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epiglottis projecting upwards in close contact with the base of the tongue. According
to Stuart (On the mechanism of the closure of the larynx, Proc. Roy. Soc. London, 1892), it is
effected by the two arytenoid cartilages being drawn together and also forwards, so that their
upper ends are brought in contact with the posterior surface of the epiglottis. These
movements of the arytenoid cartilages are produced by the contraction of the arytenoid and
thyro-arytenoid muscles.

It is remarked by Henle that the muscles " which lie in the space enclosed by the laminae
of the thyroid cartilage, and above the cricoid, may be regarded in their totality as a kind of
sphincter, such as is found in its simplest form embracing the entrance of the larynx in
reptiles." In the human larynx there is a marked predominance of adductor over abductor
fibres (acting upon the vocal cords).

THE TRACHEA AND BRONCHI.

The trachea is that part of the common air passage of both lungs, which com-
mences above at the larynx and divides below into two smaller tubes, right and left
bronchi, one for each lung.

The trachea has a median position in the neck and thorax, and extends from the
lower border of the cricoid cartilage of the larynx, on a level with the lower part
of the sixth or upper part of the seventh cervical vertebra, into the upper part of the
thorax, where it is crossed in front and on the left side by the arch of the aorta and
then bifurcates into the two bronchi at about the level of the disc between the fourth
and fifth dorsal vertebrae. It usually measures from 4 in. to 4 in. (10 to 11 cm.)
in length, and from f in. to 1 in. (2 to 2| cm.) in width ; but its length varies
according to the position of the neck. It is usually stated on the authority of Aeby
that the trachea gradually increases in calibre from above downwards, but according



THE TRACHEA.



163



to Braune and Stahel it presents a series of alternate dilatations and contractions.
Thus it is smallest at its commencement ; from this point it gradually: increases
until about midway between its two ends, from here it diminishes to within about
3 cm. of its bifurcation towards which it again enlarges. Its average diameter is




Fig. 183. HORIZONTAL SECTION OF TRACHEA, (ESOPHA-
GUS, AND THYROID BODY. (J. S. ) 1

1, oesophagus ; 2, cavity of trachea ; 3, cartilaginous
ring of trachea ; 4, thyroid body ; 5, inferior thyroid
artery ; 6, recurrent laryngeal nerve.



greater in the male than in the female. As ^

the trachea passes obliquely downwards and

somewhat backwards it gradually recedes

from the anterior surface of the neck in its

course towards the thorax. If the" face be

directed forwards the distance from the upper end of the trachea to the top of the

sternum is about 2 to 3 in., and this is increased by fully an inch when the head is

thrown back. In front and at the sides the trachea is rendered cylindrical, firm,

Fig. 184. OUTLINE SHOWING THE GENERAL FORM OF THE

LARYNX, TRACHEA, AND BRONCHI, AS SEEN FROM

BEFORE. (Allen Thomson.) -^

h, the great cornu of the hyoid bone ; e, epiglottis ;
t, superior, and t', inferior cornu of the thyroid cartilage ;
c, middle of the cricoid cartilage ; t r, the trachea, showing
sixteen cartilaginous rings ; b, the right, and &', the left
brosichus. In this and also in the succeeding figure the
right bronchus is represented as somewhat more horizontal
than is usually described (see p. 164).

and resistant, by a series of cartilaginous rings ;
these, however, are deficient behind, so that the
posterior portion is flattened and entirely mem-
branous (fig. 183). Near its bifurcation the
trachea is somewhat expanded laterally.

PECULIARITIES ACCORDING TO AGE. In the
foetus the trachea is flattened before and behind, its
anterior surface being- even somewhat depressed ; the
ends of the cartilages touch ; and the sides of the
tube, which now contains only mucus, are applied to
one another. The effect of respiration is at first to
render the trachea open, but it still remains somewhat
flattened in front, and only later becomes convex. In
consequence of the high position of the larynx in the
infant the cervical part of the trachea is relatively
longer at this period of life than in the adult, but
this increase in length is somewhat diminished by a
higher position of the manubrium sterni. The point
of bifurcation of the trachea is generally about a
vertebra higher at birth than in the adult. In an
infant six months old the trachea will admit a tube
4 mm. in diameter ; at two years one of 5 mm. ; and
at six years one of 6 or 7 mm. Ossification of its car-
tilaginous rings usually commences in the male at
about forty years of age, and in the female about sixty
(Chievitz).

