282 ANIMAL PHYSIOLOGY. 
But when we consider the lymphatic system new light will, 
it is hoped, be thrown upon the subject of the nature and the 
influences which modify the capillaries. One thing will be 
clear from what has been said, that even normally the capil- 
laries must exert an influence of the nature of a resistance, 
owing to their peculiar vital properties; and, as we have 
already intimated, such considerations should not be excluded 
from any conclusions we may draw in regard to tubes that are 
made up of living cells, whether arteries, veins, or capillaries, 
though manifestly the applicability to capillaries with their 
less modified or more primitive structure is stronger. 
It has now become clear that the circulation may be modi- 
fied either centrally or peripherally; that a change is never 
purely local, but is correlated with other changes; that the 
whole is, in the higher animals, directly under the dominion 
of the central nervous system; and that it is through this 
part chiefly that harmony in the vascular as in other sys- 
tems and with other systems is established. To have ade- 
quately grasped this conception is worth more than a knowl- 
edge of all the details. 
SPECIAL CONSIDERATIONS. 
Pathological_Changes may take place either in the sub- 
stance of the cardiac muscles, in the valves, or in the blood-ves- 
sels, of a nature unfavorable to the welfare of the body. Some 
of these have been incidentally referred to already. 
Hypertrophy, or an increase in the tissue of the heart, is 
generally dependent on increased resistance, either within or 
without the heart, in the region of the arterioles or capillaries. 
Imperfections of the aortic valves may permit of regurgitation 
of blood, entailing an extra effort if it is to be expelled in addi- 
tion to the usual quantity, which again leads to hypertrophy ; 
but this is often succeeded by dilatation of the chambers of the 
heart one after the other, and a host of evils growing out of 
this, largely dependent on imperfect venous circulation, and 
increased venous pressure. And it may be here noticed that 
arterial and venous pressures are, as a general rule, in inverse 
proportion to each other. 
If the quantity of blood in the ventricle, in consequence 
of regurgitation, should prove to be greater than it can lift 
(eject), the heart ceases to beat in diastole; hence some of the 
sudden deaths from disease of the aortic valves, 
4 
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