CIT. XXIT.] PRESSOR AND DEPRESSOR NERVES 305 



It can also be excited by the action of poisons in the blood which 

 circulates through it ; thus, strophanthus or digitalis causes a marked 

 rise of general arterial pressure due to the constriction of the peri- 

 pheral vessels brought about by impulses from the centre. 



It is also excited by venous blood, as in asphyxia; the rise of 

 blood-pressure which occurs during the first part of asphyxia is due 

 to constriction of peripheral vessels ; the fall during the last stage of 

 asphyxia is largely due to heart failure. When asphyxia is brought 

 about by the cessation of artificial respiration in a curarised animal, 

 waves are often observed on the blood-pressure curve synchronous 

 with the normal rate of respiration. The respiratory centre is 

 making ineffectual efforts to produce breathing movements, and thus 

 it affects its neighbour, the vaso-motor centre, in a parallel manner. 

 Such waves are known as Traube-Hering waves. One, however, 

 frequently sees in tracings of blood-pressure in anaesthetised animals 

 larger waves which arise from a slow rhythmic action of the vaso- 

 motor centre, and which are much slower in their rhythm than 

 those due to respiration. Fig. 266 represents a tracing from a dog 

 which shows these waves (Mayer's waves). The tracing shows three 

 sets of waves, first the oscillations due to the heart-beats, next in 

 size those produced by the respiratory movements, which in their 

 turn are superposed on the prolonged Mayer waves. 



The Vaso-motor centre may be excited reflexly. The afferent 

 impulses to the vaso-motor centre may be divided into pressor and 

 depressor. 



Most sensory nerves are pressor nerves. The sciatic or the vagus 

 nerves may be taken as instances ; when they are divided and their 

 central ends stimulated, the result is a rise of blood -pressure due to 

 the stimulation of the vaso-motor centre, and a consequent constric- 

 4 tion of the arterioles . all over the body, but especially in the 

 splanchnic area. Fig. 267 shows the result of such an experiment. 

 It is necessary in performing the experiment to administer curare 

 as well as an anaesthetic to the animal, in order to obviate reflex 

 muscular struggles, which would themselves produce a rise in arterial 

 pressure. 



Many sensory nerves also contain depressor fibres ; these produce 

 the opposite effect. The most marked bundle of these is known as 

 the depressor nerve. In most animals this is bound up in the trunk 

 of the vagus ; but in some, such as the rabbit, cat, and horse, the nerve 

 runs up as a separate branch from the heart (or, according to some 

 recent observations, from the commencement of the aorta), and joins 

 the vagus or its superior laryngeal branch, and ultimately reaches the 

 vaso-motor centre. When this nerve is stimulated (the vagi having 

 been previously divided to prevent reflex inhibition of the heart), a 

 marked fall of arterial blood -pressure is produced (see fig. 268). 



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