NERVOUS CONTROL OF THE BLOOD-VESSELS 1111 



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, 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. 



SPINAL CENTRES 



The great fall of blood pressure observed after section of the 

 cord in the lower cervical region is not permanent. After one or 

 two hours the pressure begins to rise, and, if the animal be kept 

 alive, may attain a height only a little inferior to that found in normal 

 animals. 



If the spinal cord of such an animal be destroyed, the blood 

 pressure sinks practically to zero and the circulation comes to an end, 

 because the animal has been, so to speak, bled to death into its own 

 dilated blood-vessels. In addition to the chief vaso-motor centre 

 in the medulla there is a series of subsidiary centres in the spinal 

 cord, centres which we may probably locate in the portions of grey 

 matter situated in the lateral horns of the cord and giving origin 

 to the fibres which go to make up the white rami communicantes. 

 By means of these spinal centres a certain degree of adaptation is 

 possible between the blood-supply of the various parts of the trunk. 

 The important co-ordination between the state of the blood-vessels 

 and the condition of the central pump, the heart, is, however, wanting, 

 since the blood-vessels are now cut off. from the cardiac centres and 

 from the part of the central nervous system which receives the afferent 

 impulses carried by the vagi. 



The spinal centres, like the chief vaso-motor centre, are susceptible 

 to changes in the composition of the blood supplied to them. If an 

 animal be kept alive by means of artificial respiration for a little time 

 after division of the cord just below the medulla, the blood pressure 

 slowly rises as the spinal centres begin to take on their automatic 

 functions. If artificial respiration be now discontinued the asphyxia 

 excites the centres of the cord. The motor discharge to the skeletal 

 muscles reveals itself in a single prolonged spasm, since the respiratory 

 centre is unable to take any part in directing the motor discharges. 

 Simultaneously with the spasm of the skeletal muscles general 



