94 THE CONTRACTILE TISSUES. 



Similar currents may be observed when the longitudinal surface is not 

 the natural but an artificial one ; indeed, they may be witnessed in even a 

 piece of muscle, provided it be of cylindrical shape and composed of 

 parallel fibres. 



These " muscle-currents" are not mere transitory currents, disappearing 

 as soon as the circuit is closed ; on the contrary, they last a very consider- 

 able time. They must, therefore, be maintained by some changes going on 

 in the muscle by continual chemical action, in fact. They disappear as 

 the irritability of the muscle vanishes, and are connected with those nutri- 

 tive, so-called vital, changes which maintain the irritability of the muscle. 



Muscle-currents, such as have just been described, may, we repeat, be 

 observed in any cylindrical muscle suitably prepared, and similar currents, 

 with variations which need not be discussed here, may be seen in muscles 

 of irregular shape with obliquely or otherwise arranged fibres. And Du 

 Bois-Reymond, to whom chiefly we are indebted for our knowledge of 

 these currents, has been led to regard them as essential and important prop- 

 erties of living muscle. He has, moreover, advanced the theory that muscle 

 may be considered as composed of electro-motive particles or molecules, 

 each of which, like the muscle at large, has a positive equator and neg- 

 ative ends, the whole muscle being made up of these molecules in some- 

 what the same way (to use an illustration, which must not, however, be 

 strained or considered as an exact one) as a magnet may be supposed to 

 be made up of magnetic particles, each with its north and south pole. 



There are reasons, however, for thinking that these muscle-currents have 

 no such fundamental origin, that they are, in fact, of surface, and, indeed, 

 of artificial origin. Without entering into the controversy on this question, 

 the following important facts may be mentioned : 



1. When a muscle is examined while it still retains uninjured its natural 

 tendinous terminations, the currents are much weaker than when artificial 

 transverse sections have been made; the natural tendinous end is less nega- 

 tive than the cut surface. But the tendinous end becomes at once negative 

 when it is dipped in water or acid indeed, when it is in any way injured. 

 The less roughly, in fact, a muscle is treated the less evident are the 

 muscle-currents ; and it is maintained that if adequate care be taken to 

 maintain a muscle in an absolutely natural condition, no such currents as 

 those we have been describing exist at all that natural living muscle is 

 isoelectric, as it is called. 



2. The surface of the uninjured inactive 1 ventricle of the frog's heart, 

 which is practically a mass of muscle, is isoelectric ; no current is obtained 

 when the electrodes are placed on any two points of the surface. If, how- 

 ever, any part of the surface be injured, or if the ventricle be cut across 

 so as to expose a cut surface, the injured spot or the cut surface becomes 

 at once more powerfully negative toward the uninjured surface, a strong 

 current being developed which passes through the galvanometer from the 

 uninjured surface to the cut surface or to the injured spot. The negativity 

 thus developed in a cut surface passes off in the course of some hours, but 

 may be restored by making a fresh cut and exposing a fresh surface. 



The temporary duration of the negativity after injury, and its renewal 

 upon fresh injury, in the case of the ventricle, in contrast to the more per- 

 manent negativity of injured skeletal muscle, is explained by the different 

 structure of the two kinds of muscle. The cardiac muscle, as we shall 

 hereafter see, is composed of short fibre-cells ; when a cut is made a cer- 

 tain number of these fibre-cells are injured, giving rise to negativity, but 

 the injury done to them stops with them, and is not propagated to the cells 

 1 The necessity of its being inactive will be seen subsequently. 



