THE LUNGS AND RESPIRATION.
Besides the heart and the great vessels the chest contains the lungs,
the chief organ of respiration, which, with the rest of the respiratory
system, will now be treated. The nose and mouth, through which the air
first enters the body, have already been spoken of. From them the air
passes through the larynx to the trachea, thence to the bronchi, and so
to the lungs, where the supply of oxygen for the tissues is taken from
the air by the hemoglobin of the blood.
=The Larynx.=--The larynx lies in front of the pharynx at the upper
and fore part of the neck, where it causes a considerable projection,
known as _Adam’s apple_. It is a triangular box, base up, flattened
at the back, in front, and at the sides, but becoming cylindrical
below. Above it opens into the bottom of the pharynx and below into the
trachea. It is lined with mucous membrane. Its opening at the base of
the tongue is closed during swallowing by a little door-like valve of
fibro-cartilage, the _epiglottis_, to prevent the entrance of food.
Nine _cartilages_ go to make up the larynx, of which the most important
are the thyroid and cricoid cartilages and the epiglottis already
mentioned. The _thyroid_ is the largest and is open behind, its two
alæ or wings meeting in an acute angle in front and forming the Adam’s
apple, always more prominent in the male than in the female. It is
attached above to the hyoid bone and has cornua or horns on either
side, top and bottom. The _cricoid_ or ring-like cartilage resembles
a seal ring with the stone placed posteriorly. It is stronger than
the thyroid and forms the lower part of the cavity of the larynx.
Inside and resting on the upper border of the cricoid are the two
smaller _arytenoid_ or pitcher-like cartilages, pyramidal in shape,
and surmounting these again the two _cornicula laryngis_. The two
_cuneiform cartilages_ are in the free borders of the folds of mucous
membrane which extend from the apex of the arytenoids to the sides of
the epiglottis. Numerous small muscles serve to bind these various
cartilages together.
At the angle of the thyroid cartilage in front are attached the
epiglottis at the top and just below that the _superior_ or _false
vocal cords_, two folds of mucous membrane enclosing the _superior
thyro-arytenoid ligaments_. Lower still are found the _inferior_ or
_true vocal cords_, which are formed by the _inferior thyro-arytenoid
ligaments_ covered with a thin, tightly fitting mucous membrane. Both
sets of vocal cords as well as the epiglottis may be seen by means
of a head and a throat mirror. Between the true vocal cords is a
narrow triangular interval called the _glottis_. It is by means of the
vibrations of these cords that sound is produced. The false vocal cords
cannot produce sound, though they can modify it indirectly. Quality of
voice, as treble, base, etc., depends upon the size of the larynx and
the length and elasticity of the vocal cords. Modulation is produced by
changing the form of the cavity of the mouth and nose. In whispering
the lips take the place of the vocal cords and produce sound by the
vibration of their muscular walls.
Instead of tracheotomy _laryngotomy_ is sometimes done in the
depression between the thyroid and the cricoid, which may be felt on
the living. _Foreign bodies_ sometimes get into the larynx and have
to be removed, or the mucous membrane may become inflamed, causing
_laryngitis_. _Syphilis_ attacks the larynx, and _tuberculosis_ and
_cancer_ of the larynx occur, these last two being generally fatal.
_Edema of the glottis_ may also occur.
[Illustration: FIG. 47.--The larynx, trachea and bronchi (After
Sobotta.)]
=The Trachea.=--The trachea is a membranous tube extending down from
the larynx for about four and a half inches to the fourth or fifth
dorsal vertebra, where it divides into the right and left bronchi. It
is formed of sixteen to twenty imperfect cartilaginous rings, open
behind, enclosed in a double elastic fibrous membrane, and is lined
with ciliated mucous membrane. The rings are for strength and in
the interval at the back where they are wanting there is one layer
of longitudinal and another of transverse unstriped muscle fibers.
The passage is kept clear by the action of the cilia, which sweep up
and out any particles of dust that become entangled in the mucus.
_Tracheotomy_ is generally done about one inch below the cricoid, just
above the sternal notch, incision being made through the cartilage.
[Illustration: FIG. 48.--The upper thorax of a child eight years old,
showing the thyroid and thymus glands. (Sobotta.)]
Extending up on either side of the upper trachea in the neck are the
two lobes of the _thyroid gland_, the isthmus, which connects the
lobes, covering the trachea below anteriorly. The function of the gland
is obscure, but it has an internal secretion of great importance in
the metabolic processes. Its removal or disease is followed by general
disturbances of mind and body. The injection of thyroid extract has
proved effective as treatment. In _goiter_ the gland becomes enlarged.
The _thymus gland_ lies below the thyroid gland at birth, in front of
and at the sides of the trachea, and runs down behind the sternum. It
is largest at the end of the second year, after which it atrophies,
being almost absent at puberty. It, too, is ductless and its function
is not well understood.
=The Bronchi.=--The two bronchi, of which the right is the larger and
shorter, resemble the trachea in structure. As they enter the root
of the lung they divide, the right into three and the left into two
branches, one for each lobe, after which they divide and subdivide,
the bronchioles becoming smaller and smaller and finally ending in the
_infundibula_, pouch-like places lined with air cells, in which cilia
keep the air in motion. As they grow smaller the bronchioles gradually
become wholly membranous.
