rough which, and its subdivisions, it is distributed to every
part of the system. The semilunar valves (17) prevent its returning.

_Observation._ The parts of the circulatory organs most liable to
disease are the valves of the heart, particularly the mitral. When
these membranous folds become ossified or ruptured, the blood
regurgitates, and causes great distress in breathing. The operations
of the system are thus disturbed as the movements of the steam engine
would be if its valves were injured, or did not play freely.

357. The difference between the functions of the pulmonary artery and
aorta is, the former communicates with the right ventricle of the
heart, and distributes only impure blood to the lungs; the other
connects with the left ventricle of the heart, and distributes pure
blood to the whole body, the lungs not excepted. At the extremity of
the divisions of the aorta, as well as the pulmonary artery, are found
capillary vessels. This curious net-work of vessels connects with the
minute veins of the body, which return the blood to the heart.

_Observation._ The function of the veins of the systemic circulation
is similar to the office of the arteries in the lungs, and that the
veins of the pulmonic circulation transmit to the heart the pure, or
nutrient blood, and thus supply the arteries of the general system
with assimilating fluid.

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What is the effect when the blood is not changed in the lungs? 356.
Describe the circulation of the blood from the left auricle to the
general system. What part of the circulatory organs is most liable to
disease? What is the effect when the valves are diseased? 357. Give
the difference in the functions of the pulmonary artery and aorta.
Show the relation between the functions of the arteries and veins both
of the pulmonic and systemic circulation.

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358. The veins that receive the blood from all parts of the body,
follow nearly the same course as the arteries. The myriads of these
small vessels beneath the skin, and others that accompany the
arteries, at last unite and form two large trunks, called _ve'na ca'va
as-cend'ens_, and _de-scend'ens_.

_Observation._ A peculiarity is presented in the veins which come from
the stomach, spleen, pancreas, and intestines. After forming a large
trunk, they enter the liver, and ramify like the arteries, and in this
organ they again unite into a trunk, and enter the ascending