THE MUSCULAR SYSTEM 40 1 



fresh ; Unstriped muscle requires less frequent stimulation than 

 striped. 



All the ordinary voluntary muscular contractions of the body 

 have usually been regarded as tetanic in nature — viz., as a series 

 resulting from a succession of impulses passed into the muscle so 

 rapidly that there is no interval for relaxation. Other investi- 

 gations have supported the view that in all probability a voluntary 

 contraction is a prolonged single contraction, caused by one long 

 constant stimulus. On the other hand, it has been urged, on the 

 basis of experiments made by stimulating the motor areas of the 

 cerebral cortex with stimuli of varying frequency, that a motor 

 cell cannot discharge a single impulse ; it must be a series, and 

 the normal rate in man under the influence of the will is ten per 



Fig. 119. — Summation Curves of Muscle Contraction (Waller). 



The lower curve is one obtained by stimulating the muscle ten times every 

 second ; the intervals of relaxation are clearly seen, though there is a slight 

 summation shown by the slanting rise of the tracing. In the middle curve 

 the shocks were twenty per second, and the relaxation is only of a very 

 partial kind. The upper curve is that of tetanus. 



second. Until recently the chief evidence in favour of an 

 ordinary voluntary contraction being tetanic was that supplied 

 by the musical note furnished by a contracting muscle. This note 

 Helmholtz considered corresponded to twenty vibrations per 

 second. The electrical oscillations of the string galvanometer 

 support the view that the shortest possible voluntary contrac- 

 tions are tetanic in character. 



Elasticity and Extensibility. — Elasticity is the property a body 

 possesses of returning to its shape after stretching ; extensibility 

 is the power a material possesses of stretching. A piece of steel 

 is elastic, a piece of putty is extensible. If a steel spring or a . 

 piece of elastic be tested by loading them with weights, the 

 stretching is proportional to the weight employed ; but if muscle 

 be gradually weighted, it is found that the greatest degree of 



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