MOTOR NERVES AND MUSCULAR CONTRACTION. 259 



to the needs of any degree of muscular contractility the vol- 

 untary (conscious or unconscious) movement requires. In 

 other words, the arteries, and to a certain extent the veins, become 

 constricted or dilated proportionately as the muscle is contracted 

 or retracted, and the activity of the motor mechanism is thus 

 concurrently adjusted to the functional requirements of the 

 moment. 



If this conception is not erroneous, the general process 

 it represents certainly constitutes a marvelously simple way 

 of accomplishing many of the most important functions of 

 the organism, since those ascribed to both vasoconstrictor and 

 vasodilator nerves are thus performed without, indeed, leaving 

 a single reason for the presence of either of these nerves as in- 

 dependent entities. 



This normally suggests the question: Do vasodilators 

 actually exist in muscular vessels? That there are vasodilators 

 in certain organs: the submaxillary and other salivary glands, 

 the tongue, the penis, etc., is thought to have been experi- 

 mentally demonstrated; but their existence in the muscular 

 vessels has not been shown, the evidence adduced, as was the 

 case with the constrictors, being purely inferential. We have 

 seen that section of the sciatic, followed by stimulation after 

 some time had elapsed, caused dilation of the vessels of the 

 extremities, while earlier it had caused constriction. Close 

 analysis also showed that actual dilation of the arteries could 

 be ascribed only to an active dilator action. The only remain- 

 ing feature that requires elucidation, therefore, is the manner 

 in which this is carried out. Does the above-described process 

 account for these contradictory phenomena as well as would 

 vasodilator nerves? That the elastic lamina of the vessel can 

 fulfill precisely the same function as the latter seems evident. 



The impulse-wave being accepted as the governing factor, 

 each point of the muscular coat is caused to recede or approach 

 from the axial line of the vessel just sufficiently to bring the 

 caliber of that vessel to the required limits. Hence the pres- 

 ence of the muscular coat over the elastic lamina. The cause 

 and purposes of the latter^ longitudinal corrugations now be- 

 come apparent: while perfectly elastic, it is somewhat larger 

 in diameter than the muscular layer, and, lying, as it does, 



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