68 A MANUAL OF VETERINARY PHYSIOLOGY 



with a striking instance of this when dealing with the action of 

 the depressor nerve on blood-pressure, through the medium of 

 alterations in the arteries which supply the splanchnic area, 

 brought about by means of the splanchnic nerve. 



Blood-Pressure. — From what has been said, it is hardly neces- 

 sary to define blood-pressure as the pressure exercised upon the 

 blood in the elastic vessels, resulting from the action of the 

 heart and the peripheral resistance. 



If the peripheral resistance is great through a contraction of 

 the arterioles, the amount of fluid passing into the veins is 

 reduced in quantity ; a larger bulk of fluid will in consequence 

 exist between the pump and its outlets, and the blood-pressure 

 rises. If, on the other hand, the blood is passing freely through 

 the dilated arterioles, the blood-pressure falls. When the heart 



Fig. 27. — Diagram to illustrate the Slope of Pressure along the]| 

 Vascular System (Stewart). 



A, Arterial ; C, capillary ; V, venous tract. The interrupted line represents the 

 line of mean pressure in the arteries, the wavy line indicating that the pres- 

 sure varies with each heart-beat. The line passes below the abscissa axis 

 (line of zero in atmospheric pressure) in the veins, indicating that at the end 

 of the venous system the pressure becomes negative. 



is more active, or when the arterioles contract, the blood-pressure 

 rises ; when the heart is less active or the arterioles dilated the 

 blood-pressure falls. 



The mean pressure in the arteries is highest close to the aorta 

 and lowest in the region of the periphery ; the fall in pressure 

 from the aorta to the periphery is gradual. At the minute 

 arterioles the fall in pressure is rapid, and in the veins gradual 

 and very slow ; in fact, owing to causes to be dealt with in the 

 chapter on Respiration, a negative pressure may exist in the 

 great veins near the heart. Fig. 27 exhibits in a graphic form 

 the fall in blood-pressure in the different regions of the vascular 

 system, 



