54 



A MANUAL OF VETERINARY PHYSIOLOGY 



auricular nerve. The phenomenon of inhibition is not shown 

 immediately on the application of the stimulus ; there is at least 



one contraction of the heart 

 before it slows or ceases to 

 beat. This delay is known 

 as the latent period (Fig. 24) . 

 In some animals, such as the 

 cat, even the stronger stimu- 

 lation of the vagus only slows 

 the heart ; it does not stop 

 it ; whereas with the dog 

 relatively weak stimulation 

 may bring the heart to a 

 standstill. Fig. 25 shows the 

 effect on blood-pressure of 

 weak and stronger stimula- 

 tion of the vagus. 



Inhibition does not last 

 long ; it may, of course, 

 cause death, but as a rule it 

 is overcome and the heart 

 starts again. Moderate 

 stimulation of the vagus 

 sufficient to slow the rate 

 may be tolerated for some 

 time. In the frog the same 

 escape from inhibition may 

 be seen. The heart subse- 

 quently makes up for lost 

 time by working at a 

 greater rate or strengthen- 

 ing its contractions, until 

 it regains its usual rate of 

 working. When an in- 

 hibited heart behaves in 

 this way, it is spoken of 

 as secondary augmentation, 

 and the phenomena, 

 though best seen in the 

 frog, may also, though in 

 a less marked degree, be 

 seen in the mammal. 

 Reflex inhibition of the 

 heart may arise through a sensory surface, as in painful im- 

 pressions, or a blow on the abdomen, and in other ways. 

 Afferent impulses — that is, impulses conveyed to a nerve 



Fig. 22. — Nerves on-the Surface of the 

 iIorse's Heart (Pettigrew). 



a, Nerve descending to the auricle ; b, c, 

 coronary vessels ; e, d, vessels in an- 

 . terior ventricular furrow. 



Fig. 23. — Nerves on the Surface of the 

 Left Ventricle of the Calf (Petti- 

 grew). 



The nerves take a spiral direction, like^the 

 U muscle fibres ; c, apex of ventricle. ~ . 



