O74 ANIMAL PHYSIOLOGY. 
nerve terminals, or the ganglia; or does it affect the heart 
through the central nervous system ? 
Resort to comparative physiology is important in such cases, 
if only to foster caution and avoid narrow views. 
THE HEART IN RELATION TO BLOOD- PRESSURE. 
It is plain that all the other conditions throughout the cir- 
culatory system remaining the same, an increase in either the 
force or the frequency of the heart-beat must raise the blood-press- 
ure. But, if the pressure were generally raised when the heart 
beats rapidly, it would fare ill with the aged, the elasticity of 
their arteries being usually greatly impaired. Asa matter of 
fact any marked rise of pressure that would thus occur is pre- 
vented as a rule, and in different ways, as will be seen; but, so 
far as the heart is concerned, its beat is usually the weaker the 
more rapid it is, so that the cardiac rhythm and the blood- 
pressure are in inverse proportion to each other. 
By what method is the heart’s action tempered to the condi- 
tions prevailing at the time in the other parts of the vascular 
system ? 
The matter is complex. It is possible to conceive that there 
is a local nervous apparatus which regulates the beat of the 
heart according to the intra-cardiac pressure, which latter again 
will depend on conditions outside of the heart itself—the arte- 
rial pressure, in fact. It is possible to understand that, apart 
-from any nervous elements at all, the cardiac cells regulate 
their own action in obedience to the impressions made upon 
them. 
But, inasmuch as the heart is not regulated perfectly in the 
mammal according to the blood-pressure, when the vagi nerves 
are cut, and considering the dominance of the central nervous 
system, it does not seem likely that it should resign the con- 
trol of so important a matter. Experiment bears this out. 
There is some evidence for believing that not only may the 
vagus itself act as an afferent sensory nerve, but that the de- 
pressor nerve, to be shortly referred to more particularly, is 
also such a sensory nerve. 
However, such a view does not exclude previously men- 
tioned factors, and there can be little doubt that in forms below 
mammals the muscular tissue is to some degree self-regulative; 
and it is not likely that this quality is wholly lost even in the 
highest mammals. 
