ventricle. 4, The left
ventricle. 5, 6, 7, 8, 9, 10, The vessels[13] through which the blood
passes to and from the heart.]

[Illustration: Fig. 67. A back view of the heart. 1, The right auricle.
2, The left auricle. 3, The right ventricle. 4, The left ventricle. 5, 6,
7, The vessels that carry the blood to and from the heart. 9, 10, 11, The
nutrient vessels of the heart.]

[13] All vessels that carry blood to the heart, are called _veins_.
All vessels that carry blood _from_ the heart, are called
_arteries_.

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With what is it surrounded? What is its use? How much fluid does this
membrane contain when healthy?

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_Observation._ In health, there is usually about a tea-spoonful of
fluid in the pericardium. When these parts are diseased, it may be
thrown out more abundantly, and sometimes amounts to several ounces,
producing a disease called dropsy of the heart. But all the unpleasant
sensations in the region of the heart are not caused by an increased
amount of fluid in the pericardium, as this disease is not of frequent
occurrence.

335. The heart is composed of muscular fibres, that traverse it in
different directions, some longitudinally, but most of them in a
spiral direction. The human heart is a double organ, or it has two
sides, called the right and the left. The compartments of the two
sides are separated by a muscular _sep'tum_, or partition. Again, each
side of the heart is divided into two parts, called the _Au'ri-cle_
(deaf ear) and the _Ven'tri-cle_.

[Illustration: Fig. 68. A section of the heart, showing its cavities and
valves. 3, The right auricle. 4, The opening between the right auricle
and right ventricle. 5, The right ventricle. 6, The tricuspid valves. 7,
The pulmonary artery. 9, The semilunar valves of the pulmonary artery.
10, The septum between the right and left ventricle. 12, The left
auricle. 13, The opening between the left auricle and left ventricle. 14,
The left ventricle. 15, The mitral valves. 16, The aorta. 17, The
semilunar valves of the aorta.]

336. The AURICLES differ in muscularity from the ventricles. Their
walls are thinner, and of a bluish color. These cavities are a kind of
reservoir, designed to contain the blood arriving by the veins.

337. The VENTRICLES not only have their walls thicker than the
auricles, but they differ in their internal structure. From the
interior of these cavities arise f