RESPIRATORY WAVE IN BLOOD-PRESSURE CURVE. 307 



centres in the medulla cannot have any influence when the cord 

 is cut. 



The blood pressure in the capillaries cannot be directly measured 

 by the means above described ; it is difficult to estimate, and 

 very variable. The slightest change of pressure in the corre- 

 sponding veins or arteries causes the pressure in the capillaries to 

 rise or fall. Thus variations in pressure are constantly occurring 

 in the capillaries, which cause an alteration in the rate of flow, 

 or even a retrograde stream in some parts of the network. 



The regulation of the blood supply, and, therefore, of the 

 pressure in the capillaries, is under the control of the small 

 arterioles which supply them ; a slight relaxation of the muscle 

 of the arterioles causes great increase in the amount of blood 

 flowing through the capillaries, as can readily be seen with the 

 microscope. 



The blood pressure in the veins must be less than that in the 

 capillaries, and, as has been said, must diminish as the heart is 

 approached, where in the great veins (superior cava) the pressure 

 is said to be rather below that of the atmosphere ( 3 to 5 mm., 

 mercury). During inspiration the minus pressure may become 

 much less, whilst, on the other hand, it is only by very forced ex- 

 piration that it ever becomes equal to or at all above that of the 

 atmosphere. 



This is a most important fact, as the suction considerably 

 helps the flow of blood from the veins, and also the current of 

 fluid from the thoracic duct that bears the chyle from the intes- 

 tines and the fluid collected from the tissue drainage back to the 

 blood. 



The pressure of the blood in the veins may then be said to be 

 generally nil, since the veins are nowhere overfilled with blood. 

 The pressures, on the other hand, that can be registered and 

 measured depend upon forces communicated from without, namely: 

 (1) gravity ; (2) the elastic pressure of the surrounding tissue ; 

 and (3) the pressure exerted by the muscle during contraction. 

 This pressure is increased by any circumstance which impedes 

 the flow of blood through the right side of the heart, through 

 any large vein, or through the pulmonary circulation ; but when 



