5* 



A MANUAL OF VETERINARY PHYSIOLOGY 



communication with the centre which presides over the vascular 

 system, a centre by whose varying activities the arteries of the 

 body are made smaller (constricted) or larger (dilated) , according 

 to the needs of the system. If the heart is labouring and its 

 muscular structure becoming weakened, impulses pass up the 

 depressor to the vasomotor centre, resulting in impulses being 

 sent out which cause the abdominal arteries to dilate and hold 

 more blood. By this means the peripheral resistance is dimin- 

 ished, the blood-pressure falls, and the heart is eased, since it 

 now has less work to do in ejecting its contents. 



If the depressor nerve be divided, no effect follows ; if the end 

 in contact with the heart be stimulated, there is no result ; '|but 



if the central or upper 

 end be stimulated, the 

 blood-pressure falls (see 

 Fig. 26). 



By some the depres- 

 sor nerve has been de- 

 scribed as the sensory 

 nerve of the heart, as 

 there are signs of pain 

 when it is stimulated 

 in an animal not under 

 an anaesthetic. But 

 this view is not gene- . 

 rally accepted, and as 

 a matter of fact the 

 heart may be handled, 

 pinched, pricked, or 

 otherwise injured, with- 

 out provoking the least 

 sign of pain on the part 

 of the animal. Colin's 

 experiments in this 

 direction on horses appear quite conclusive. Not only is it 

 considered that the external surface is insensible to pain, but 

 the internal surface also ; for, as previously noted, the ex- 

 perimental introduction of foreign bodies into the cavities 

 of the heart appears to produce no pain. Under pathological 

 conditions the results are otherwise ; foreign bodies, so common 

 in the heart of the cow, cause great suffering, therefore there 

 must be sensory nerves, though normally their excitability is 

 probably low. 



It is supposed that in the heart or aorta the sensory nerve 

 endings are stimulated by contraction, and that the impulses so 

 obtained are conveyed to the medulla by the depressor, and 



Fig. 26. — Blood-Pressure Tracing : Rabbit 

 (Mercury Manometer) (Stewart). 



Central end of depressor stimulated at 1 ; stimu- 

 lation stopped at 2. Time trace, seconds. 



