NERVOUS CONTROL OF THE BLOOD-VESSELS 1005 



in the whole animal, provided that sufficient oxygen is supplied, very large 

 percentages of carbon dioxide may be inhaled without causing fatal dilata- 

 tion of the heart, the effect of the adrenaline discharged into the blood-stream 

 serving to counteract the injurious influence of carbon dioxide on the heart 

 muscle. These two chemical influences, the local production of carbon 

 dioxide and the discharge of adrenaline into the general circulation, must 

 always be kept in mind in trying to account for the behaviour of the blood- 

 vessels under the most various conditions. Thus in Fig. 470 is shown the 

 effect of temporary stimulation of the splanchnic nerve on the blood-pressure 

 and on the volume of the hind limb of the cat. It will be noticed that the 

 volume of the hind limb increases passively with the rise of pressure and then 

 diminishes much below its previous amount. This diminution is due to the 

 discharge of adrenaline into the blood- stream as the result of stimulation of 

 the splanchnic nerve and is absent if the suprarenals have been previously 

 destroyed. The curve shown in Fig. 471, which with the foregoing one was 

 taken by Bayliss, to indicate a local adaptation of the blood-vessels to their 

 internal pressure, is probably brought about by the local production of carbon 

 dioxide (v. Anrep). Temporary occlusion of the abdominal aorta is here 

 shown to cause first a diminution of the volume of the hind limb, followed by 

 a marked increase. During the period of obstruction the circulation of the 

 hind limb was interrupted and there was therefore accumulation of carbon 

 dioxide in the tissues and around the blood-vessels. This caused a relaxa- 

 tion of the blood-vessel walls and a corresponding increased volume of the 

 limb when the blood was allowed once more to flow by release of the 

 aortic obstruction. 



