238 THE CIRCULATION OF THE BLOOD 



of the vaso- motor center and, therefore, a diminution of the tension in the 

 blood vessels. This diminishes the overstrain on the heart in propelling 

 blood into the already too full or too tense arteries. It has been shown by 

 Porter and Beyer that the fall in blood pressure, following stimulation of the 

 depressor nerve, will still occur, even when the abdominal vaso-constriction 

 is kept constant by a simultaneous stimulation of the splanchnics. It is 

 therefore evident that the inhibitory effect of depressor- nerve stimulation is 

 a general one and not confined to the splanchnic area alone. 



The action of the depressor nerve in causing an inhibition of the vaso- 

 motor center illustrates the more unusual effect of afferent impulses; that is, 

 inhibition of the vaso-constrictor tone. As a rule, the stimulation of the 

 central end of an afferent nerve, such as the sciatic or the internal saphenous, 

 produces the reverse, i.e., a pressor effect, and increases the tonic influence 

 of the center which by causing constriction of the arterioles raises the blood 

 pressure. Thus the reflex effects of stimulating an afferent nerve may be 

 either to constrict or to dilate the arteries. These reflexes may be general 

 enough to influence the general blood pressure or they may be limited to 

 definite local areas, but the local effects are the all-important ones, since 

 by these the local regulation of the blood flow is accomplished. 



Traube-Hering Curves. The vaso-motor center sends out rhythmical 

 impulses by which undulations of blood pressure of a large and sweeping 

 character are produced, quite independent of the so-called respiratory un- 

 dulations. The action of this center in producing such undulations is dem- 

 onstrated in the following observations. In an animal under the influence 

 of curari and with both vagi cut, and a record of whose blood pressure is 

 being taken, if artificial respiration be stopped, the blood pressure rises 

 sharply at first. After a time the rhythmical undulations shown in figure 206 

 occur. These variations are called Traube's or Traube-Hering curves. 

 There may be upward of ten of the respiratory undulations in one Traube- 

 Hering curve. They continue until the vaso-motor center is asphyxiated 

 and the heart exhausted, when the pressure falls. The undulations cannot 

 depend upon anything but the vaso-motor center, as the mechanical effects 

 of respiration have been eliminated by the curari and by the cessation of 

 artificial respiration, and the effect of the cardio-inhibitory center has been 

 removed by the division of the vagi. The rhythmic rise of blood pressure 

 is most likely due to a rhythmic constriction of the arterioles followed by a 

 rhythmic fall of pressure and relaxation, both being due to the action of the 

 vaso-motor center. The vaso-motor center, therefore, is capable of pro- 

 ducing rhythmical undulations of blood pressure. 



Vaso -dilator Nerves. Claude Bernard discovered (1856) that the 

 blood flow was increased through the salivary glands by stimulation of the 

 nerves (the chorda tympani for the submaxillary, and the tympanic branch 

 of the glossopharyngeal for the parotid), thus proving that the arteries have 



