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A TEXTBOOK OF PHYSIOLOGY 



autonomic system. They arise from a centre in the medulla oblongata 

 by the middle of the lowermost group of vagus roots, and the pre- 

 ganglionic fibres have their cell-stations in the ganglion cells of the 

 heart. These ganglion cells lie chiefly in the subpericardial tissue, 

 in the posterior wall of the auricles between and around the orifices 

 of the vense cavse and pulmonary veins, and between the aorta and 

 pulmonary artery. The right nerve goes particularly to the ganglion 

 cells in the neighbourhood of the simi-auricular node, through 



FIG. 76. CARDIAC NERVES OF DOG. (Foster.) 



which certain of the post-ganglionic fibres act. The inhibitory 

 fibres run chiefly in this nerve. The centre is in tonic action, and 

 constantly bridles the heart's action, and this when the vagi are 

 divided, the frequency of the heart increases and the blood-pressure 

 rises. During stimulation of the peripheral end of the vagus the 

 arterial pressure falls and the vena cava pressure rises (Fig. 77). 

 The vagus centre is reflexly excited by the inhalation of chloroform, 

 ammonia, or other vapour irritant to the air-passages; also by 

 the want of oxygen in the blood, as in asphyxia. It may be 