Relations of the trachea to neighbouring parts. The windpipe is nearly
everywhere invested by a loose areolar tissue, abounding in elastic fibres, and is very
moveable on the surrounding parts. Both in the neck and thorax, it rests behind
against the gullet, which intervenes between it and the vertebral column, but

M 2




164



ORGANS OF RESPIRATION AND VOICE.



towards its lower part projects somewhat to the left side. The recurrent laryngeal
nerve ascends to the larynx on each side in the angle between these two tubes.

In the neck the trachea is situated between the common carotid arteries ; at its
upper end it is embraced by the lateral lobes of the thyroid body, the middle part
or isthmus of which lies across it just below the larynx. It is covered in front by
the sterno-thyroid and sterno-hyoid muscles, between which, however, there is left
an elongated lozenge-shaped interval in the middle line : this interval is covered in
by a strong process of the deep cervical fascia, while, more superficially, another
layer not so strong crosses between the sterno-mastoid muscles. The inferior

Fig. 185. 00TLINK SHOWING THE GENERAL FORM OF THE

LARYNX, TRACHEA, AND BRONCHI, AS SEEN FROM

BEHIND. (Allen Thomson. ) |

h, great cornu of the hyoicl bone ; t, superior, and t',
inferior cornu of the thyroid cartilage ; e, the epiglottis ; a,
points to the back of both the arytenoid cartilages, which
are surmounted by the cornicula ; c, the middle ridge on
the back of the cricoid cartilage ; t r, the posterior mem-
branous part of the trachea ; b, b', right and left bronchi :
their relative direction, as shown in this figure, is not that
most frequently met with.

thyroid veins and the arteria thyroidea ima,
when that vessel exists, also lie upon its anterior
surface below the isthmus of the thyroid body ;
while just above the level of the upper edge of
the sternum the innominate artery is occasionally
found crossing obliquely in front of it.

In the thorax, the trachea is covered by the
manubrium sterni, together with the sterno-
thyroid and sterno-hyoid muscles and the re-
mains of the thymus gland ; behind these, by
the left innominate vein, then by the com-
mencement of the innominate artery and left
carotid, which pass round to its sides ; and
lastly by the arch of the aorta and the deep
cardiac plexus of nerves. Placed between the
two pleurae, the trachea is contained in the
superior mediastinum, and has on its right side
the pleura and pneumo-gastric nerve ; on the
left side are the arch of the aorta, the left carotid
and subclavian arteries and the left recurrent
laryngeal together with some cardiac nerves.

The right and left bronchi (figs.
184, 185, Z>, b') proceed each to the root of

the corresponding lung, and then undergo division. Previous to this they exactly
resemble the trachea on a smaller scale ; being rounded and firm in front and at the
sides, where they are provided with imperfect cartilaginous rings, and flattened and
membranous behind. The right bronchus is larger than the left, the area of a
section at right angles to the long axis of the right bronchus being in the proportion
of 100 to 78 for a corresponding section of the left bronchus. It is also more nearly
vertical so that on looking down the windpipe towards the bifurcation, the right
bronchus appears to be a more direct continuation of the trachea than the left.
This is, however, by no means constantly the case. Further it differs from the left
in giving off on its outer side about \ in. to 1 in. from its origin a branch which
passes to the upper lobe of the right long. As this branch comes off above the
place where the right pulmonary artery crosses the bronchus it is called eparterial ,




STRUCTURE OF THE TRACHEA AND BRONCHI.



165



the continuation of the bronchus below this branch being called hyparterial. On
the left side there is no eparterial branch and the bronchus passes downwards and
outwards for about two inches before giving off any branches (see p. 177"andTfig. 195).

The right bronchus is embraced above by the vena azygos major, which hooks
forwards over it to end in the vena cava superior. The right vagus descends behind
it. At the root of the lung the eparterial branch is above the pulmonary artery,
the hyparterial part is crossed close to its origin by the pulmonary artery ana
lower down the superior pulmonary vein gets in front of it. The left bronchus
inclines downwards and outwards beneath the arch of the aorta to reach the root of
the left lung where the left pulmonary artery lies first in front of and then above it.
It crosses over the front of the oesophagus and the descending aorta. Close to the
hilum of the lung the upper left pulmonary vein is in front of it.

The combined sectional area of the two bronchi is about one-fifth greater than
that of the trachea.

The distribution of the branches of the bronchi within the lungs will be
described in connection with those organs.



