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sternum. From this point the two pleurae are in close contact down to the level of
the upper border of the fifth costal cartilage, where they tend to separate from one
another. On the right side the line of reflection continues nearly straight down to
the lower end of the body of the sternum, where it begins to turn outwards. On the
left side, according to Luschka, it normally diverges from the median line at the
upper border of the fifth costal cartilage, so that at the level of the sternal end of the
fifth costal cartilage it, is 1*5 cm., at the sixth 2 cm., and at the seventh 3'5 cm.
external to the left border of the sternum. Xot unfrequently, however, the lateral
deviation of the left pleura opposite the lower end of the sternum is not so marked




Fig. 191. HORIZONTAL SECTION OF THE THORAX OF A JIAN, AGED 57, AT THE LEVEL OF THE ROOTS OF

THE LUNGS, SEEN FROM ABOVE. (J . S. ) ^

i., s., superior and inferior lobes of lungs; E, eparterial bronchus; A.M., anterior mediastinum ;
R.P.C., right pleural cavity ; P.C., pericardial cavity; A. A., ascending aorta; PA., pulmonary artery ;
R.P.A., its right branch ; R.P.V., L.P.V., right and left pulmonary veins; A.V., azygos major vein;
other letters as in fig. 189.

as described by Luschka. Thus, Sick, in seventeen out of twenty-three adults, found
the reflection of the pleura at the level of the sternal end of the fifth costal cartilage
was either behind the sternum or at its left edge. This was also the case in ten
out of the twenty-three at the sixth cartilage and in nine of them at the seventh
costal cartilage. Brooks' observations also support this view. In four out of seven
cases in which the pleura was quite healthy he found the reflection was entirely
behind the sternum, while in one it was at the left edge of this bone. He also
states that in all the cases the two pleurae were close to one another as far down as
the ensiform cartilage, so that the area of pericardium uncovered by the pleura was
reduced to a minimum.

The lower border of the costal pleura is reflected on to the diaphragm opposite a
line passing from the lower end of the sternum outwards behind the seventh costal



172 ORGANS OF RESPIRATION AND VOICE.

cartilage nearly as far as its rib ; here it leaves the seventh cartilage, and continuing
to pass obliquely downwards and backwards, crosses the eighth, ninth, tenth, and
eleventh ribs, and reaches the twelfth rib near its vertebral end. If the twelfth rib
be very short the whole of its anterior surface m:iy be covered and the pleura pass
as low as the transverse process of the first lumbar vertebra (Pansch). When the
rib is well developed it is seldom in contact with the pleura external to its inner
half. On the lateral wall of the chest the pleura will generally be found rather
lower on the left than on the right side. Luschka estimates that in the midaxillary
line the pleura reaches to the lower border of the ninth rib on the right side and to
the lower border of the tenth on the left. The inferior limit of the pleura does not




Fig. 192. HORIZONTAL SECTION OF THOBAX OP A MAN, AGED 57, IMMEDIATELY ABOVE THE PULMONARY

VALVES, AND THE RIGHT AURICULAR APPENDIX, SEEN FROM ABOVE. (J. S.)

3', third costal cartilage ; fi.i. , superior and inferior lobes of lungs ; p, on right anterior pulmonary
valve ; R, tip of right auricular appendix ; L, left auricular appendix ; L.A., left auricle ; a, b, c,
branches of right pulmonary artery and vein and of right bronchus ; l.p.c., left pleural cavity ; other
letters as in fig. 189.

extend to the attachment of the diaphragm, but leaves a portion of the circumference
of this muscle in contact with the costal parietes. Owing to the height of the
diaphragm on the right side (corresponding to the greater convexity of the liver)
the right pleural sac is shorter than the left ; it is at the same time wider, as the
pericardium does not pass so far to the right as to the left of the median plane.

The upper part of the pleura, together with the apex of the corresponding lung,
rises dome-like into the root of the neck forming the pleura cervicalis. It reaches
from one to two inches above the anterior end of the first rib and half an inch to
one and a-half inches above the clavicle, but not higher than the neck of the first
rib. The subclavian artery, as it arches outwards, lies in a groove on its inner and
anterior aspect a little below its highest point. Externally it comes in contact with
the scalenus anticus and medius.



THE I'LEUR.E.



173



A small slip of muscle arising 1 from the transverse process of the last cervical vertsbra is
described by Sibson as expanding into a dome-like aponeurosis or fascia, which covers or
strengthens the pleural cul-de-sac, and is attached to the whole of the mner_edge of the
first rib.

