THE NERVOUS CONTROL OF THE BLOOD VESSELS 1031 



During the period of increased pressure, waves are often observed on the 

 bloo4-pressure curve. These are of two kinds. In completely curarised animals 

 we may observe oscillations of blood pressure corresponding with the respira- 

 tory rhythm before the administration of curare, or if the vagi are cut, presenting a 

 rhythm similar to that usual in animals with divided vagi. They are certainly due to 

 irradiation of impulses from the excited respiratory centre to the vaso-motor centre in 

 the medulla. In fact if the curarisation is not complete, a slight twitch of the diaphragm, 

 insufficient by itself to have any mechanical influence on the circulation, may be observed 

 to accompany each rise on the blood-pressure curve. Besides these curves others are 

 occasionally seen which must arise in a slow rhythmic variation of the constrictor 

 impulses sent out from the vaso-motor centre. These waves are known as the Traube 

 curves and are not to be confused with the waves on an ordinary pressure curve due to 

 respiration, being much slower in their rhythm than the latter. They are observed not 



FIG. 466. Blood-pressure tracings showing Traube curves. (C. J. MARTIN.) 



only during asphyxia, but may occur in blood-pressure tracings from normal dogs, and 

 are frequent in dogs poisoned with morphia. Fig. 466 represents tracings obtained from 

 a dog under the influence of morphia and curare. The upper curve, taken while artificial 

 respiration was being carried on, shows the three forms of curves the oscillations due 

 to the heart beat next in size those due to the respiratory movements, which in their 

 turn are superposed on the slow prolonged curves. The lower curve is taken immediately 

 after cessation of the artificial respiration, and shows only the heart beats and the 

 Traube curves. The presence of these waves may generally be ascribed to a state of 

 abnormal excitation of the vaso-motor centre. This excitation may arise in various 

 ways. A very frequent cause is the one just described, viz. increased venosity of the 

 blood supplied to the centre.. Well-marked Traube curves are often observed in cases 

 of haemorrhage. In spite of the loss of blood, the vaso-motor centres maintain a normal 

 arterial blood pressure by means of vascular constriction. As the bleeding continues, 

 this means becomes inadequate, and at this point the ' efforts ' of the centres take on a 

 rhythmic character, giving well-marked Traube curves, just as the arm of a man holding 

 up a weight begins to shake before he is obliged to give way through fatigue. If the 

 bleeding still continues, the pressure sinks steadily and the curves disappear. The 

 curves may also be often observed during operations involving exposure of the cord, 

 and may possibly be ascribed in this case to abnormal irritations ascending the posterior 

 columns. 



The vaso-motor centre may also be directly affected by drugs such as digitalis or 

 strophanthus, both of which cause a rise in general blood pressure from stimulation 

 of the centre. 



