The heart is a hollow muscular basket which begins to beat about ten days after conception and stops only with death. The cavity consists of four chambers. Two of them, called auricles, receive blood from the veins. Two, called ventricles, push blood into the arteries.
Although the heart is an automatic motor, it cannot govern its own speed. Two nerves and two body chemicals do that. The accelerator nerve and adrenalin (hormone of the adrenal glands) speed up the heart. The vagus nerve and acetylcholine (hormone produced by the vagus nerve) slow down the...
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