The raediastinal portions of the two pleural sacs constitute, as already described,
the lateral boundaries of the mediastinal space. The layers forming the sides of the
anterior mediastinum pass backwards from the sternum to the pericardium in close
relation with one another except below, where a triangular interval is sometimes
found between them. At the front of the pericardium the two layers separate, each
passing round its own side of the pericardium to the front of the root of the corre-




Fig. 193. HORIZONTAL SECTION OP THE THORAX OF A MAN, AGED 57, AT THE LEVEL OF THE NIPPLES,

SEEN FROM ABOVK. (J.S.) ^

??., nipple ; M, middle lobe of right lung ; R.A., right auricle ; R.V., right ventricle ; L.A., left
auricle ; L.V., left ventricle ; R.P.V., right posterior valve of aortic orifice ; r.p.c., right pleural cavity ;
other letters as in fig. 189.

sponding lung, where it becomes continuous with the visceral pleura. This part of
the pleura is often termed pericardial ; it is the lateral limit of the middle medias-
tinum. The phrenic nerve courses to the diaphragm between it and the pericardium.
The pleura which goes from the side of the vertebral column to the back of the root
of the lung bounds the posterior mediastinum. On the right side it lies in relation
with the vena azygos major, the right vagus, and the O3sophagus ; on the left side
with the descending aorta, and low down with the oesophagus. The two pleurae
are here united by an interpleural ligament passing behind the oesophagus and in
front of the aorta. At the level of the superior mediastinum the pleura passes back-
wards from the anterior to the posterior chest wall without being reflected over the
lungs. On the right side it covers the right innominate vein and superior vena
cava, the innominate artery and the right phrenic and vagus nerves, and the



174 ORGANS OF RESPIRATION AND VOICE.

trachea ; and on the left the oesophagus and thoracic duct, the left common carotid
and subclavian arteries, and the corresponding phrenic and vagus nerves.

In certain situations the surface of the parietal pleura directed towards the
pleural cavity is not in contact with the visceral pleura, two portions of parietal
pleura being in apposition. The part of the pleural cavity bounded in this way by the
parietal pleura is called the supplemental or complemental pleural space. This space
varies in its extent according to the condition of the lungs, being diminished in
inspiration and increased during expiration ; but in all probability it cannot be
completely obliterated even by forcible inspiration. It is found on both sides
between the chest 'wall and the diaphragm, and on the left side between the chest
wall and the pericardium, opposite the lower part of the body of the sternum. It
is best marked posteriorly, as here the costal and diaphragmatic portions of the
pleura are in contact from about the tenth to the twelfth rib.

Structure. The pleura possesses the usual characters of serous membranes.
The costal part is the thicker, and may be easily raised from the ribs and intercostal
spaces. It is strengthened here by a layer of subserous areolar tissue of considerable
thickness. On the pericardium and diaphragm the pleura is thinner and more firmly
adherent ; but it is thinnest and least easily detached upon the surface of the lungs.
A difference is also noticeable in the character of the superficial epithelial layers, for
while on the pleura cosialis this consists of the ordinary flattened cells, on the pleura
pulmonalis the cells are less distinctly flattened and more granular and polyhedral,
but they become flattened out when the lung is distended (Klein). Lymphatic
vessels are abundant in and beneath the pleura as in other serous membranes, and
they communicate in many parts, by means of stomata, with the cavity of the
membrane. In the pleura cosialis the stomata are only found over the intercostal
spaces, not over the ribs (Dybkowsky).

Beneath the serous covering there is placed a thin layer of subserous areolar
tissue mixed with a large number of elastic fibres. It is continuous with the areolar
tissue in the interior of the lung, and has been described as a distinct coat under the
name of the second or deeper layer of the pleura. In the lungs of many animals,
such as the lion, seal, and leopard, this subserous layer forms a very strong
membrane, composed principally of elastic tissue ; in others, as the guinea-pig, a
network of plain muscular fibres is found, which have a general radiating direction
from the apex (Klein). A close plexus of lymphatic vessels is also met with in this
sub-pleural tissue : these vessels communicate on the one side by means of stomata
with the pleural cavity, and on the other, as will be afterwards noticed, with a net-
work of similar vessels in the inter-alveolar septa of the lungs. A uniform network
of capillary blood-vessels covers the surface of the lung. These are supplied from
branches of the bronchial arteries. They are less closely arranged than the blood-
vessels of the pulmonary alveoli, and are thus, as well as by their position, easily dis-
tinguishable from them in specimens of injected lung.

