256 VETERINAEY PHYSIOLOGY 



the rabbit stimulation of the 5th cranial nerve by the inhala- 

 tion of ammonia vapours has this action, and in all animals 

 stimulation of the abdominal nerves produces the same effect. 

 This reflex stimulation of the centre is used to determine its 

 position in the medulla. It can be induced after removal of 

 the brain above the medulla, but destruction of the medulla 

 entirely prevents it. 



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

 by stimulating the local inhibitory mechanism in the heart; 

 and when this has been poisoned by atropine, they cannot 

 act. According to the observation of Gaskell, they excite 

 in the heart anabolic changes, since the electric current of 

 injury is increased when they are stimulated, indicating that 

 the difference between the living part of the heart and the 

 injured part is increased. 



4. Result of Action. 



(a) The output of blood from the heart is diminished, and 

 thus less blood is forced into the arteries, and the blood 

 pressure falls (fig. 126). 



(6) The rhythm of both auricles and ventricles is slowed, 

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

 ventricles, and the ventricles may show a contraction rhythm 

 independent of that of the auricles (fig. 121, A.). 



(c) 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, either as a result 

 of decrease in the force of contraction or as a mechanical 

 result of the accumulation of blood due to the decreased output 

 per unit of time. In the heart of the tortoise excitability 

 and conductivity are decreased, and the auricular contraction 

 may fail to pass to the ventricles. 



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

 mentors and accelerators of the heart's action. When they are 

 cut the heart may beat slower. When the peripheral end is 

 stimulated, the rate and force of the heart are increased. 



1. Course of, the Fibres. These are small medullated fibres. 

 They leave the spinal cord by the anterior roots of the 2nd, 

 3rd and 4th dorsal nerves, passing to the stellate ganglion 

 where they have their cell stations (fig. 120). From the cells 

 in this ganglion non-medullated fibres run on in the annulus 



