THE VASOMOTOR NERVES. 619 



A most suggestive example of the regulating action of the depressor nerve 

 is given by Sewall. When the carotids in a rabbit are clamped a variable 

 and not very large rise of arterial pressure is observed. If, however, the 

 depressor nerves are first cut, clamping the carotids causes an extraordinary 

 rise of arterial pressure. When the carotids are closed we may suppose that 

 the resulting anemia of the medulla stimulates the vasoconstrictor center 

 and thus tends to raise arterial pressure, but this effect is neutralized because 

 as the pressure rises the depressor fibers of the heart are stimulated. It 

 seems evident that during life the depressor fibers must exert a very important 

 regulating effect upon the circulation. 



A similar nerve has been described anatomically in man, while 

 in animals like the dog, in which it is not present as a separate 

 anatomical structure, it probably exists within the trunk of the 

 vagus. If this latter nerve is cut in the dog and the central end 

 is stimulated a depressor effect is usually obtained. 



Vasoconstrictor Centers in the Spinal Cord. From the description of the 

 vasoconstrictor mechanism given above the probable inference may be made 

 that throughout the thoracic region the cells of origin of the preganglionic 

 fibers may, under special conditions, act as subordinate vasoconstrictor centers 

 capable of giving reflexes and of exhibiting some tonic activity. Numerous 

 experiments tend to support this inference When the spinal cord is cut^ in 

 the lower thoracic region there is a paralysis of vascular tone in the posterior 

 extremities. If, however, the animal is kept alive the vessels gradually re- 

 cover their tone, although not connected with the medullary center. The re- 

 sumption of tone in this case may be attributed to the nerve cells in the lower 

 thoracic and upper lumbar region, since vascular paralysis is again produced 

 when this portion of the cord is destroyed. Finally, Gpltz has shown that 

 when the entire cord is destroyed, except the cervical region (p. 154), vascular 

 tone may be restored eventually in the blood-vessels affected. In this case the 

 resumption of tonicity must be referred either to the properties of the muscular 

 coats of the arteries themselves, or to the activity of the sympathetic nerve 

 cells that give rise to the postganglionic fibers. However this may be, it seems 

 quite clear that under normal conditions the great vasoconstrictor center in 

 the medulla is the important seat of tonic and of reflex activity. If the con- 

 nections of this center with the blood-vessels are destroyed suddenly for 

 example, by cutting the cervical cord blood-pressure falls at once to such a 

 low level, 20 to 30 mms. Hg., that death usually results unless artificial means 

 are employed to sustain the animal. 



Rhythmical Activity of the Vasoconstrictor Center. 



Throughout life the vasoconstrictor center is in tone the intensity 

 of which varies with the intensity and character of the reflex im- 

 pulses playing upon it. Under certain unusual conditions the 

 center may exhibit rhythmical variations in tonicity which make 

 themselves visible as rhythmical rises and falls in the general 

 arterial pressure (Fig. 256), the waves being much longer than 

 those due to the respiratory movements. These waves of blood- 

 pressure are observed often in experiments upon animals, but 

 their ultimate cause is not understood. They are usually desig- 

 nated as Traube-Hering waves, although this term, strictly speak- 

 ing, belongs to waves, synchronous with the respiratory move- 

 ments, that were observed by Traube upon animals in which 

 the diaphragm was paralyzed and the thorax was opened. 

 These latter waves are also due to a rhythmical action of the 