THE LUNGS.

Each lung is irregularly pyramidal or conical, with the base downwards (fig. 194).
The broad, concave base is of a semilunar form, and rests upon the arch of the
diaphragm. It is bounded by a thin margin, which is received in the angle between
the ribs and the diaphragm, and reaches much lower down behind and at the outer
side than in front. The apex is blunt, and, as already mentioned, reaches into the
root of the neck, above the first rib, where it is separated from the first portion of
the subclavian artery by the pleural membrane. The apex is generally marked by
a groove where the subclavian artery crosses it. The outer surface, which moves
upon the thoracic parietes, is smooth, convex, and of great extent, corresponding



THE LUNGS.



175



with the arches of the ribs and costal cartilages. The inner surface is concave, and
in part adapted to the convex pericardium. The posterior border is rounded, and is
received into the deep groove formed by the ribs at the side of the vertebral column ;
measured from above downwards, it is the longest part of the lung. The anterior
border is thin and overlaps the pericardium, forming a sharp edge, which, opposite
the middle of the sternum, is separated during inspiration from the corresponding
margin of the opposite lung only by the two thin layers of the mediastinal septum.
Upon the inner surface, somewhat above the middle of the lung, and considerably
nearer to the posterior than the anterior border, is the hilum or fissure, where the
bronchi and great vessels enter the lung. These structures form the root of the lung.
The left lung is divided into two lobes by a long and deep fissure, which can be
traced on the surface of the lung from the upper and posterior part of the hilum
upwards and backwards on the inner surface, and reaches the posterior border at
about the level of the fourth rib ; the fissure then passes obliquely' downwards and
forwards over the outer surface to the lower border, which it joins near its anterior





Fig. 194. VIEW OF THE LUNGS FROM BEFORE, DRAWN FROM THE MODELS OF His.
a, b, c, upper, middle, and lower lobes of the right lung ; d, e, upper and lower lobes of the left lung.

end. This part of the fissure is somewhat more oblique than the adjacent ribs ; thus,
beginning at the fourth rib it gradually leaves it to gain the inner surface of the
fifth rib, which it finally crosses to reach the sixth costal cartilage a little below the
apex of the heart. From the lower border of the lung the fissure can be seen to
pass up the inner surface to the lower part of the hilum. The fissure extends from
the surface deeply into the lung, reaching close to the hilum, and practically dividing
the lung into two distinct parts. The upper lobe is the smaller. It forms the apex,
the whole of the anterior border, and the greater part of the concavity for the heart.
To the lower and larger lobe belong the greater part of the thick posterior border
and almost the whole of the diaphragmatic surface except a small area in front. The
highest part of the lower lobe is found at the posterior border of the lung, where it
usually reaches to the fourth rib.

In the right lung there are two fissures dividing it into three lobes, called upper,
middle, and lower. One of these fissures closely corresponds in its position and
direction with the one on the left side, except that it is rather more vertical, and joins
the lower border of the lung farther outwards. It may be regarded as the main



176 ORGANS OF RESPIRATION AND VOICE

fissure, and separates the lower lobe from both the upper and middle lobes. The
additional fissure is seen on the outer surface to pass from the main fissure nearly
horizontally inwards and join the anterior border at the level of the fourth costal
cartilage. From this border it can be traced on the inner surface backwards to the
hilum. Like the main fissure it extends deeply into the lung, and it almost
completely cuts off the middle from the upper lobe.

Varieties. Irregularities in the number and position of the lobes of the lung are not
very uncommon. Absence' of a lobe owing to its non-development is very rare, but cases of
its partial union with the adjacent parts of the lung owing to defective formation of the
fissures are often seen. Accessory lobes often result from fissuring of the primary ones. An
accessory lobe on the right side situated below the root of the lung, and apparently corre-
sponding to the-lobva impar of various mammals, has been frequently observed. Several cases
have also be.en recorded of an accessory lobe above the root of the lung, which was constricted
at its base by the vena azygos major.

In spite of its. comparatively small size, it appears not improbable, from the researches of
Aeby, which will be afterwards more fully referred to, that the middle lobe of the right lung
is the morphological equivalent of the whole upper lobe of the left lung, and that the upper
lobe of the right lung is not represented on the left side.

