144 



PRACTICAL PHYSIOLOGY 



depressor nerve. On excitation the blood-pressure slowly falls, and 

 remains at a lower level so long as the excitation is maintained. The 

 rhythm of the heart is as a rule unaffected. The second vagus nerve 

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



RDSkTATLOCKl-ONDON 



FIG. 141. Bering's apparatus for demonstrating the action of the respiratory pump. 

 A, Glass bell, thorax ; B, air-tight base ; K, diaphragm ; C, trachea leading to lungs ; I, 

 manometer ; B, tube opening into A ; F, heart with valves V. The action of the dia- 

 phragm pumps air in and out of the lungs and water through the heart. The lungs and 

 heart are thin rubber bags. 



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. A swing board can be 

 improvised thus : through two staples on the under surface of the board 

 on a level with the point of insertion of the carotid cannula, an iron 

 rod is passed and the end of this clamped to two stout retort stands. 

 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 



