1110 



PHYSIOLOGY 



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 respira- 

 tion, being much slower in their rhythm than the latter. They are 

 observed not only during asphyxia, but may occur in blood-pressure 



FIG. 460. Blood-pressure tracings showing S. Mayer curves. (C. J. MARTIN.) 



tracings from normal dogs, and are frequent in dogs poisoned with 

 morphia. Fig. 460 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 super- 

 posed on the slow prolonged curves, i.e. the Mayer curves. The lower 

 curve is taken immediately after cessation of the artificial respira- 

 tion, 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 hemorrhage. 

 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 