The left lung has a deep notch in its anterior border, into which the apex of the
heart (enclosed in the pericardium) is received. Besides these differences the right
lung is shorter than the left, owing to the diaphragm rising higher on the right side
to accommodate the liver, whilst the left lung is the narrower, owing to the heart
and pericardium encroaching on the left half of the thorax. On the whole, how-
ever, as is seen on a comparison of their weights, the right is the larger of the two
lungs.

At the summits and posterior borders the extent of the lungs corresponds with
that of the pleural sacs which contain them, but in front and below the relation is
variable, inasmuch as the anterior margins below the level of the third or fourth
costal cartilages pass forwards most completely between the mediastinal and costal
pleura during inspiration, and retire to a variable degree from between them in
expiration ; and in like manner the inferior margins descend, during inspiration,
between the costal and diaphragmatic pleurae ; probably at no time do they ever
descend completely to the line of reflection between those membranes.

The lower edge of the right lung usually extends to the sixth rib in the
mamillary line, to the eighth in the midaxillary, and to the tenth in the post-scapular
line. The left lung is often nearly a rib lower than the right.

In consequence of the notch in the lower part of the anterior border of the left lung, an
area of the heart, on the left side of the median plane, is uncovered by lung. This area,
which is of importance clinically, is often called the area of prascordial dulness, or the area
of superficial cardiac dulness. It is irregularly triangular in shape, the three fixed points
being, (a) one at midsternum opposite the fourth costal cartilages, (b) another at the apex
beat, (r) a third at midsternum at the junction of the body of the sternum with the ensi-
form cartilage. The line joining the first two of these points is irregularly curved with
the convexity directed upwards and outwards. The line joining the second with the third
point is slightly curved with the convexity directed downwards and to the right. At the
inner part of its lower border cardiac dulness frequently merges in hepatic dulness.

EOOTS OF THE LUNGS. The root of each lung is composed of the bronchus or
sub-division of the air-tube, and the large blood-vessels, together with nerves,
lymphatic vessels, and glands, connected together by areolar tissue, and enclosed by
the reflection of the pleura.

The roots of the lungs are situated at the level of the bodies of the fifth, sixth,
and seventh, and often also the eighth, dorsal vertebrae. The root of the right lung-
lies behind the superior venacava and part of the right auricle, and below the azygos
vein, which arches over it to enter the superior cava. That of the left lung passes
below the arch of the aorta, and in front of the descending aorta. The phrenic
nerve descends in front of the root of each lung, and the pneumogastric nerve



THE LUNGS.



177



behind, whilst the ligamentum latum pulmonis is continued from the lower border.
The bronchus, together with the bronchial arteries and veins, the lymphatics, and
lymphatic glands, are placed on a plane posterior to the great blood-vessels, whilst
the pulmonary veins are in front of the arteries. The pulmonary plexuses of nerves
lie on the anterior and posterior aspect of the root, beneath the pleura, the posterior
plexus being the larger of the two.

On the right side the undivided portion of the bronchus is usually altogether
above the right pulmonary artery ; on the left side the undivided portion of the
bronchus, which is considerably longer than on the right side, extends to below the



Ur.




r.lir.



Fig. 195. SKETCH SHOWING THE LOWER END OF THE TRACHEA, ITS DIVISION INTO THE TWO BRONCHIAL

TRUNKS, AND THE COURSE AND CHIEF BRANCHES OF THESE WITHIN THE LUNGS FROM BEFORE

(after Aeby).

a, upper, 6, middle, c, lower, lobe of the right lung ; V, upper, c', lower lobe of the left lung :
r.p., right pulmonary artery; l.p., left artery; r.br., right bronchial trunk; l.br. , left bronchial
trunk ; ep. , on the right side, eparterial branch supplying the upper lobe ; v.h. 1 , first ventral hypar-
terial bronchus supplying the middle lobe on the right side, the upper lobe on the left ; v.h.' 2 , v.h. 3 ,
v.h. 4 , the remaining ventral hyparterial branches distributed in the lower lobe on each side ; d.h. 1 ,
d.h.^, d.h. 3 , d.h. 4 , the four dorsal hyparterial branches distributed on both sides in the posterior and
inner part of the lower lobe ; h, accessory bronchus arising close to the first dorsal hyparterial bronchus
on the right side, and representing the one which supplies the azygos lobe in some animals. The
main branches of the pulmonary vessels are distributed like the bronchi. Within the lung the arterial
trunks run behind the bronchial branches, the venous trunks in front.

level of the left pulmonary artery, which crosses it. On both sides the pulmonary
veins are below the corresponding arteries.

