sobbing, although indicating opposite states of the mind, are produced
in very nearly the same manner. In hiccough, the contraction is more
sudden and spasmodic than in laughing or sobbing. The quantity of oxygen
consumed during sleep is estimated to be considerably less than that
consumed during wakefulness.

[Illustration: Fig. 45.
View of the pulmonary circulation.]

It is difficult to estimate the amount of air taken into the lungs at
each inspiration, as the quantity varies according to the condition,
size, and expansibility of the chest, but in ordinary breathing it is
supposed to be from twenty to thirty cubic inches. The consumption of
oxygen is greater when the temperature is low, and during digestion. All
the respiratory movements, so far as they are independent of the will of
the individual, are controlled by that part of the brain called the
_medulla oblongata_. The respiratory, or breathing process, is not
instituted for the benefit of man alone, for we find it both in the
lower order of animals and in plant life. Nature is very economical in
the arrangement of her plans, since the carbonic acid, which is useless
to man, is indispensable to the existence of plants, and the oxygen,
rejected by them, is appropriated to his use. In the lower order of
animals, the respiratory act is similar to that of the higher types,
though not so complex; for there are no organs of respiration, as the
lungs and gills are called. Thus, the higher the animal type, the more
complex its organism. The effect of air upon the color of the blood is
very noticeable. If a quantity be drawn from the body, thus being
brought into contact with the air, its color gradually changes to a
brighter hue. There is a marked difference between the properties of the
venous and the arterial blood.