GENERAL PHYSIOLOGY OF MUSCLE 



25 



When, instead of using make and break shocks from an 

 induction coil, a muscle is excited with the constant current, 

 it contracts at each closure or opening of the current, but is 

 relaxed during the passage of the current. This law usually holds 

 good if a current of medium strength is employed, but if the 

 strength of the current exceeds certain limits, the make or break 

 of the current is immediately followed by a tetanus (closure or 

 opening tetanus). This fact, which was first noted by Wundt, 

 can also be observed on man, by sending a strong galvanic current 

 into a muscle, or even a comparatively weak current when the 

 muscle is degenerated. 



Curarised muscles react more readily to the closure and open- 

 ing of a constant current than to the 

 more transitory make and break shocks 

 of an induced current. Hence in ex- 

 amining the rate of transmission of 

 contraction, the constant current is pre- 

 ferable. 



The excitation at make of the con- 

 stant current is greater than at break, 

 as can be seen by varying the amount 

 of current passed through the muscle, 

 by means of a rheochord. 



Von Bezold, Engelmann, and Hering 

 showed that the "law of contraction" 

 which Pfliiger formulated for nerve (see 

 Chap. IV.) holds for muscle also : the 

 closing contraction always starts from 

 the negative pole, while the opening 

 contraction is set up at the positive 

 pole ; in other words the make excita- 

 tion is kathodal, the break excitation is 



anodal. This law may be demonstrated by placing two myograph 

 levers far apart on a curarised muscle, to the two ends of which 

 the two electrodes are applied. At make and break of the 

 current the two contractions are recorded at brief intervals, but 

 the kathodal always precedes the anodal at the closure, and the 

 anodal the kathodal at the opening, of the current. 



V. In order to understand the changes in form which the 

 muscle undergoes during activity, it is necessary to examine the 

 structure of the muscle fibre under the microscope, and the 

 changes which it undergoes during contraction. 



Each muscle fibre consists of soft protoplasm enclosed in an 

 elastic tubular sheath, the sarcolemma. This membrane is so 

 resistant that it is uninjured by a pull strong enough to rupture 

 the muscle substance (Fig. 17). Oval nuclei parallel with the 

 long axis of the fibre generally lie immediately under the sarco- 



Fi 



}. 17. Sarcolemma of mammalian 

 muscle. (Schafer.) Highly magni- 

 fied. The sarcolemma is left clear, 

 owing to rupture of the muscular 

 substance. 



