CH. XXI.] THE VASO-MOTOR CENTRE 303 



of vaso-motor tone, and of tone in certain viscera after destruction of extensive 

 tracts of the spinal cord, or the persistence of peristaltic action in the intestine 

 after cutting through all its nerves, are cases in point. (See further, under 

 Intestinal Movements, and Spinal Visceral Reflexes). 



The observations of W. M. Bayliss on the vaso-diiator nerves of dogs are of 

 considerable interest. He could find no vaso-dilator fibres to the hind limb in the 

 abdominal sympathetic chain ; but the only fibres excitation of which produced 

 vascular dilatation there, are contained in the posterior roots. He also found fibres 

 in the posterior roots of the 12th and 13th thoracic nerves, which act as vaso-dilators 

 of the small intestine. Not only is vaso-dilatation the result of mechanical, electri- 

 cal, or thermal stimulation of these roots, but experiments are adduced which show 

 that in normal reflexes, such as occur when the depressor nerve is stimulated, the 

 dilator impulses travel by the same route. This raises the question whether the 

 posterior roots contain true efferent fibres. The facts of degeneration show that 

 they do not. Bayliss is therefore driven to the conclusion that the same nerve 

 terminations in the periphery serve both to take up sensory impressions, and to 

 convey inhibitory impulses to the muscular structures in which they end. In other 

 words, we have here another example which may be added to those previously 

 mentioned (p. 173), that nerve-fibres may convey impulses in both directions. The 

 term antidromic is used by Bayliss to express the fact that impulses may travel in 

 the reverse direction to that in which they usually pass. 



The Vaso-motor centre can be excited directly, as by induc- 

 tion currents; the result is an increase of arterial blood -pressure 

 owing to an increase of the contraction of the peripheral arterioles. 



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. We shall study asphyxia 

 more at length in connection with respiration. During the period of 

 decreased pressure, waves are often observed on the blood-pressure 

 curve which arise from a slow rhythmic action of the vaso motor 

 centre. The centre alternately sends out stronger and weaker con- 

 strictor impulses. They are known as the Traube-ffering waves, and 

 are much slower in their rhythm than the waves on the tracing 

 which are due to respiration. They are not peculiar to asphyxia, but 

 are frequently seen in tracings from normal animals. Fig. 298 

 represents tracings obtained from a dog under the influence of 

 morphine and curare. The upper curve, taken while artificial respira- 

 tion was being carried on, shows the three sets of waves, first the 

 oscillations due to the heart-beats, next in size those due to the 

 respiratory movements, which in their turn are superposed on the 

 prolonged Traubs-Hering waves. The lower tracing was taken 

 immediately after the cessation of the artificial respiration, and shows 

 only the heart-beats and the Traube-Hering waves. 



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



