216 



THE CIRCULATION OF THE BLOOD 



sor 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 



FIG. 205. Blo9<i- Pressure Record (lower) and Respiratory Record (upper) Obtained from a 

 Dog upon Stimulating the Central End of the Divided Vagus, Both Vagi being Cut. The marked 

 fall in blood pressure is due to the effect of stimulating the depressor fibers contained in the vagus 

 trunk of the dog. (New figure by Dooley and Dandy.) 



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

 pressure. Thus the effect 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, but the local effects are the all-im- 

 portant ones, since by these the local regulation of the blood flow is accom- 

 plished. 



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

 cal impulses by which andulations 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 de- 

 monstrated 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 mav be upward of ten of the respiratory undulations in one Traube- 



