418 VETERINARY PHYSIOLOGY 



the vessels. Some means must be provided of preventing a 

 too great rise of arterial pressure and of preventing an over- 

 distension of the heart when the venous return is too great. 

 This is effected by the action of the central nervous 

 system. 



III. The Influence of the Central Nervous System 



on the Heart. 



In the frog, a branch from the vagus connects the central 

 nervous system with the heart. When the branch is cut no 

 effect is produced, showing that it is not constantly in action ; 

 but, when the lower end is stimulated, the heart is generally 

 slowed or brought to a standstill. Sometimes the effect is 

 not produced. The reason for this is that the cardiac 

 branch of the vagus in the frog is really a double nerve, 

 derived in part from the spinal accessory, and in part from 

 fibres which reach the vagus from the superior thoracic 

 sympathetic ganglion. If the spinal accessory nerve, or the 

 medulla oblongata from which it springs be stimulated, the 

 heart is always slowed ; and if the sympathetic fibres are 

 stimulated, it is quickened. Generally, stimulation of the 

 cardiac branch containing these two sets of fibres simply 

 gives the result of stimulating the former, but sometimes 

 the stimulation of the latter masks this effect {Practical 

 Physiology). 



In the mammal three sets of nerve fibres pass to the 

 heart : — 



Ist. The superior cardiac branch of the vagus starts 

 from near the origin of the superior laryngeal nerve, and 

 passes to the heart to end in the endocardium (fig. 181, S.G.). 



2nd. The inferior cardiac branch of the vagus leaves the 

 main nerve near the recurrent laryngeal, and passes to join 

 the superficial cardiac plexus in the heart (fig. 181, I.C.). 



2rd. The 8yin])athetic nerve fibres come from the superior 

 thoracic and inferior cervical ganglia, and also end in the 

 suparficial cardiac plexus (fig. 181, >Si.). 



l.s^. The Superior Cardiac Branch of the Vagus is an ingoing 

 nerve. Section produces no effect ; stimulation of the lower 



