256 PHYSIOLOGY CHAP. 



out in Mosso's laboratory with a sphygmonmnometer applied to 

 the fingers of a healthy man are only partially in agreement with 

 Pawlow's. 



He found that when a man is removed from all external and 

 internal influences liable to produce disturbance of the vasomotor 

 functions, the lateral pressure of the digital arteries fluctuates 

 constantly within 24 hours between a minimum of 65 and a 

 maximum of 100 mm. Hg (average 80-85). The greatest decrease 

 occurs after meal times, the greatest rise during hours most 

 removed from meals. The daily curve of blood pressure must 

 accordingly run an opposite course to that of pulse frequency and 

 temperature. As a matter of fact the pulse is accelerated when 

 arterial pressure falls, and becomes slower when it rises, which is 

 apparently the expression of some compensatory mechanism. 



The depressor effect of meals is certainly due to active vascular 

 dilatation of the digestive organs, and possibly to the entrance of 

 a small amount of peptones into the circulation. Colombo found 

 that the highest fall of pressure (which may amount to 20 mm. Hg) 

 is apparent two hours after meals, when the absorption of the 

 products of digestion is beginning. 



The introduction of a large quantity of milk, contrary to what 

 might be expected, produces a rise in pressure, in consequence of 

 the overloading of the circulation, which cannot apparently be 

 compensated by depression of the vascular tone. 



Alcohol, chloroform, opium, warm baths, sleep, lower the blood 

 pressure ; cold baths and coffee raise it. 



Gymnastics and massage of the limbs and back produce a rise, 

 massage of the abdomen a fall, in blood pressure, probably because 

 they influence the vascular tone in a different degree and in 

 different proportions. 



The pressure in the veins, in consequence of their ready 

 distension, is less subject to fluctuation than arterial pressure. 

 Theoretically it may be assumed that all the circumstances that 

 produce a rise or fall in arterial pressure, cause, or may cause, a 

 change in venous pressure in the opposite direction. Venous 

 pressure is specially affected by : 



(a) Increment or decrement in amount of blood. 



(j) Eespiratory movements (to be discussed later). 



(c) Position of the body (according to the laws of hydrostatics). 



VI. Observations on the velocity of the circulation have 

 been made in two opposite directions. On the one hand, it has 

 been attempted to determine the mean velocity with which the 

 blood moves in any given artery ; on the other, to establish the 

 form of the pulsatory changes of velocity. 



At present we must confine ourselves to the methods employed 

 for ascertaining the first point. 



By mean velocity of the blood in an artery is meant the length 



