MOTOR NERVES AND MUSCULAR CONTRACTION. 



261 



A diseased sciatic means impairment not only of its conductiv- 

 ity, but also of that of its subdivisions. The current may or 

 may not, under these conditions, continue to follow the nerve 

 stimulated after the segment buried in the tissues is reached; 

 selecting the best conductors, it will, if it follow the course of 

 the vessels, mainly stimulate the layer offering the least re- 

 sistance: i.e., the thickest and softest, as regards inherent pro- 

 portion of fluids. As this characterizes the elastic layer, it 

 cannot but receive the brunt of the stimulus. We are no 

 longer dealing with muscular vibrations, but with a corrugated 

 elastic tube the normal tendency of which is to expand, level 

 its folds, and increase its lumen. The current, by inducing 

 erethism, encourages this, and produces, by increasing the 

 caliber of the vessels, "flushing and increased temperature of 

 the foot/' Indeed, electricity is a poor substitute for the 

 physiological impulse-wave and sometimes a misleading one. 



We can fully agree with Professor Foster, therefore, when 

 he says: "There is no adequate evidence that these vasodilator 

 fibers serve as channels for tonic dilating impulses or influ- 

 ences," and express our personal opinion that neither vasocon- 

 strictor nor vasodilator nerves exist as separate entities in volun- 

 tary muscles, loth functions being fulfilled through the agency of 

 their motor nerves. 



The following general deductions seem to be warranted 

 by the facts presented: 



The functional activity of a voluntary muscle is dependent 

 upon the following correlated factors: 



1. As to nervous stimulus: an impulse-wave transmitted 

 from a cerebral center through motor nerves, which adjusts the mus- 

 cle to a fixed degree of contraction. As the vibratory rhythm of the 

 impulse and that of the muscle always correspond, any variation of 

 rhythm by the brain-center correspondingly modifies the muscular 

 contraction. 



There are no independent vasoconstrictor or vasodilator nerves 

 in voluntary muscles, both functions being fulfilled by the motor 

 nerves through the filaments distributed to the muscular coat of 

 the vessels of these muscles, and under the influence of the same 

 impulse-wave that adjusts and fixes the tatter's contraction or 

 retraction. 



