GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 145 



of a man may be interpreted from the movements of his shadow, but they 

 are very different phenomena; the activity of the nerve and muscle is indi- 

 cated by the electrical changes accompanying it, but they may be independent 

 processes. Certainly the irritating change which is transmitted along the 

 nerve and which excites the muscle to action, although accompanied by elec- 

 trical changes, is not itself an electric current. 



Electrical energy is exhibited not only by active nerve and muscle, but 

 during the activity of a great variety of forms of living matter. It may be 

 detected in gland-cells, in the cells of many of the lower animal organisms, 

 and even in plant-cells. The amount of electrical energy developed in animal 

 tissues may be far from trivial. Although delicate instruments are necessary 

 to observe the electrical changes in nerve and muscle, as the great internal 

 resistance of the tissues causes the currents to be small, we find in certain 

 fish special electric organs, which appear to be modified muscle tissue, and 

 which are capable of discharging a great amount of electrical energy when 

 excited through their nerves. So intense is the action of this electrical 

 apparatus that it can be used as a weapon of defence and offence. Got eh 

 and Burch state that the electric organ of the malapterurus electricus can 

 give a shock having an electric potential of 200 volts. 1 



1. Methods of Ascertaining the Electrical Condition of a Muscle or a Nerve. — 

 If the electric tension of any two parts of an object differs, the instant they are joined an 

 electric current will flow from the point where the tension is greater to that where it is less. 

 The presence, direction of flow, and strength of an electric current can be detected by an 

 instrument called a galvanometer. If any two parts of a muscle or nerve, as e, e, Figure 

 62, be connected by suitable conductors with the coils, c, c, of a galvanometer, and if there 

 be a difference in the electric potential of the two parts examined, an electric current will 

 be indicated by the instrument. In such tests all extra sources of electricity are to be 

 avoided, therefore the electrodes applied to the muscle must be non-polarizable. 



The Giilriiitu/iu fer — An ordinary form of galvanometer consists of a magnet suspended 

 by an exceedingly delicate fibre of silk, or quartz, and one or more coils, composed of many 

 windings of pure copper wire, placed vertically near the magnet and in the plane of the mag- 

 netic meridian. If an electric current be allowed to flow through the wire, it influences the 

 magnetic field about it, and, if the coils he close to the suspended magnet, causes the 

 magnet to deviate from the plane of the magnetic meridian in one or the other direction, 

 according to the direction of the flow of the current. In the more delicate instruments the 

 influence of the earths magnetism is lessened by the use of two magnets of as nearly as pos 

 sible the same Strength, placed so as to point in opposite directions, and fastened at the 

 extremities of a light rod. As each magnet tends to point toward the north, they mutually 

 Oppose each other, and therefore I lie effect of I lie earth's magnetism is partly compensated. 

 Still another magnet may he brought near this " astatic " combination, and by opposing the 

 action of the earth's magnetism make the arrangement even more delicate. In the Thomp- 

 son galvanometer, the rod connecting the needles hears a slightly concave mirror, from 

 which a beam of lighl can lie reflected on a scale. Or a scale may be placed so that iis 

 image falls on the mirror, and the slightest movement of the magnet may be read in the 

 mirror by a telescope. 



The ordinary galvanometer is influenced by changes in the magnetism of the earth. 

 and by earth currents which may he due to an escape of electricity from neighboring 

 street-car circuits, etc. These disturbances may interfere with the use of the instrument, 

 1 Proceedings of the Royal Society, 1900, vol. lxv. p. I 12. 

 Vol. ir.— in 



