546 CIRCULATION OF BLOOD AND LYMPH. 



vessel, and it is easy to see that when stimulated to greater con- 

 traction through the constrictor fibers it must cause a narrowing 

 of the artery. It is not so evident how the nerve impulses carried 

 by the dilator fibers bring about a widening of the artery. At 

 one time peripheral sympathetic ganglia in the neighborhood of 

 the arteries were used to aid in the explanation, but, since histo- 

 logical evidence of the existence of such ganglia is lacking, the 

 view that seems to meet with most favor at present is as follows: 

 The dilator fibers end presumably in the muscle of the walls of 

 the arteries, and when stimulated their impulses inhibit the tonic 

 contraction of this musculature and thus indirectly bring about a 

 relaxation. Dilatation caused by a vasodilator nerve fiber always 

 presupposes therefore a previous condition of tonic contraction in 

 the walls of the artery, this tonic condition being produced either 

 by the action of vasoconstrictor fibers or possibly by the intrinsic 

 properties of the muscle itself. In the nerves of the limbs, as 

 stated above, both vasoconstrictor and vasodilator effects may be 

 detected by stimulation. It has been shown that the separate 

 fibers may be differentiated by certain differences in properties. 

 Thus, if the peripheral end of the cut sciatic nerve is stimulated 

 by rapidly repeated induction shocks a vasoconstrictor effect is 

 obtained as shown plethysmographically by a diminution in volume 

 of the limb. If, however, the same nerve is stimulated by slowly 

 repeated induction shocks the dilator effect will predominate,* 

 indicating a greater degree of irritability on the part of these latter 

 fibers. After section of the sciatic nerve the vasodilators degen- 

 erate more slowly than the vasoconstrictors, and they retain 

 their irritability when heated or cooled for a longer time than 

 the constrictors.f 



Vasodilator Center and Vasodilator Reflexes. Since the 

 vasodilator fibers form a system similar to that of the vasocon- 

 strictors, it might be supposed that, like the latter, their activity 

 is controlled from a general center, forming a vasodilator center in 

 the brain similar to the vasoconstrictor center. What evidence 

 we have, however, is against this view. In the dog with his spinal 

 cord severed in the lower thoracic region the penis may show normal 

 erection when the glans is stimulated, a fact that indicates a 

 reflex center for these dilator fibers in the lumbar cord. For the 

 other clear cases of vasodilator fi,bers we have no reason at present 

 to believe that they are all normally connected with a single group 

 of nerve cells located in a definite part of the nervous system. The 

 dilator fibers in the facial, glossopharyngeal, and cervical sympa- 

 thetic (distributed through the trigeminal) all arise probably in the 



* Bowditch and Warren, " Journal of Physiology, " 7, 439, 1886. 



t Howell, Budgett, and Leonard, " Journal of Physiology, " 16, 298, 1894. 



