250 THE CIRCULATION OF THE BLOOD 



muscles, 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 203. 



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.) 



Vaso-dilator Centers. No distinct medullary center has yet been shown 

 to regulate the vaso-dilator nerve activity. Such centers, if they exist, 

 should be influenced by isolating them from their efferent paths, on the one 

 hand, or by stimulation by afferent channels, on the other. The former 

 method of study has revealed nothing that can be compared to the tonic ac- 