[Illustration: FIG. 49.--Relation of lungs to other thoracic organs.
(Ingals.)]
=The Lungs.=--The lungs themselves, two in number, lie each in a serous
sac or _pleura_, similar in structure to the pericardium and serving
a like purpose. The outer layer of the pleura is reflected back over
the thoracic wall and diaphragm. There is no pleural cavity in health
between the two layers of the pleura, the two surfaces being in close
contact, though moistened with lymph to prevent friction during
respiration. In inflammation of the pleuræ or pleurisy, they become
thickened and roughened and friction results, as is shown by the sounds
heard through the stethoscope. Friction causes effusion and fluid
collects. This generally absorbs again, but occasionally the serous
fluid becomes pustular and _empyema_ results.
In front, between the two pleuræ, which are wholly separate, is the
_mediastinal space_ or _mediastinum_, which extends from the sternum
to the spinal column and contains all the thoracic viscera except the
lungs and heart, that is, the trachea, esophagus, thoracic duct, and
many large vessels and nerves.
[Illustration: FIG. 50.--Diagrammatic representation of the termination
of a bronchial tube in a group of infundibula: _B_, Bronchial tube;
_LB_, bronchiole; _A_, atrium; _I_, infundibulum; _C_, alveoli. (de
Nancrede.)]
Roughly speaking, the lungs begin at the sterno-clavicular articulation
above, the apex coming up above the level of the first rib, and extend
downward together to the fourth cartilage, where the lower margins
gradually separate, the lowest lung limit being the eleventh rib in
the vertebral region. Each lung is conical. The apices extend upward
and the bases, which are broad and concave, rest upon the diaphragm.
The right lung is divided by a fissure into three lobes, the left into
two. The root consists of a bronchus and pulmonary arteries, veins,
lymphatics, and nerves. The tissue itself is composed of an aggregation
of lobules, each consisting of a terminal bronchiole with its alveoli
or air cells, blood-vessels, and nerves, a lung in miniature. The blind
pouches which the air cells surround are called _infundibula_ and are
separated by delicate membranous septa in which lie the capillaries of
the pulmonary artery, thus exposing the blood to the air on two sides.
The lung itself is supplied by the bronchial arteries from the thoracic
aorta and by branches of the sympathetic and pneumogastric nerves.
At birth the lungs are pinkish-white in color but in later life they
are marked with slate-colored patches, due to the deposit in the lung
tissue of particles of dirt breathed in. They are light, spongy, and
highly elastic, and will float in water, crepitating upon pressure
owing to the air in the tissue.
At birth, also, the lungs are solid, so that the first air has to
overcome adhesions between the collapsed walls of the bronchioles and
air sacs, but after they are thus gradually unfolded, in that they are
of extensible material and open to the air above, atmospheric pressure
from within keeps them distended to the full extent of the chest, which
is air tight. They never collapse afterwards unless puncture of the
chest wall, as in stabbing, causes collapse, in which case the lung
shrivels into a small ball.
=Respiration.=--That the organic materials used by the body as food may
give up their energy they must be broken up, and for this oxygen is
needed. The supply of oxygen for the purpose is brought to the tissues
by the blood, which acquires it in the lungs, and the waste product
of combustion, carbon dioxide, is carried off in the same manner. The
lungs are, therefore, adapted to take in large quantities of air and
to keep up a rapid exchange of oxygen and carbon dioxide in the blood.
This process of supplying oxygen to the tissues and of removing carbon
dioxide and other waste is ordinarily an involuntary act, though it can
be regulated temporarily, and is known as respiration or breathing.
There are two periods to respiration: 1. _inspiration_ or the drawing
in of air, and 2. _expiration_ or the expulsion of air from the lungs,
the former process being a little shorter than the latter. A pause
follows each expiration before there is another inspiration. At birth
the normal rate of respiration is 42, but it grows slower as the child
grows older, being 26 at the age of five or six, while in the adult it
averages 17 to 20 times a minute. It is slower during sleep and more
rapid during physical activity. The average amount of air taken in with
every inspiration is 30 cubic inches and the minimum air space per
individual should be 3000 cubic feet per hour.
Breathing is of two kinds, _diaphragmatic_ or _abdominal_ and _chest_
or _rib_ breathing, the former usually being more pronounced in men
than in women, probably because of centuries of tight dressing on the
part of the latter. As a rule, however, both diaphragm and ribs come
into play; for in inspiration, which is an active movement, the thorax
becomes enlarged from before backward, laterally, and vertically.
The ribs are raised by the external intercostals chiefly, though the
internal intercostals aid somewhat, and swinging out upon the vertebræ,
widen the chest as well as deepen it. The diaphragm, which is dome-like
when relaxed, becomes flattened in contraction and so increases the
size of the chest from above downward. As the chest enlarges, the lungs
expand, the air in them becomes rarefied, and more air rushes in.
