NERVOUS CONTROL OF THE BLOOD-VESSELS 1105 

 from all portions of the body, from the higher centres of the brain 

 and the .cortex cerebri, and especially by afferent impulses travelling 

 by the. vagi from the viscera of the chest and abdomen. Whether 

 in the absence of all afferent stimuli the centre would be active is 

 doubtful ; all we know is that the sum of the stimuli arriving at the 

 centre produces a state of average continued activity, which is respon- 

 sible for the maintenance of arterial tone and for the regulation of the 

 arterial blood pressure. 



The centre may also be affected directly by changes in its blood- 

 supply, or in the composition of the blood flowing through it. Thus 

 anything which interferes with the oxygenation of the centre, whether 

 obstruction to respiration, absence of oxygen in the air breathed, or a 

 failure of the blood-supply, as by ligature of the cerebral arteries, calls 

 forth an increased state of activity of the centre. This can be best 

 studied by observing the changes in the blood pressure produced in a 

 curarised animal by the cessation of artificial respiration. 



The changes occurring in the blood pressure in asphyxia depend 

 partly on the stimulation of the vaso-motor and vagus centres by the 

 venous blood, and partly on the affection of the heart itself. These 

 phenomena are best observed in a curarised animal, and we will first 

 consider them with both vagi cut, in order to shut out the action of 

 the vagus centre. The blood pressure is registered by means of a 

 mercurial manometer in connection with the carotid artery. On 

 leaving off the artificial respiration, the blood pressure remains at the 

 same height for twenty or thirty seconds, the only change noticed 

 being the absence of the respiratory oscillations. At this point the 

 blood pressure suddenly rises rapidly (Fig. 457, A), and in another ten 

 seconds may reach a height twice as great as it was previously. The 

 heart beats a little more forcibly in consequence of the increased cardiac 

 tension, but its frequency is almost unaltered. The blood pressure 

 remains at this height for about a minute, and then gradually falls, the 

 heart-beats becoming smaller and smaller, until the pressure has sunk to 

 a point very little above the abscissa line (level of no pressure). This fall 

 in pressure is due to the failure of the heart. The heart, badly supplied 

 with oxygen, cannot overcome the high resistance presented by the 

 contracted arterioles ; it gets overfilled, and gradually loses the 

 power of expelling any of its contents. If, when the blood pressure 

 has sunk to its lowest point, the heart be rapidly cut out of the body, 

 it will begin to beat fairly forcibly, being relieved of the excessive 

 internal tension. The vessels, however, remain constricted until the 

 death of the animal. This is shown by two facts. If, while the 

 pressure is sinking, artificial respiration be recommenced, the heart 

 supplied with oxygen at once begins to beat more forcibly, and the 

 blood pressure may rise to an even greater height than immediately 



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