CHAP, iv.] THE VASCULAR MECHANISM. 155 



is, by means of the forceps, at present shut off from the heart. On 

 removing the forceps a direct communication is at once established 

 between the tube and the artery below ; in consequence the blood from 

 the heart flows through the artery into the tube. 



This experiment shews that the blood as it is flowing into the 

 carotid is exerting a considerable pressure on the walls of the 

 artery. At the moment when the forceps is removed, there is 

 nothing but the ordinary pressure of the atmosphere to counter- 

 balance this pressure within the artery, and consequently a 

 quantity of blood is pressed out into the tube ; and this goes on 

 until the column of blood in the tube reaches such a height that 

 its weight is equal to the pressure within the artery, whereupon 

 no more blood escapes. The whole column continues to be raised 

 a little at each heart beat, but sinks as much during the interval 

 between each two beats, and thus oscillates, as we have said, 

 above and below a mean level. In a rabbit this column of blood 

 will generally have the height of about 90 cm. (3 feet) ; that is to 

 say, the pressure which the blood exerts on the walls of the carotid 

 of a rabbit is equal to the pressure exerted by a column of rabbit's 

 blood 90 cm. high. This is equal to the pressure of a column 

 of water about 95 cm. high, and to the pressure of a column of 

 mercury about 70 mm. high. 



If a like tube be similarly introduced into a corresponding 

 vein, say the jugular vein, it will be found that the column of 

 blood v similarly formed in the tube, will be a very low one, not 

 more than a very few centimeters high ; and that while the level 

 of the column may vary a good deal, owing as we shall see later 

 to the influence of the respiratory movements, there will not, as 

 in the artery, be oscillations corresponding to the heart beats. 



We learn, then, from this simple experiment, that in the carotid 

 of the rabbit the blood, while it flows through that vessel, is 

 exerting a considerable mean pressure on the arterial walls, equi- 

 valent to that of a column of mercury about 70 mm. high, but that 

 in the jugular vein the blood exerts on the venous walls a very 

 slight mean pressure, equivalent to that of a column of blood a few 

 centimeters high, or of a column of mercury three or four milli- 

 meters high. We speak of this mean pressure exerted by the 

 blood on the walls of the blood vessels as blood pressure, and we 

 say that the blood pressure in the carotid of the rabbit is very 

 high (70 mm. Hg.), while that in the jugular vein is very low (only 

 3 or 4 mm. Hg.). 



In the normal state of things, the blood flows through the 

 carotid to the arterial branches beyond, and through the jugular 

 vein towards the heart ; the pressure exerted by the blood on the 

 artery, or on the vein is a lateral pressure on the walls of the 

 artery and vein respectively. In the above experiment the pres- 

 sure measured is not exactly this, but the pressure exerted at the 

 end of the artery (or of the vein) where the tube is attached. We 



