is now connected with the iron end of the needle, and the copper 

 shank of that wire with the copper end of the needle ; but somewhere in the 

 last-named connection the galvanometer is inserted. Any difference in tem- 

 perature between the metallic junction immersed in the water, and that em- 

 bedded in the living tissues to be examined, creates a current, either one way 

 or the other, according to which junction is hotter than the other ; and any 

 elevation or fall of temperature in the one, such as might be produced by the 

 acts of contraction and relaxation of a muscle, would cause proportionate, and 

 measurable, changes in the strength of the electric current. 



As thus determined, the quantity of heat evolved in contracting 

 muscle in warm-blooded animals, has been found sufficient to raise its 

 temperature by 1 or 2 ; in the frog the elevation of temperature is 

 rather less than J-. This effect may be partly due to friction, but it 

 is supposed to be chiefly owing to chemical combinations taking place 

 in the muscle, incidental and essential to the act of contraction. 

 Probably these chemical changes consist in an oxidation of the con- 

 stituents of the muscular tissue ; for exhausted muscle contains more 

 creatin, creatinin, urea, and inosinic acid, than muscle in a state of 

 rest (Helmholz) ; the substance of quiescent muscle is neutral ; that 

 of muscle, after frequent contractions, is acid (Du Bois-Reymond) ; 

 the interchange of oxygen and carbonic acid is doubled in active mus- 

 cle. It has recently been stated that the temperature of a muscle is 

 lowered at the beginning of a contraction, but that, after a few sec- 

 onds, a rise of its temperature takes place, which, in a tetanized 

 muscle, continues after contraction has passed off. Such a lowering 

 of temperature, if established, might indicate an absorption of heat, 

 or an increase of the latent heat of the muscular substance during its 

 commencing action ; whilst the subsequent elevation of temperature 

 might be due partly to increased chemical changes taking place after 

 contraction had ceased, and partly to the greater activity of the capil- 

 lary circulation. The amount of heat evolved is said to be propor- 

 tionate to the work performed (Meyerstein and Thiry). 



The living muscular tissue has also important electrical relations, 

 'hus, it is a good conductor of electricity, and it is also extremely 

 insitive to that agent, being very easily excited to contraction by it. 



