ee 6: - 
the system; 1. “Variations i in the blood pressure. 2. Variations in the 
resistance to the outflow of that fiuid from the arteries. 
As the capacity of the arteries, including the ventricles, varies 
directly as the blood pressure, and as the flow of blood from the capil- 
- laries does the same, the frequency of the heart’s beats is dependent 
on the resistance to the capillary outflow, and not at all on the blood 
pressure ; in other words, the heart always recommences to beat when 
the blood pressure in the x eras arteries has fallen a certain inva- 
riable proportion. 
Variations in blood pressure result from: 1. Absorption into, and 
excretion from, the vascular system of fluids. 2. Changes in the 
capacity of the arterial system, which occur on the contraction or 
relaxation of the muscular arteries. 3. Changes in the amount of Page 28. 
available blood, which result from the hemastatic dilatation of some 
of the yielding vessels on altering the position of the body. As 
changes in the first of these cannot be very sudden, and those in the 
latter are never very considerable, the mean blood pressure in health 
varies but little during short intervals. 
Variations in peripheral resistance result from: 1. Different de- 
grees of tonicity or patency of the muscular arteries. 2. Different 
resistances in the venous system. The former may occur indepen- 
dently in one or other system of vessels, as the cutaneous or the 
alimentary ; also mechanically from pressure on a part of the body. 
The latter are insignificant in health. 
The heart depends for its power of doing work on chemical pro- 
perties in the blood it pumps into the systemic vessels, and as the 
blood reaches it direct from those vessels, the cardiac intramural 
circulation varies with the changes in the former; and the length of 
the systole varying only as the square root of the time of diastole, the 
degree of cardiac nutrition varies directly as the systemic blood pres- 
sure, and as the square root of the diastolic time. The coronary 
arteries supplying the whole heart, the work done by the right 
ventricle is governed by that done in the left; thus the supply of 
blood in the left auricle is always rendered sufficient for the require- 
ments of the systemic circulation; though, as there is no reason for 
believing that the resistance in the pulmonary vessels varies with that 
of the systemic, there must be some peculiarities in the former circu- Page 29. 
lation (which may explain the variations in the ratio of the number of 
pulse-beats to respirations, in some cases). 
The auricular contraction is a very small force, and its function is 
most probably to close the tricuspid and mitral valves. 
The heart commencing its systole as a whole, it is highly probable 
that the impulse for action is given by a force which affects both 
