194 



CIRCULATION OF THE BLOOD 



excitation of the vagus, the fall in pressure to a reflex dilatation of the blood 

 vessels. 



The natural assumption with regard to the normal action of the depressor, 

 is that it is stimulated by dilatation of the aorta, when for example the pres- 

 sure there becomes very high so that it is difficult for the heart to empty itself. 

 The blood vessels are dilated as the result of the depressor impulses, and the 

 heart, working now against less resistance, empties itself more easily. Since 

 the heart beats more slowly also as the result of the vagus reflex, it has a 

 better opportunity to recover after the previous overexertion. These conclu- 

 sions have been confirmed by recent observations ; e. g., when high pressure 



FIG. 70. Behavior of the blood pressure on stimulation of the depressor nerve. To be read 

 from right to left. The two vertical lines indicate the time of stimulation. | | = ten 



seconds. 



is produced artificially in the arch of the aorta, an action current appears in 

 the trunk of the depressor (Tschermak), and if both depressors are cut when 

 the pressure is high, the pressure rises still further (Pawlow). 



According to Cyon, the depressor has a third root, central stimulation of 

 which produces an acceleration of the heart; this root is connected with the 

 superior cervical ganglion. 



The heart does not appear to have nerves which mediate tactile sensations ; 

 on the other hand reflexes may be started from its afferent nerves which take 

 effect in the skeletal muscles. 



The heart may be influenced reflexly by a great many other nerves. If one 

 vagus be cut, and the other be left intact, central stimulation of the cut nerve 

 produces a slowing of the pulse, which disappears when the other is cut. 

 Among the different afferent fibers of the vagus, those coming from the lungs 

 are most effective, those from the heart much less so, and those springing 

 from points below the lung fibers are still less effective (Brodie and Eussell). 

 The inhibitory nerves of the heart are excited also by stimulation of the 



