MOTOR NERVES AND MUSCULAR CONTRACTION. 249 



striction being the fundamental requirement of normal activity, 

 the conductivity of both nerves must be equal; hence, this posi- 

 tion may be taken as a working basis. But quite another result 

 is to be expected when, as is actually the case, the sciatic is to 

 be considered as a factor of the problem. A large motor nerve 

 plus the constrictor plus the dilator no longer represents bal- 

 ance as to conductivity, and our analysis must now include, as 

 an element, the fact that the energy distributed to the vaso- 

 dilator nerve will, all else being equal, be as its circumference 

 is to that of the sciatic and the vasoconstrictor combined. 

 When the great size of the sciatic is recalled, it becomes evi- 

 dent that the dilator will at best receive an insignificant pro- 

 portion of the current. Under these circumstances what ex- 

 perimental results could we expect? Section would obviously 

 cause dilation, since the dilator nerves would be cut, and the 

 tonic contraction of the vessels would also be annulled through 

 section of the constrictor. Electrical stimulation of the pe- 

 ripheral stump of all the nerves, therefore, could have but one 

 result, constriction, since the dilators receive practically no 

 current. This agrees perfectly with observed facts. But why 

 the opposite result also observed? This renders it necessary 

 to analyze what evidence there is as to the actual existence 

 of vasoconstrictors in striated muscles. 



"With regard to the vasoconstrictor fibers/' says Professor 

 Foster, "the only evidence that they exist in muscles is that 

 when the nerve of a muscle is divided the blood-vessels of the 

 muscle widen, somewhat like blood-vessels of the ear after 

 division of the cervical sympathetic. This suggests the pres- 

 ence of vasoconstrictor fibers carrying the kind of influence 

 which we called tonic, leading to an habitual moderate constric- 

 tion; it cannot, however, be regarded by itself as conclusive 

 evidence." We have seen that vascular constriction is unmis- 

 takably associated with the sympathetic system: its only source 

 elsewhere. No fibers of the sympathetic are associated with 

 skeletal muscular tissue. In fact, Professor Foster, referring 

 to the latter, says: "The presence of any vasoconstrictor fibers 

 at all has not at present been satisfactorily established. When 

 a muscle contracts there is always an increased flow of blood 

 through the muscle/' thus simultaneously suggesting the pos- 



