88 A MANUAL OF VETERINARY PHYSIOLOGY 



results are explained by saying that the cord possesses vaso- 

 motor subcentres, and that, given time, these are capable of 

 carrying on the work single-handed. It is even possible to 

 carry the inquiry a stage further, and by destroying all nervous 

 connection, isolate the vessels from their innervation ; even 

 then the vascular tone may be recovered, and it is supposed 

 that it is developed in the wall of the vessel itself, possibly in 

 response to the stimulating influence of variations in their 

 internal blood-pressure. 



It is possible that the nature of the stimulus conveyed by 

 afferent nerves to the vasomotor centre may determine the 

 nature of the reflex which follows, and this is considered probable 



Fig. 38. — Plethysmograms : Hind - Limb of Cat (after Bowditch and 



Warren). 



To be read from right to left. On the left hand is shown the effect of slow stimu- 

 lation of the sciatic (1 per second) ; on the right hand the effect of rapid 

 stimulation (64 per second). In the first case the limb swelled owing to 

 excitation of the vaso-dilators ; in the second it shrank through excitation 

 of the vaso-constrictors. 



from the fact that the electrical stimulation of muscle always 

 produces a rise in blood-pressure, whereas the mechanical stimula- 

 tion by rubbing always causes a fall. The effect of stimulating 

 the divided sciatic was recently referred to as producing con- 

 striction of the bloodvessels and a rise in pressure ; but if the 

 sciatic nerve be cooled and then stimulated, exactly the con- 

 verse effects are produced — viz., a dilatation of the arterioles and 

 a fall in pressure. Not only are the results obtained dependent 

 on the nature of the stimulus, but the state of the centre itself 

 is of the utmost importance in determining the results of reflex 

 vasomotor stimulation. When an animal is deeply under the 

 influence of chloroform or chloral, the functions of the vasomotor 