STRUCTURE OP THE TRACHEA AND BRONCHI.

TKACHEA. The trachea consists of an elastic framework of incomplete cartila-
ginous rings or hoops united by fibrous tissue, and at one part by plain muscular
tissue. It is lined throughout by a mucous membrane, and provided with glands.




Fig. 186. LONGITUDINAL SECTION OP THE HUMAN TRACHEA, INCLUDING PORTIONS OF TWO CARTILAGINOUS
RINGS. (Klein and Noble Smith.) MODERATELY MAGNIFIED.

a, ciliated epithelium ; b, basement membrane ; c, superficial part of the mucous membrane, con-
taining the sections of numerous capillary blood-vessels and much lymphoid tissue ; d, deeper part of
the mucous membrane, consisting mainly of elastic fibres ; e, submucous areolar tissue, containing the
larger blood-vessels, small mucous glands (their ducts and alveoli are seen in section), fat, &c. ; /,
fibrous tissue investing and uniting the cartilages ; g, a small mass of adipose tissue in the fibrous layer ;
/*, cartilage.

The cartilages are from sixteen to twenty in number. Each forms a curve of
rather more than two-thirds of a circle, resembling the letter C (fig. 183). The depth



166



ORGANS OF RESPIRATION AND VOICE.



from above downwards is three or four millimeters, and the thickness 1 mm. The
outer surface of each is flat, but the inner is convex from above downwards, so as to
give greater thickness in the middle than at the upper and lower edge. The cartilages
are held together by strong fibrous tissue, which is elastic and yielding to a certain
extent, and not only occupies the intervals between them, but is prolonged over
their outer and inner surfaces, so that they are, as it were, imbedded in the tissue.

The cartilages terminate abruptly behind by rounded ends, but the fibrous tissue
is continued across between them, and completes the tube behind ; it is here looser
in its texture.

The first or highest cartilage, which is connected by the fibrous membrane with
the cricoid cartilage of the larynx, is broader than the rest, and often divided at one
end. Sometimes it coalesces to a greater or less extent with the cricoid or with the
one below. The lowest cartilage, placed at the bifurcation of the trachea, is
peculiar in shape ; its lower border being prolonged downwards, and at the same
time bent backwards so as to form a curved projection between the two bronchi.

Fig. 187. A PART OF THE SECTION REPRESENTED IN
THE PRECEDING FIGURE MORE HIGHLY MAGNIFIED.

(Klein and Noble Smith.)
The letters represent the same parts as in that figure.

The cartilage next above this is slightly
widened in the middle line. Sometimes
the extremities of two adjacent cartilages are
united, and not unfrequently a cartilage is
divided at one end into two short branches,
the opposite end of that next it being like-
wise bifurcated so as to maintain the
parallelism of the entire series. The use of
these cartilaginous hoops is to keep the wind-
pipe open, a condition essential for the free
passage of air into the lungs.

Within the fibrous membrane at the
posterior flattened part of the trachea, is a
continuous pale reddish layer of unslriped
muscular fibres, which pass across, not only
between the ends of the cartilages, but also

opposite the intervals ; they doubtless serve to narrow the tube by approximating
the ends of the cartilages. Those opposite the hoops are attached to the extremities
of the latter, and encroach also for a short distance upon their inner surface.
Outside the transverse fibres are a few fasciculi having a longitudinal direction.

The submucous tissue consists of loose areolar tissue which serves to connect the
mucous membrane with the fibrous layer and the cartilaginous rings. It contains
mucous glands and a quantity of adipose tissue is often found in it.

The mucous membrane is smooth and of a pale pinkish white colour
in health, although when congested or inflamed, it becomes intensely purple
or crimson. It contains a considerable amount of lymphoid tissue. Underneath the
epithelium is a basement membrane (figs. 186, 187, b), well marked in the human
trachea, through which nerves and processes from the subjacent connective tissue
cells here and there pass into the epithelium. Throughout the mucous membrane
a number of fine elastic fibres are found, but in the deeper parts the elastic fibres
are very large and numerous (d). Along the posterior membranous part, they are
more abundant than elsewhere, and are there collected into distinct longitudinal




STRUCTURE OF THE TRACHEA.



167



bundles, which produce visible elevations or flutiugs of the mucous membrane.
These bundles are particularly strong and numerous opposite the bifurcation of the
trachea.

