212 HANDBOOK OF PHYSIOLOGY. 



ometer the mercury will fall in the arm furthest away from the vein and 

 will rise in the arm nearest the vein, the action being that of suction 

 rather than pressure forward. In the large veins of the neck the ten- 

 dency to suck in air is especially marked, and is the cause of death in 

 some surgical operations in that region. The amount of pressure in the 

 brachial vein is said to support 9 mm. of mercury, whereas the pressure 

 in the veins of the neck may fall to a negative pressure of rather more 

 than inch or about to ^ inch or 3 to 8 mm. 



The variations of venous pressure during systole and diastole of the 

 heart are very slight, and a distinct pulse is never seen in veins except 

 under extraordinary circumstances. From observations upon the web 

 of the frog's foot, the tongue and mesentery of the frog, the tails of 

 newts, and small fishes (Roy and Brown), as well as upon the skin of 

 the finger behind the nail (Kries), by careful estimation of the amount 

 of pressure required to empty the vessels of blood under various condi- 

 tions, it appears that the blood-pressure is subject to variations in 

 the capillaries, apparently following the variations of that of the 



Fig. 178. Normal arterial tracing: obtained with Fick's kymograph in the dog. 

 (Burdon-Sanderson .) 



arteries ; and that up to a certain point, as the extravascular pressure is 

 increased, so does the pulse in the arterioles, capillaries, and venules be- 

 come more and more evident. The pressure in the first case (web of 

 the frog's foot) has been found to be equal to about 1 to f inch or 14 to 

 20 mm. of mercury; in other experiments to be equal to about -J to -J of 

 the ordinary arterial pressure. 



The arterial blood-pressure may be made to vary by variations of 

 either of the two chief factors upon which the pressure in the vessels 

 depends, viz., the cardiac contractions and the peripheral resistance. 

 Thus, increase of blood-pressure may be brought about by either (a) a 

 more frequent or more forcible action of the heart, or (b) by increase of 

 the peripheral resistance; and on the other hand, diminution of the 

 blood-pressure may be produced, either by (a) a diminished force or fre- 

 quency of the contractions of the heart, or by (b) a diminished periphe- 

 ral resistance. These different factors, however, although varying con- 

 stantly, are so combined that the general arterial pressure remains fairly 

 constant; for example, the heart may, by increased force or frequency 

 of its contractions, distinctly increase the blood-pressure, but this in- 

 creased action is almost certainly followed by diminished peripheral 



