284 VETERINARY PHYSIOLOGY 



various states of the brain, accompanied by emotions, may 

 stimulate part of the vaso-dilator mechanism, as in the act 

 of blushing. 



Again, it has been shown that stimulation of the central 

 end of the depressor nerve (superior cardiac branch of the 

 vagus) causes a dilatation of the arterioles chiefly in the 

 abdominal cavity, but also throughout the body generally. 

 This is the most generalised vaso-dilator reflex known 

 (see p. 254). 



(b) Position. While the dominant vaso- constrictor centre 

 is in the medulla, the vaso-dilator centres seem to be dis- 

 tributed throughout the medulla and spinal cord. 



II. PRESSURE IN THE CAPILLARIES 



This may be determined by finding the pressure required 

 to blanch the skin or to occlude the capillaries of some 

 transparent membrane. 



It has already been shown that the pressure is less than in 

 the arteries and greater than in the veins. 



Like the pressure in the arteries, it depends upon the two 

 factors 



1st. Eorce of inflow. 



2nd. Resistance to outflow. 



1st. Variations in the Force of Inflow. The capillary 

 pressure may undergo marked local changes through the 

 vaso-motor mechanism. Wherever the function of a part is 

 active, dilatation of the arterioles and an increased capillary 

 pressure exists, and, when the influence of vaso-dilator nerves 

 is withdrawn, the capillary pressure falls. 



But the capillary pressure may also be modified by the 

 heart's action, inasmuch as the arterial pressure, by which 

 blood is driven into the capillaries, depends upon this. In 

 cardiac inhibition not only is arterial pressure lowered, but 

 capillary pressure may also fall. In augmented heart action 

 both arterial and capillary pressure are raised. 



2nd. Variations in Resistance to Outflow. Normally the 

 flow from capillaries to veins is free and unobstructed; but, 

 if the veins get blocked, or if the flow in them is retarded by 

 gravity, the capillaries get engorged with the blood which 