The epithelium consists of a layer of long columnar ciliated cells, often very
irregular at their fixed end, where they are impressed by smaller cells, between
which they penetrate to reach the basement membrane. The cilia serve to drive
the mucous secretion upwards towards the larynx. Between these ciliated cells, are
found others, also elongated ; they are prolonged at one end towards the surface, whilst
the other end, which is not unfrequently forked, reaches to the subjacent membrane.
These intermediate cells secrete mucus, which is to be seen in them in various
stages of formation, and some of them are to be seen converted into goblet-cells by
the extrusion of their mucinoid contents (fig. 188). A few lymph corpuscles are
also found amongst the epithelial cells, as in other epithelia.

The trachea is provided with numerous small mucous (/lands. The largest are
situated at the back part of the tube, either close upon the outer surface of the
fibrous layer, or occupying little recesses formed between its meshes. Smaller
glands are found between the cartilaginous rings, upon and within the fibrous
membrane, and still smaller ones close beneath the mucous membrane. They are

Fig. 188. CILIATED EPITHELIUM CELLS FROM THE TRACHEA OF

THE RABBIT ; HIGHLY MAGNIFIED. (E. A. S. )

m 1 , m 2 , m 3 , mucus -secreting cells, lying between the ciliated
cells, and seen in various stages of mucin-formation.



racemose glands, and their cavities are lined by a
columnar epithelium : the excretory ducts pass through
the muscular layer and the mucous membrane, on
the surface of which their orifices are perceptible.

Vessels and Nerves. The arteries of the
trachea are principally derived fiom the inferior thy-
roid. The larger branches run for some distance
longitudinally, and then join a superficial capillary
plexus with polyhedral meshes. The veins enter the
adjacent plexuses of the thyroid veins. A rich plexus of
lymphatics may readily be injected in the mucous

membrane and submucous tissue, but the lymphoid follicles, so common in the
alimentary mucous membrane, and also in the walls of the smaller bronchi, are
rarely present. When found it is generally surrounding the ducts of the glands
as they pass through the mucous membrane. The nerves come from the trunk
and recurrent branches of the pneunw-gastric, and from the sympathetic system.
There are said to be numerous ganglia upon them, especially outside the muscular
layer at the back of the tube.

In the dog, cat, sheep, and rabbit, the upper half of the trachea is said to be
supplied chiefly by the superior laryngeal nerve, through the anastomosis between
the superior and inferior nerves in the larynx (Kandarazi).

BRONCHI. The general structure of the undivided portions of the bronchi
corresponds with that of the trachea in every particular. Their cartilaginous rings,
which resemble those of the trachea in being imperfect behind, are, however, shorter
and narrower. The number of these rings on the right side varies from six to eight,
whilst on the left the number is from nine to twelve.

The bronchi are supplied by the bronchial arteries and veins, and the nerves are
from the same source as those of the lower part of the trachea.




168



ORGANS OF RESP1KATION AND VOICE.



THE MEDIASTINUM THORACIS.

The greater part of the thorax is occupied by the lungs, each of which is invested
by a serous membrane, the pleura. The right and left pleural cavities are separated




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THE LUNGS AND PLEURA. 169

by a median partition called the mediastinum. This consists of a layer of pleura
on each side, with the heart, great vessels, pericardium, and other structures inter-
posed. The interval between the right and left pleurae, which is occupied by the
structures referred to, is known as the mediastinal space. The whole mediastinum
is subdivided into the following parts : The middle mediastinum, which is co-
extensive with the pericardium ; the anterior mediastinum, the part in front of the
pericardium ; the posterior mediastinum, the part behind the pericardium ; and the
superior mediastinum, which is situated above the pericardium, and extends upwards
as far as the root of the neck.

The superior mediastinum may be considered as bounded below by a plane
passing from the lower border of the body of the fourth dorsal vertebra behind to
the junction of the manubrium with the body of the sternum in front. Its upper
limit corresponds with the superior aperture of the thorax. In front are the
manubrium and the lower ends of the sterno-hyoid and sterno-thyroid muscles ; and
behind are the upper four dorsal vertebrje and the lower ends of the longus colli
muscles.

It contains the trachea, oesophagus and thoracic duct ; the whole of the arch of
the aorta, the innominate artery, and the thoracic parts of the left common carotid
and subclavian arteries ; the innominate veins and upper part of the superior vena
cava ; the phrenic and pneumo-gastric nerves, the left recurrent, and the cardiac
nerves ; and the cardiac lymphatic glands and remains of the thymus gland.