When the lungs are full they relax and the muscles relax after their
contraction, so that expiration is a passive movement, due largely to
the elastic relaxation of lungs and muscles, the air being driven out
by the lessened capacity of the lungs.
_Difficult Breathing._--In heart and lung troubles, where too little
oxygen is carried to the tissues, dyspnœa or difficult breathing
results and may even advance to _asphyxia_, a condition in which no air
is obtained. In difficult or labored respiration the pectoral muscles
are used in inspiration and the scaleni, which pass from the vertebræ
of the neck to the sternum, develop and become powerful. The levatores
of the ribs may also assist, and even the muscles of the neck and arms
may help out, while in forced expiration the abdominal muscles are
called into play. The glottis opens and closes rhythmically as the air
enters and leaves the lungs, and the nostrils add their mite in the
struggle for oxygen. Finally there may be scarcely a muscle in the body
that is not striving to aid the respiration, and general convulsions
may result, followed by exhaustion and death.
_Air._--In ordinary breathing the lungs are not used to their full
capacity and the air ordinarily used is known as _tidal air_. In forced
inspiration the lungs are filled to their fullest extent and the air
then taken in in excess of the tidal air is known as _complemental
air_. In like manner, the difference between the air ordinarily
breathed out and that breathed out in forced expiration is known as
_supplemental air_. The sum of these three is the _vital capacity_ of
the lungs, while beyond this there is probably some air that is never
expelled, the stationary or _residual air_.
_Respiratory Sounds._--The entrance and exit of the air is accompanied
by respiratory sounds or murmurs, which vary according to their
position in the trachea, the bronchi, or the bronchioles and are
modified in diseases of the lungs and bronchi, when they are often
called _râles_.
_Changes in Air in Lungs._--In passing through the nose and the rest
of the respiratory tract the air is warmed to body temperature and
saturated with moisture. After its entrance into the lungs various
changes take place in it through the mingling of the tidal with the
residual air. Thus, it gives up about 4 or 5 per cent. of its oxygen
and acquires some 4 per cent. additional carbon dioxide, while the
amount of nitrogen remains about the same. By its giving up more oxygen
than it receives carbon dioxide, its volume is slightly diminished.
Exhaled air also contains traces of ammonia and certain organic
matters, generally the results of decomposition, which give a bad odor
to the breath and are more dangerous in a close room than the mere
lack of oxygen or the presence of carbon dioxide. Indeed, the amount
of oxygen may be very much diminished, being reduced even to 5 or 6
per cent. instead of the normal 21 per cent., without being noticed
or giving rise to any immediate bad results. Yet the importance of
ventilation is very evident.
_Effect on Blood._--Respiration causes changes also in the blood,
the venous blood being purple and the arterial bright red. This
difference in color is due to the absence or presence of oxygen, which
is not absorbed or dissolved by the blood but forms a rather unstable
compound, _oxyhemoglobin_, with the hemoglobin of the blood. As the
oxygen is removed in the passage of the blood through the body, there
results in venous blood reduced hemoglobin, which is of a purplish
color. Upon exposure to the air, however, it absorbs oxygen once more
and resumes its scarlet color. If carbon monoxide gets into the blood,
as in cases of gas poisoning, it drives off the oxygen and forms a more
stable compound with the hemoglobin, whence the difficulty in restoring
a person so poisoned.
_Nervous Mechanism._--Nervously, respiration is controlled in three
ways: 1. by the phrenic nerve to the diaphragm; 2. by some fibers
of the vagus or pneumogastric, and 3. by the respiratory center in
the bulbous portion of the spinal cord. Injury to the respiratory
center means the ceasing of respiration and death. Stimulation of the
respiratory center seems to depend upon the character of the blood. If
it is well oxygenized, the breathing is slow and quiet; if there is a
lack of oxygen, dyspnœa results. Probably certain chemical substances
in the blood, which are ordinarily rapidly burned up by the oxygen but
which accumulate in its absence, serve to stimulate the respiratory
center, thus adjusting the effort to get oxygen to the need of it.
Respiration may be stopped by stimulating the mucous membrane of the
nose, as with strong ammonia.
_Variations._--Certain variations from the ordinary respiration might
be mentioned here. A deep inspiration followed by a long expiration is
known as a _sigh_ and a very deep inspiration through the mouth only
as a _yawn_. _Hiccough_ results from a sudden inspiratory contraction
of the diaphragm during which the glottis is suddenly closed. In
_sobbing_ the inspirations are short and rapid with a prompt closing of
the glottis between. Both _coughing_ and _sneezing_ consist of a deep
inspiration followed by complete closure of the glottis and then its
sudden opening and the forcible expulsion of air. Coughing, however,
is generally caused by an irritation or obstruction of the larynx or
trachea and the air is expelled through the mouth, while sneezing is
caused by irritation of the nasal passages and the air is driven out
through the nose. _Laughing_ and _crying_ also resemble one another in
that each is an inspiration followed by a series of short, spasmodic
expirations, during which the glottis is open and the vocal cords in
characteristic vibration. They differ, however, in rhythm and in the
facial expression that accompanies them.