MUSCLE 



637 



When a strong current is closed through a muscle there is an 

 immediate sharp contraction (initial contraction). The muscle then 

 promptly relaxes, but incompletely. When the current is opened, 

 there is another contraction (Fig. 223). The force of the initial 

 contraction, as measured by the resistance necessary to prevent it, 

 is greater than that of the tonic contraction which follows it. 



A second law of great theoretical importance is that of polar 

 stimulation. At make the stimulation occurs only at the kathode ; at 

 break only at the anode. This is true both for muscle and nerve, but 

 it is most directly and simply demonstrated on muscle. A long 

 parallel-fibred curarized muscle is supported about its middle ; the 

 two ends, which hang down, are connected with levers writing on a 

 revolving drum, and a current is sent longitudinally through the 

 muscle. It is not difficult to see from the tracings that^at make the 

 lever attached to the 

 kathodic end moves 

 first, and that the 

 other lever only 

 moves when the con- 

 traction started at 

 the kathode has had 

 time to reach it in its 

 progress along the 

 muscle. Similarly, 

 at break the lever 

 connected with the 

 anodic end moves 

 first. The law of 

 polar excitation 

 holds both for stri- 

 ated and for smooth 

 muscle. Not only is 

 there no excitation 

 of unstriped muscle 

 at the anode on clo- 

 sure of the current, 

 but a previously ex- 

 isting contraction 

 disappears. For 

 skeletal muscle the 



make is stronger than the break contraction. 

 that this is the case for smooth muscle. 



FIG. 223. TONIC CONTRACTION DURING AND AFTER 

 FLOW OF VOLTAIC CURRENT. 



Curve from frog's gastrocnemius. At M constant 

 current closed, at B broken. Contracture continues 

 after opening of current. Time trace, ' two-second 

 intervals. 



It has not been proved 



The Muscular Contraction. When a muscle contracts, its 

 two points of attachment, or, if it be isolated, its two ends, come 

 nearer to each other ; and in exact proportion to this shortening 

 is the increase in the average cross-section. The contraction is 

 essentially a change of form, not a change of volume. The most 

 delicate observations fail to detect the smallest alteration in 

 bulk (Ewald). Living fibres kept contracted by successive 

 stimuli can be examined under the microscope ; or fibres may be 

 ' fixed ' by reagents like osmic acid, and sometimes a very good 

 opportunity of studying the microscopic changes in contraction 

 is given by a group of fibres in which the ' fixing ' reagent has 



