ELEMENTARY DEMONSTRATIONS 135 



is now exposed and divided. The heart accelerates, and the arterial 

 pressure rises. This is very marked in the morphinised dog. The 

 vagus centre tonically controls the rhythm of the heart. 



The Effect of Posture. The animal is placed on a swing board, with 

 the arterial cannula in the axis of rotation. On dropping the animal 

 into the vertical posture, with the head up, the arterial pressure falls. 

 It may rise again to, or even beyond, the normal level in the cat. In 

 the hutch rabbit the pressure falls, until the medullary centres become 



FIG. 116. Arterial pressure ; effect of asphyxia. Animal anaesthetised and curarised. At A 

 the artificial respiration was stopped. The large oscillations are Traube-Hering curves. (L.H.) 



paralysed from anaemia. The weight of the blood in the vertical 

 posture is supported by the taut skin, the tone of the skeletal muscles, 

 and the tone of the arterial system. 



The blood is largely returned to the heart by the action of the skeletal 

 muscles, aided by the valves in the veins, and the respiratory pump. 



If the spinal cord be divided in the lower cervical region, or the 

 administration of chloroform be pushed, these mechanisms are paralysed, 

 and the blood congests in the lower parts, and the heart fails to fill. In 

 such case the circulation is immediately restored by placing the animal 

 in the horizontal posture. 



Asphyxia. The trachea is clamped. Note the sequence of events. 



1st stage : Respirations deeper and more ample ; heart accelerated and 

 more forcible. In the normal animal loss of consciousness now occurs 

 and convulsive movements. 



2nd stage: Respiration convulsive, less frequent; blood presure rising; 

 heart slow. At the end of second minute the pupils dilate and emission 

 takes place of urine and faeces. The veins are congested with black blood. 



3rd stage : The inspirations, which have occurred at longer and longer 

 intervals, finally cease. The heart beats slowly and with great force. 

 Finally the heart accelerates, and the blood pressure falls to zero. 



