HEART 421 



The reflex stimulation of the centre may be used to 

 determine its position. It can be induced after removal of 

 the brain above the medulla, but destruction of the medulla 

 entirely prevents it. 



(8) The action of this centre may be checked or inhibited. 

 This happens when the diastolic filling is markedly increased 

 as a result of increased venous inflow, and since it does 

 not occur after section of the vagi it is a central effect. 



3. Mode of Action. — These inhibitory fibres appear to act 

 by stimulating the intra-cardiac nervous mechanism. When 

 these peripheral neurons have been poisoned by atropine, the 

 vagus cannot act {Practical Physiology). Nicotine, which 

 poisons the synapses between the vagus nerve and the terminal 

 neurons, also prevents stimulation of the vagus from slowing 

 the heart, but, as is shown by experiments on the heart of 

 the frog, direct stimulation of the terminal neurons does 

 act. 



Gaskell found that stimulating the inhibitory fibres causes 

 a positive variation of the current of injury, indicating that 

 the difference between the living part of the heart and the 

 injured part is increased (p. 213), and he concluded that 

 they excite anabolic changes in the heart. 



4. Result of Action. 



(a) The rate of both auricles and ventricles is slowed, 

 but the effect on the auricles is more marked than upon the 

 ventricles. The rigl^t vagus has usually the most marked 

 effect upon the rate of the heart (fig. 182, A.). 



(b) The force of contraction of the auricles is decreased. 

 In the ventricles the systole becomes less complete and the 

 cavities become more and more distended, partly as 

 a result of decrease in the force of contraction, partly 

 as a mechanical result of over-distension due to the 

 decreased output. 



(c) Conduction, especially from auricles to ventricles, is 

 decreased so that the ventricles may not contract with each 

 auricular contraction, and a condition of heart-block may be 

 established. This is usually best marked upon stimulation 

 of the left vagus. 



C Sympathetic Fibres.— The outgoing fibres are the aug- 



