MUSCLE-NERVE PHYSIOLOGY, 50* 



equally distant or iso-electrio points (fig. 319, 6, 7, 8). The cut ends 

 are always negative to the equator. These currents are constant for some 

 time after removal of the muscle from the body, and in fact remain us 

 long as the muscle retains its life. The;/ are in all probability due to 

 chemical changes going on in the muscles. 



The currents are diminished by fatigue and are increased by an increase 

 of temperature within natural limits, if the uninjured tendon be used as 

 the end of the muscle, and the muscle be examined without re- 

 moval from the body, the currents are very feeble, but they are at once 

 much increased by injuring the muscle, as by cutting off its tendon. 

 The last observation appears to show that they are right who believe 

 that the currents do not exist in uninjured muscles in situ, but that in- 

 jury, either mechanical, chemical or thermal, will render the injured 

 part electrically negative to other points on the muscle. In a frog's 

 heart it has been shown, too, that no currents exist during its inactivity, 

 but that as soon as it is injured in any way they are developed; the in- 

 jured part being negative to the rest of the muscle. The currents which 

 have been above described are called either natural muscle currents or 

 currents of rest, according as they are looked upon as always existing in 

 muscle or as developed when a part of the muscle is subjected to injury ; 

 in either case, up to a certain point, it is agreed that the strength of the 

 currents is in direct proportion to the injury. 



Muscle in Activity. 



The property of muscular tissue, by which its peculiar functions are 

 exercised, is its Contractility, which is excited by all kinds of stimuli ap- 

 plied either directly to the muscles, or indirectly to them through the 

 medium of their motor nerves. This property, although commonly 

 brought into action through the nervous system, is inherent in the 

 muscular tissue. For (1.) it may be manifested in a muscle which is 

 isolated from the influence of the nervous system by division of the nerves 

 Supplying it, so long as the natural tissue of the muscle is duly nourished; 

 and (2.) it is manifest in a portion of muscular fibre, in which, under 

 the microscope, no nerve-fibre can be traced. (3.) Substances such as 

 l^urari, which paralyze the nerve-endings in muscles, do not at all dimin- 

 ish the irritability of the muscle itself. 



(4.) When a muscle is fatigued, a local stimulation is followed by a 

 contraction of a small part of the fibre in the immediate vicinity without 

 any regard to the distribution of nerve-fibres. 



The Conditions which Affect the Irritability of Muscle that 

 is, its readiness of response to stimuli are numerous. The chief causes 

 of variation in irritability are the following: 



Blood-Supply. The irritability of muscles is also soon lost, unless a 



