608 CIRCULATION OF BLOOD AND LYMPH. 



4. In the nervi erigentes. Eckhard first gave conclusive proof that the 

 erection of the penis is essentially a vasodilator phenomenon. The 

 fibers arise from the first, second, and third sacral spinal nerves, pass 

 to the hypogastric plexus as the ner^'i erigentes, and thence are dis- 

 tributed to the erectile tissues of the penis. 



The General Properties of the Vasodilator Nerve Fibers.— 

 Unlike the vasoconstrictors, the vasodilators are not in tonic 

 activity; at least, no experimental proof has been given that they 

 are. In the case of the erectile tissue of the penis and the dilators 

 of the glands it would seem that the fibers are in activity only 

 during the functional use of the organ, at which time they are 

 excited retiexly. There has been much discussion in physiology as 

 to the nature of the action of the dilator fibers. The muscular 

 coat of the small arteries runs transversely to the length of the 

 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 incomplete, 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 they inhibit the tonic contrac- 

 tion of this musculature and thus bring about a relaxation. These 

 fibers in fact inhibit the tonic contraction of the vascular muscle 

 just as the vagus fibers inhibit the tone of the cardiac muscle. 

 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 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 ob- 

 tained, 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. t 



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



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