DISTRIBUTION OP THE BRONCHI WITHIN THE LUNGS. As already mentioned,
the right bronchus gives off a branch near its origin which is distributed in
the upper lobe of the lung (fig. 195, ep). This branch, which comes off above the
place where the right pulmonary artery crosses the bronchus (eparterial branch), is
not represented on the left side in man, and it is hence inferred by Aeby that the
lobe of the lung to which it is distributed is also absent on the left side, and that
the upper lobe of the left lung is in reality the homologue of the middle lobe
of the right lung. All the other branches of the right bronchus, and all the

VOL. III., PT. 4. N



178



ORGANS OF RESPIRATION AND VOICE.



branches of the left bronchus, come off below the place where the corresponding
pulmonary artery crosses the air-tube (hyparterial).

In many animals the bronchi, instead of dividing, as in man they appear to do,
into nearly equal branches at the root of the lung, pass down in the form of
main trunks towards the extremity of the lower lobe, giving off branches at
intervals in two directions, viz., dorsally and ventrally. The character of the
ramification of the hyparterial bronchial trunk as it is continued in the lung is
therefore bipinnate and not dichotomous. In addition to these two rows of dorsal
and ventral branches, accessory branches are occasionally met with coming off from
the main trunk in its passage through the lower lobe. These usually arise from
the front, and are intermediate in position between the dorsal and ventral series.




Fig. 196. CAST OP THK INTERIOR or THE TRACHEA AND BRONCHI, WITH THEIR CHIEF RAMIFICATIONS

WITHIN THE LUNG. (Aeby. )

This cast shows a type of division frequently met with, the right bronchus being almost in continua-
tion of the line of the trachea.

, eparterial branch ; b, c, hyparterial branches (ventral and dorsal).

generally taking origin near one or other of these. Of the accessory bronchi the
only one that claims especial notice is that which arises near the second ventral
branch of the right bronchial trunk, and which in some animals (monkeys) supplies
a special small lobe placed mesially behind the pericardium, and termed by Owen the
azyyos [lobe. In some animals well-developed eparterial branches arise from both
bronchi, and supply corresponding lobes in the two lungs. This arrangement repre-
sents the bilaterally symmetrical type of bronchial distribution. In some (sheep,
ox), the eparterial bronchus to the right upper lobe springs directly from the
trachea ; and a similar condition has in rare cases been met with in the human subject.



THE LUNGS.



179



In the human lung the same character of bronchial ramification can be made out
(figs. 195, 196, 197). From the continuation of the bronchus four dorsal and as
many ventral hyparterial branches are given off in succession in each Tung. Of these
the ventral or outer are much the larger, and the first ventral branch supplies the
middle lobe of the right and the upper lobe of the left lung. But the subordination
of the branches to the trunk becomes obscured in consequence of the size of the
ventral branches, which are as large in most cases as the trunk itself ; the latter




Fig. 197. CAST OP THE INTERIOR OF THE TRACHEA AND BRONCHI, WITH THEIR CHIEF RAMIFICATIONS

WITHIN THE LUNGS. ( Aeby. )

This cast shows a type of division less frequent than the last, the right and left bronchi being at
about a right angle with one another.

a, eparterial branch ; b, ventral hyparterial branches ; V accessory (azygos) branch ; c, dorsal
hyparterial branches.

can nevertheless be detected pursuing with but little deviation a course towards the
posterior and lower extremity of the inferior lobe.

According to Hasse the larger branches of the bronchi distributed to the upper
lobe of the left lung and the upper and middle lobes of the right lung are directed out-
wards, upwards, and forwards ; while those of the lower lobes of both lungs pass
downwards, inwards, and backwards. These directions agree with the movements of
the chest walls, which in their upper and anterior parts expand in an upward,
forward, and outward direction, while below the descent of the diaphragm increases
the chest cavity in a direction downwards and inwards.

N 2



180 ORGANS OF RESPIRATION AND VOICE.

On the whole there is a gradual increase in the combined sectional area of the
system of air-tubes in proceeding from the loAver end of the trachea to the termina-
tions of the bronchial tubes in the lungs ; the increase being only interrupted at one



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