544 CIRCULATION OF BLOOD AND LYMPH. 



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 hi 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 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 hi 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. 227. Rhythmical vasomotor waves of blood-pressure (Traube-Hering waves). 

 The upper tracing (l).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. 227), 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. 



