The Bloodvessels. 66 



reduction is sudden, and in the veins the pressure is very 

 low (see Fig. 2). 



In the carotid of the rabbit the pressure is equivalent to 

 3 feet of blood ; in the capillaries it has fallen to 1 or 1 5 

 inches, and in the veins to 1| to 2 inches only. It is pos- 

 sible for the blood pressure to be negative; such is observed 

 in the great veins opening into the heart and also in the 

 auricles, the negative pressure being produced by the act of 

 inspiration ; by its presence the flow of blood into the heart 

 is facilitated. 



The blood pressure in the veins is from one-twentieth to 

 one-tenth of that in the corresponding arteries ; these 

 results have been obtained on vessels near the heart. The 

 greater distance the veins are from the heart the greater 

 the pressure. M'Kendrick states that experiment has 

 shown the blood pressure in the external facial vein of 

 the sheep to be 15 inches, in the brachial 2 inches, and in 

 the crural vein 5*8 inches of blood. 



Capillary pressure is very difficult to ascertain. Du Bois 

 Reymoni calculates it as being one-half that in the large 

 arteries ; it is probable that in many places it is much less 

 than this. 



The larger undulations obtained in blood-pressure ex- 

 periments are not all variations in pressure due to the 

 heart-beat, but largely in part to the respiratory move- 

 ments, the expiratory effort having on the whole the effect 

 of reducing the pressure in the vessels, whilst, conversely, 

 inspiration raises the pressure within them. 



The conditions affecting the rise and fall of blood pressure 

 have been dealt with on p. 57 ; we have here only to notice 

 the effect of blood-letting upon it. Blood pressure, contrary 

 to what we might anticipate, is not affected until two- fifths 

 of the blood in the body have been removed. 



We have described the recoil of the bloodvessel following 

 on distension. This elastic recoil produces what is known 

 as Tension, between which and blood pressure a clear dis- 

 tinction must be drawn. Blood pressure is the pressure of 

 the blood on the coats of a vessel, whereas tension is the 



