268 PHYSIOLOGY. 



inhibit the main vasomotor center and permit the arterioles to dilate, 

 and, opening the flood-gates, thus relieve the systolic strain of the 

 muscular fibers of the heart. The depressor nerve is not in constant 

 action and is not easily fatigued. 



It has been stated that the depressor nerve acts like a safety-valve 

 to the heart. Sewall and Steiner found on division of the depressor 

 nerves that with few exceptions the blood-pressure rose 1 to 3 centi- 

 meters of mercury. This would seem to indicate a tonic action of 

 these nerves. They also clamped the carotids in rabbits, and this 

 anaemia of the medulla stimulated the vasoconstrictor center and 

 elevated blood-pressure. If, however, they divided the depressors and 

 then occluded the carotid arteries, the rise of arterial tension was much 

 greater. These data prove in the first experiment that the activity of 

 the depressors prevented a great rise of pressure by the inhibition of 

 the vasoconstrictor center and necessarily dilatation of the arterioles 

 of the splanchnic area. 



Von Cyon has shown that iodothyrin augments the irritability of 

 the depressor nerve. 



The depressor is greatly called into play in the heart of the bicycle- 

 rider, where the abdominal reservoir of blood is compressed by the 

 active abdominal muscles, and the blood is driven into the thoracic 

 cavity and the heart is swollen with blood. The depressor cannot well 

 dilate the abdominal vessels, for they are compressed in bicycle-riding 

 by the violent compression of the muscles of the abdomen. 



After an injection of pyocyanine, irritation of the depressor fails 

 to lower blood-pressure. After the use of strychnia, the irritation of 

 the depressor, causes a rise instead of a fall of arterial tension. 



This nerve facilitates the escape of blood from the ventricles. At 

 the same time it indirectly reduces the flow of blood in the auricles and 

 the work of the heart lessens. The slow heart-beat contributes to 

 diminish the work of the heart. The defence of the heart from excess 

 of intra-cardiac pressure is the slowing of the heart and the vaso-dila- 

 tion especially of the intestinal arterioles. 



Accelerators. The accelerators act upon the tonus and rhythm of 

 the heart. In 186'6 E. and M. Cyon proved that in a curarized dog 

 with the depressors, vagi, cervical sympathetics and splanchnics divided 

 that electric irritation of the cut cord was followed by a considerable 

 acceleration of the heart without a change in blood pressure. The 

 increase of heart-beats often amounts to 20 per cent. In this experi- 

 ment the cord must exert its influence on the heart through the 

 inferior cervical and the first thoracic ganglia. After they extirpated 



