556 



PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



be stimulated, it will be found that the blood pressure will steadily fall 

 until it may be not more than one-third of the normal height (Fig. 235). 

 During the production of this decrease of blood pressure no change 

 occurs in the rate of pulsation of the heart. It must be, therefore, due to 

 the diminution of the resistance in the peripheral vascular system. If the 

 splanchnic nerves be divided previous to this experiment, no subsequent 

 decrease in pressure will take place. The conclusion from this is evi- 

 dent. When this nerve, which has been termed the " depressor of Lud- 

 wig and Cyon " (the superior cardiac branch of the vagus), is stimulated 

 the impulses pass to the vaso-motor centre in the medulla, and there so 

 alter the impulses coming from it that it is unable to maintain the nor- 

 mal degree of contraction of the abdominal vessels, through the failure of 

 the influence which it normally exerts on these vessels through the 

 splanchnic nerves. The abdominal vessels, consequently, dilate and 



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Fig. 235.— Blood-Pressure Tracing Obtained by Stimulating the De- 

 pressor Nerve in a Rabbit. (Foster.) 



The current entered the nerve at C and was shut off at 0. The intervals on the line T represent seconds. 



draw off so much blood from the general arterial system that the 

 blood pressure may be reduced two-thirds or more. On the other hand, 

 if in an animal placed under curare the central end of the divided 

 sciatic nerve be stimulated, directly opposite effects will be produced. 

 Without any change in the heart's rate of pulsation the pressure will 

 gradually rise until it may be one-third higher than before the experi- 

 ment : here, again, the increased pressure being, evidently, due to constric- 

 tion of the local arterioles, for previous division of the splanchnic nerves 

 will largely interfere with the production of an increased blood pressure. 

 These variations in blood pressure are of the greatest importance in 

 governing the general character of the circulation. 



The effects of local and general variations in vascular tone have 

 been admirably formulated by Foster : — 



Let us suppose that any artery, A, is in a condition of normal tone— 



