216 A Manual of Veterinary Physiology. 



slowly, and is not capable of performing the same amount 

 of work. If the contractions in a muscle succeed each 

 other with considerable rapidity, there is no period of 

 relaxation, and the part is thrown into a condition of 

 tetanus or cramp. During muscular contraction a sound 

 is produced, which under suitable conditions may be 

 heard. 



All these changes can be studied with considerable 

 accuracy by employing muscles isolated from the body, 

 and placed in contact with an apparatus on which they 

 can describe their movements when irritated, and at the 

 same time we can study the electrical phenomena in 

 nerves. 



Muscle Currents. — Great controversy has taken place over 

 the question as to whether in muscle, currents of electricity 

 exist independently of those which make their appearance 

 when the muscle is stimulated. It has been found, for 

 instance, that a muscle isolated from the body, and placed 

 under suitable precautions in connection with a galvano- 

 meter, demonstrated the presence of electric currents which 

 behaved in a perfectly regular manner, viz., under certain 

 conditions they were always weaker, and under other con- 

 ditions stronger, in passing from one definite point on the 

 muscle to another. They are spoken of as the natural 

 muscle currents, or the currents of rest, and they always 

 pass in a definite direction, viz., from the surface of the 

 muscle to the cut extremity (Fig. 20). If while the gal- 

 vanometer is registering the direction of the natural muscle 

 current we stimulate the muscle preparation, a backward 

 swing of the needle of the instrument indicates a current 

 passing in an opposite direction to the natural muscle 

 current: it is termed the negative variation of the muscle 

 current, or the current of action. The existence of currents 

 of rest in living muscle is denied, and it would appear that 

 they are purely post-mortem phenomena, or when obtained 

 in living muscle are the result of injury due to the needful 

 preparation for the experiment. 



But there can be no doubt that currents are developed 