The anterior mediastinum is narrow in its upper half, the two pleurae coming
nearly or quite into contact behind the second piece of the sternum. Below it is
u little broader, the left pleura receding from its fellow, and is bounded in front by
the sternum, sometimes also by the fifth and sixth, and a small portion of the
seventh left costal cartilages, and by the triangularis sterni muscle ; behind it is the
pericardium. The enclosed space contains only some areolar tissue, and in its lower
part two or three small lymphatic glands (anterior mediastinal glands).

The middle mediastinum is the enlarged central portion of the partition, con-
taining in addition to the pericardium with its contents (viz., the heart, the arch of
the aorta, the trunk of the pulmonary artery and the lower half of the superior
vena cava), the phrenic nerves and accompanying vessels, the arch of the azygos
vein, and the roots of the lungs with the bronchial lymphatic glands.

The posterior mediastinum is the part between the pericardium, the dia-
phragm, and the roots of the lungs in front, and the spine behind (from the lower
border of the fourth dorsal vertebra downwards). It contains between its pleural
layers the descending thoracic aorta ; the oesophagus with the pneumo-gastric
nerves, the azygos veins, the thoracic duct and the posterior mediastinal lymphatic
glands.

THE LUNGS AND PLETJILE.

The lungs occupy by far the larger part of the cavity of the chest, and in health
are always in accurate contact with the internal surface of its wall. Each lung
is attached at a comparatively small part of its flattened inner or mesial surface by a
part named the root. In other directions the lung is free, and its surface is closely
covered by a serous membrane, which is reflected at the root to the corresponding
side of the thorax, and named the (right or left) pleura.

THE PLEURAE.

The pleurae are two serous sacs quite distinct from each other. Each consists
of a visceral and a parietal portion, between which is the pleural cavity containing a
very small quantity of fluid, merely sufficient to lubricate the opposing surfaces.
The viscera] portion (pleura pulmonalis} covers the lung and extends into the fissures



170



ORGANS OF RESPIRATION AND VOICE.



between its lobes ; and the parietal portion lines the ribs and intercostal spaces
(pleura costalis), covers the upper convex surface of the diaphragm ( pleura dia-
pkragmatis}, enters into the formation of the mediastinum (pleura mediastmalis), and
extends upwards into the neck (pleura cervicalis).

At the root of each lung the visceral and parietal portions of the corresponding-
pleura are continuous with one another ; and at the lower border of the root is a
fold of the serous membrane, the two layers of which are continuous above with
those in front of and behind the root of the lung. It extends vertically along the
inner surface of the lung down to the diaphragm, to which it is attached by its
extremity ; this fold is named ligamentum latum pulmonis.




Fig. 190. HORIZONTAL SECTION OF THORAX OF MAN, AOED 57, IMMEDIATELY ABOVE THE BIFURCATION

OF THK TRACHEA, SEEN FROM ABOVK. (J. S.) I,

U.L., upper lobe of right lung ; U.P., L. L., upper and lower lobes of left lung ; R.B., L.B., origin of
right and left bronchi, in this specimen the termination of the trachea was lower than usual ; A, arch
of aorta ; D. A., descending aorta ; i>, obliterated ductus arteriosus ; N, left recurrent laryngeal nerve ;
L.G., lymphatic glands ; other letters as in lig. 189.

Along the mediastinal aspect of each, pleura there descends a ligamentous band, an offshoot
of the prevertebral fascia, attached above to the bodies of the cervical and first dorsal vertebras
and below to the pericardium and central tendon of the diaphragm. These bands, the " sus-
pensory ligaments of the diaphragm " of Teutleben, embrace the roots of the lungs, and in a
measure serve to fix both these and the other parts to which they are attached below.

Relations. The costal portion of the pleura lines the inner surface of the ribs
and intercostal spaces and posteriorly passes over the heads of the ribs and the
gangliated cord of the sympathetic on to the sides of the bodies of the dorsal
vertebras, where it becomes continuous with the posterior part of the mediastinal
pleura, while in front it is reflected backwards to join the anterior part of the
mediastinal pleura. This anterior line of reflection varies at different levels, and
also slightly on the two sides. Opposite the manubrium sterni it may be repre-
sented by a line passing from the sterno-clavicular articulation downwards and



THE PLEURA.



171



inwards to meet the pleura of the opposite side at the upper edge of the body of the



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