VASODILATOR NERVES 239 



not only vaso-constrictors, but also vaso-dilator nerves. Vaso-dilator nerves 

 have been described for most parts of the body. In general they are dis- 

 tributed in the same nerve trunks which bear the vaso-constrictors. 



It is not supposed that the vaso-dilators produce widening of the 

 arterioles by stimulation to active muscular contraction; in fact, the circular 



FIG. 206. Traube-Hering's Curves. (To be read from left to right.) The curves 

 i, 2, 3, 4, and 5 are portions selected from one continuous tracing forming the record of a 

 prolonged observation, so that the several curves represent successive stages of the same 

 experiment. Each curve is placed in its proper position relative to the base line, which is 

 omitted; the blood pressure rises in stages from i to 2, 3, and 4, but falls again in stage 5. 

 Curve i is taken from a period when artificial respiration was being kept up, but, the vagi 

 having been divided, the pulsations on the ascent and descent of the undulations do not 

 differ; when artificial respiration ceased, these undulations for a while disappeared, and the 

 blood pressure rose steadily while the heart-beats became slower. Soon, as at 2, new 

 undulations appeared; a little later, the blood pressure was still rising, the heart-beats still 

 slower, but the undulations still more obvious (3); still later (4), the pressure was still 

 higher, but the heart-beats were quicker, and the undulations flatter; the pressure then 

 began to fall rapidly (5), and continued to fall until some time after artificial respiration was 

 resumed. (M. Foster.) 



arrangement of the muscle fibers would seem to exclude such a deduction. It 

 is probable that there is local inhibition of the tonic contraction of the mus- 

 cles, thus allowing the mechanical factor of the general blood pressure to 

 dilate the vessels. The vaso-dilator nerves are characterized by their re- 

 sponse to slowly developed stimuli, shown by Bowditch and Warren, and by 

 the retention of irritability after degeneration of the constrictors has taken 

 place, see figure 207. 



