452 
RESPIRATION AND EXCRETION 
to all the cells of the body and back to the heart again and 
again. The heart serves as a pump to force the blood along. 
The heart is about the size of the fist and has strong muscu¬ 
lar walls. In a healthy person, it contracts regularly about 
seventy times a minute. It is 
obvious, therefore, that the work 
which the heart does is very 
great . 1 
The heart is located in the 
thoracic , or chest cavity, a little 
to the left side and between the 
lungs. It is a cone-shaped or¬ 
gan, inclosed in a membranous 
bag called pericardium (per-i- 
kar'di-um : Greek, peri, around ; 
cardia, heart). 
The heart is divided by a wall 
into right and left chambers. A 
nearly complete cross partition 
divides each side into upper cham¬ 
bers, the auricles, and the lower 
ones, the ventricles. The opening 
between an auricle and a ventricle 
is guarded by a valve, which is 
partly membranous and partly muscular. The auricles 
receive blood from the veins, while the ventricles force 
blood into the arteries. 
Artery is the name given to the blood vessels which carry 
1 “The work the heart does during the day is about equal to the energy 
expended by man in climbing to the top of a mountain 3600 feet high. 
Assuming that the man weighs about 150 pounds, this would be equal to 
an amount of energy sufficient to lift 90 tons to a height of three feet. The 
work of the left side is greater than that of the right, since the former has 
to drive the blood all over the body, while the latter has only to force it 
to the lungs which are near by. For this reason the muscle walls of the 
right ventricle are much thinner than those of the left ventricle.” — Conn 
AND BUDDINGTON. 
R . V, right ventricle; L. V 
left ventricle ; R. A., right auri¬ 
cle ; L. A., left auricle. The 
arrows indicate the direction 
that the blood takes. Describe 
its course through the heart. 
