988 



PHYSIOLOGY 



During the period of increased pressure waves are often observed on the blood- 

 pressure curve. These are of two kinds. In the first place, 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. These waves are known as 

 the Traube-Hering curves from the physiologists by whom they were first observed. 

 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 the Traube-Hering 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 Mayer curves, and are not to be confused 

 with the waves on an ordinary pressure curve due to respiration, being much slower in 



FIG. 457. Blood-pressure tracings showing S. Mayer curves. (C. J. MARTIN.) 



their rhythm than the latter. They are observed not only during asphyxia, but may 

 occur in blood-pressure tracings from normal dogs, and are frequent in dogs poisoned 

 with morphia. Fig. 457 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, i.e. the Mayer curves. The lower curve is taken imme- 

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

 the Mayer curves. The presence of Mayer curves 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 Mayer 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 Mayer 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, 



