NERVOUS CONTROL OF THE BLOOD-VESSELS 1003 



The general result therefore will be to cause dilatation of the blood-vessels 

 of the part where the carbon dioxide is produced and where it is present 

 m greatest concentration, and vascular constriction elsewhere under the 

 influence of the sensitive nervous centres. 



FLOW IN CUBIC CENTIMETRES PER MINUTE PER 100 GRM. TISSUE 



ACTION OF ADRENALINE. This substance, produced by the supra- 

 renal glands, has also a marked influence on the calibre of the blood-vessels. 

 If 1 c.c. of a 1 in 10,000 solution of this substance is injected into the jugular 

 vein, there is at once a universal constriction of the arterioles with the 

 exception of those of the brain. If the vagi are cut we obtain a simultaneous 

 augmentor action of this drug on the heart and constrictor effect on the 

 blood-vessels, so that the arterial pressure rises to an enormous extent, up 

 to 300 mm. Hg. or more. The same result occurs after section of the vaso- 

 motor nerves or after destruction of the brain and spinal cord, so that there 

 is no doubt that adrenaline acts directly on the blood-vessel wall. The 

 action of this drug as a whole is therefore largely to augment the energy of 

 the circulation. The arterial pressure rises, and the blood will be therefore 

 travelling at a much greater pace through any part of the body where the 

 vessels are maintained in a dilated condition, e.g. in an active muscle, or 

 where there are no vaso-motor nerves, as in the vessels of the brain. It is 

 therefore not surprising that we have evidence of the secretion of adrenaline 

 in increased quantities into the blood during any condition of stress. When- 

 ever the splanchnic nerve is stimulated, there is an increased production of 

 adrenaline. On this account the rise of pressure produced under these 

 circumstances shows a stepped curve, the first rise being due to the direct 

 action of the vaso-motor nerves of the blood-vessels, the second being 

 brought about by the stimulation of the suprarenals and the discharge of 

 adrenaline into the general circulation. Simultaneously with this second 

 rise of blood-pressure we notice in the curve given in Fig. 473 a diminished 

 volume of the heart due to more effective contraction of this organ. 



This diminished volume of the heart is often associated with a marked 

 quickening of the heart-rate, both effects being due to the action of adrenaline 

 on the heart. During asphyxia the rise of arterial pressure is largely brought 

 about through the intermediation of the splanchnic nerves and is therefore 

 also associated with the discharge of adrenaline. It is on this account that 



