PHYSIOLOGICAL ANATOMY.
THE ORGANS OF RESPIRATION.
THE ORGANS OF RESPIRATION are the Trachea, or windpipe, the Bronchia,
formed by the subdivision of the trachea, and the Lungs, with their
air-cells. The _Trachea_ is a vertical tube situated between the lungs
below, and a short quadrangular cavity above, called the _larynx_, which
is part of the windpipe, and used for the purpose of modulating the
voice in speaking or singing. In the adult, the trachea, in its
unextended state, is from four and one-half to five inches in length,
about one inch in diameter, and, like the larynx, is more fully
developed in the male than in the female. It is a fibro-cartilaginous
structure, and is composed of flattened rings, or segments of circles.
It permits the free passage of air to and from the lungs.
The _Bronchia_ are two tubes, or branches, one proceeding from the
windpipe to each lung. Upon entering the lungs, they divide and
subdivide until, finally, they terminate in small cells, called the
_bronchial or air-cells,_ which are of a membranous character.
[Illustration: Fig. 43.
An ideal representation of the respiratory organs. _3._ The
larynx. _4._ The trachea. _5, 6._ The bronchia. _9, 9, 9, 9._ Air-cells.
_1, 1, 1, 2, 2, 2._ Outlines of the lungs.]
The _Lungs_ are irregular conical organs rounded at the apex, situated
within the chest, and filling the greater part of it, since the heart is
the only other organ which occupies much space in the thoracic cavity.
The lungs are convex externally, and conform to the cavity of the chest,
while the internal surface is concave for the accommodation of the
heart. The size of the lungs depends upon the capacity of the chest.
Their color varies, being of a pinkish hue in childhood but of a gray,
mottled appearance in the adult. They are termed the _right_ and _left_
lung. Each lung resembles a cone with its base resting upon the
diaphragm, and its apex behind the collar-bone. The right lung is larger
though shorter, than the left, not extending so low, and has three
_lobes_, formed by deep fissures, or longitudinal divisions, while the
left has but two lobes. Each lobe is also made up of numerous _lobules_,
or small lobes, connected by cellular tissue, and these contain great
numbers of cells. The lungs are abundantly supplied with blood-vessels,
lymphatics, and nerves. The density of a lung depends upon the amount of
air which it contains. Thus, experiment has shown that in a _foetus_
which has never breathed, the lungs are compact and will sink in water;
but as soon as they become inflated with air, they spread over a larger
surface, and are therefore more buoyant. Each lung is invested, as far
as its root, with a membrane, called the _pleura_, which is then
continuously extended to the cavity of the chest, thus performing the
double office of lining it, and constituting a partition between the
lungs. The part of the membrane which forms this partition is termed the
_mediastinum_. Inflammation of this membrane is called _pleurisy_. The
lungs are held in position by the root, which is formed by the pulmonary
arteries, veins, nerves, and the bronchial tubes. Respiration is the
function by which the venous blood, conveyed to the lungs by the
pulmonary artery, is converted into arterial blood. This is effected by
the elimination of carbonic acid, which is expired or exhaled from the
lungs, and by the absorption of oxygen from the air which is taken into
the lungs, by the act of inspiration or inhalation. The act of
expiration is performed chiefly by the elevation of the diaphragm and
the descent of the ribs, and inspiration is principally effected by the
descent of the diaphragm and the elevation of the ribs.
[Illustration: Fig. 44.
A representation of the heart and lungs. 4. The
heart. 5. The pulmonary artery. 8. Aorta. 9, 11.
Upper lobes of the lungs. 10, 13. Lower lobes. 12.
Middle lobe of the right lung. 2. Superior vena
cava. 3. Inferior vena cava.]
When the muscles of some portions of the air-passages are relaxed, a
peculiar vibration follows, known as snoring. Coughing and sneezing are
sudden and spasmodic expiratory efforts, and generally involuntary.
Sighing is a prolonged deep inspiration, followed by a rapid, and
generally audible expiration. It is remarkable that laughing and
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.
The venous blood is carried, as we have previously described, to the
right side of the heart and to the lungs, where it is converted into
arterial blood. It is now of uniform quality, ready to be distributed
throughout the body, and capable of sustaining life and nourishing the
tissues. Man breathes by means of lungs; but who can understand their
wonderful mechanism, so perfect in all its parts? Though every organ is
subservient to another, yet each has its own office to perform. The
minute air-cells are for the aeration of the blood; the larger bronchial
tubes ramify the lungs, and suffuse them with air; the trachea serves as
a passage for the air to and from the lungs, while at its upper
extremity is the larynx, which has been fitly called the organ of the
human voice. At its extremity we find a sort of shield, called the
_epiglottis_, the office of which is supposed to be to prevent the
intrusion of foreign bodies.
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