564 CIRCULATION OF BLOOD AND LYMPH. 



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, Goltz has shown that 

 when the entire cord is destroyed, except the cervical region (p. 145), vascular 

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

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

 coats of the arteries themselves reacting to the stimulus of the internal pres- 

 sure, or to the activity of the sympathetic nerve cells that give rise to the 

 postganglionic fibers. Under normal conditions it seems quite clear that the 

 great vasoconstrictor center in the medulla is the important seat of [tonic and 

 of reflex activity. If the connections 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 



Fig. 235. Rhythmical vasomotor waves of blood-pressure in a dog (Traube-Hering 

 waves). The upper tracing (1) is the blood-pressure record as taken with the mercury 

 manometer; the lower tracing (2) is taken with a Hiirthle manometer. Seven distinct 

 respiratory waves of blood-pressure may be recognized on each large wave. (Dawson.) 



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. 235), 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- 



