52 A MANUAL OF VETERINARY PHYSIOLOGY 



of the experimental material. The physiology of this creature's 

 heart is known more completely than that of any other animal, 

 but what is known cannot always be applied to the mammal, 

 owing to anatomical differences in the arrangements of the nerve. 



In the frog there is a vago-sympathetic nerve in the neck, in 

 which the fibres of both nerves are anatomically mixed, though 

 functionally distinct ; the mixing up does not occur in the chest, 

 as in the mammal, but close under the skull. When this vago- 

 sympathetic is stimulated, either inhibitory or augmentor effects 

 may be obtained, according as to which one of the set of nerve- 

 fibres in the mixed nerve happen to be most efficiently stimulated. 

 The mixing of the fibres takes place in the ganglion of the vagus 

 nerve, which lies just outside the skull. If the intracranial 

 fibres of the vagus are stimulated, the effect on the heart is 

 purely inhibitory, and if the fibres of the sympathetic are stimu- 

 lated just before they enter the ganglion, the effect is entirely 

 augmentor. In the mammal the vagus and sympathetic are 

 distinct, even if, as in some animals, they run in the same sheath ; 

 while accelerator fibres do not join the sympathetic until it 

 enters the thorax. 



In the mammal the vagus arises from the medulla ; the in- 

 hibitory fibres it receives are derived from the spinal accessory 

 and join the vagus within the skull ; the cardiac branches of this 

 nerve are given off from it in the thorax. The sympathetic 

 supply to the heart comes out of the spinal cord at the second 

 and third dorsal nerves, probably others, and by means of the 

 rami communicantes passes to the stellate ganglion, thence to the 

 inferior cervical ganglion on the cervical sympathetic trunk, 

 and from this ganglion they pass to the heart. The above 

 arrangement applies especially to the dog, and is shown diagram- 

 matically in Fig. 21 ; it is not quite the same for all mammals. 

 The rich plexus of nerves on the surface of the horse's heart may 

 be seen in Fig. 22, and that of the calf in Fig. 23. 



Function of the Vagus. — If the vagus in the neck be cut and 

 its peripheral end stimulated, the rate of the heart-beat is slowed 

 and the force of the beat diminished. If stronger stimulation 

 be applied, the heart stops in a condition of dilatation, and 

 becomes swollen with blood. Slowing of the heart-rate is the 

 most prominent effect of vagus stimulation, and this in the 

 mammal is more apparent in its effect on the auricles than on 

 the Ventricles. The strength of the ventricular contraction may 

 continue undiminished at the time the auricles are suffering 

 from inhibition, and should the stimulation be sufficiently strong 

 to cause the auricles to cease contracting, the ventricles for a 

 brief time are inhibited, and then beat again independently. 

 In other words, in the mammal the vagus is essentially an 



