260 Organic Physics. 
and powerful contraction. The chemical action is explosive, as 
in gunpowder touched by fire. It seems a special arrangement 
for the production of energetic action from slight contact ir 
fluence. : 
There are other interesting features in this action of the heart 
Each of its divisions is insulated from the others. The contrac- 
tion of the auricles cannot be communicated to the ventricles, 
since they have no nervous or muscular connection. Each acts 
singly and successively. When the blood rushes from the vena 
cava into the auricle the muscles of the latter are relaxed, and the 
cavity readily receives the current. But the pressure of this it- 
flowing blood affects the nerves in the walls of the cavity, quick 
and powerful muscular contraction ensues, and the blood is driven 
onward into the ventricle. The auricle, emptied of its blood and 
relieved from pressure, immediately relaxes. At the same time 
the ventricle similarly affected, contracts in like manner, and 
drives the blood into the arteries. The ventricle relaxes as the 
arteries contract, in response to a like influence. Thus by sut- 
cessive contractions and relaxations, induced by pressure of the 
blood current upon the nerve extremities, it is driven in successWè 
pulses throughout the body.! hess: 
The heart has no opportunity for the long intervals of repos 
enjoyed by the remainder of the body. It, in company with the | 
respiratory muscles, must be continually active. And yet it g 
have its intervals of sleep, however brief they be. If repair cannot 
accompany waste, it must have an opportunity to succeed T 
The muscles of the heart have a sleeping and a waking ap 
with every successive pulse. Contact with the blood current 
awakes them to action, rapid oxidation takes place, and at the | 
same time nutrient molecules may exude from theif blood 
capillaries into the tissues. With the cessation of the blood pres- 
sure the muscle cells sink again into repose, rapid nutrition -i 
and ev 
‘ee ert o 
by a slight touch, thus showing the long continued sensitiveness of its MY” 
