Organ hardened _in situ_. _c_, _d_, Grooves for superior cervical and
dorsal veins. Arrows indicate canal for posterior vena cava between
mediastinal lobe and main part of lung.
]
The =dorsal= (or superior) =border= (Margo obtusus) is long, thick, and
rounded; it lies in the groove alongside of the bodies of the thoracic
vertebræ.
The =ventral= (or inferior) =border= (Margo acutus) is thin and short.
It presents, opposite to the heart, the =cardiac notch= (Incisura
cardiaca). On the left lung this notch is opposite to the third, fourth,
and fifth ribs, so that a considerable area of the pericardium here lies
in direct contact with the chest-wall. On the right lung the notch is
less extensive, and is an intercostal space further forward.
The =base= of the lung (Basis pulmonis) is oval in outline; its surface
(Facies diaphragmatica) is deeply concave in adaptation to the thoracic
surface of the diaphragm. Laterally and posteriorly it is limited by a
thin =basal border= which fits into the narrow recess (Sinus
phrenico-costalis) between the diaphragm and the chest-wall. The
position of this border, of course, varies during respiration. In the
deepest inspiration it may reach the bottom of this recess; in ordinary
inspiration it lies about four or five inches (ca. 10 to 12 cm.) from
the costal arch, to which it is nearly parallel; in ordinary expiration
it lies about twice as far from the costal arch.
The =apex= of the lung (Apex pulmonis) is prismatic, narrow, and
flattened transversely. It is partially marked off from the rest of the
lung by the cardiac notch. It curves downward, and is related internally
to the anterior mediastinum and behind to the anterior part of the
pericardium.
In the horse the lungs are not divided into distinct =lobes= by deep
fissures, as in most mammals. In the case of the left lung there is no
lobation, but the right lung presents a =mediastinal lobe=, separated
from the body of the lung by a fissure which forms in its dorsal part a
canal for the posterior vena cava and the right phrenic nerve, inclosed
in a special fold of the right pleura.[129]
The =root= of the lung (Radix pulmonis) is composed of the structures
which enter or leave the lung at the hilus on the mediastinal surface.
These are: (1) The =bronchus=; (2) the =pulmonary artery=; (3) the
=pulmonary veins=; (4) the =bronchial artery=; (5) the =pulmonary
nerves=; (6) the =lymph vessels=, which open into the bronchial lymph
glands. The bronchus is situated dorsally, with the bronchial artery on
its upper surface and the pulmonary artery immediately below it. The
pulmonary veins lie chiefly below and behind the artery.
The =lobulation= of the lungs is not very evident on account of the
small amount of interlobular tissue. The lobules appear on the surface
or on sections as irregular polygonal areas of different sizes.[130]
=Bronchial Branches.=—Each bronchus at its entrance into the lung gives
off a branch to the apex of the lung. It is then continued backward as
the main stem bronchus, parallel with the dorsal border of the lung,
giving off branches (Rami bronchiales) in monopodic fashion; these
branch similarly and reach all parts of the lung. The right bronchus
gives off a special branch to the mediastinal lobe.
The arrangement of the larger bronchi is not the same in the two
lungs. The right bronchus, after detaching the apical branch, gives
off a large ventral branch which shortly bifurcates. One of the
divisions passes ventrally into the part of the lung behind the
cardiac notch (homologue of cardiac lobe), while the other passes
downward and backward into the lower part of the base. The stem
bronchus then gives off the branch to the mediastinal lobe, and
divides, about six inches from the hilus of the lung, into two nearly
equal branches; the dorsal branch is a direct continuation of the
stem, the ventral one passes into the middle part of the base. The
left bronchus, after a short course, divides into two nearly equal
branches. The dorsal branch may be regarded as the continuation of the
stem; it passes backward parallel with the dorsal border of the lung,
giving off a ventral branch much further back than in the right lung.
The ventral division of the stem bronchus divides, after a very short
course, into two branches; the anterior one passes ventrally into the
homologue of the cardiac lobe, while the posterior one passes downward
and backward into the base of the lung.
The =structure= of the larger bronchial tubes is, in general, similar to
that of the trachea. Their walls contain irregular plates of cartilage
instead of rings. There is a continuous layer of unstriped muscle,
composed of circularly arranged bundles. The mucous membrane presents
numerous longitudinal folds; it contains many elastic fibers, mucous
glands, and lymph nodules, and is lined by ciliated columnar epithelium.
As the tubes diminish in size the coats become thinner and the
cartilages smaller; in tubes about 1 mm. in diameter the cartilages and
mucous glands are absent.
By repeated branching the =interlobular bronchi= are formed, and from
these arise the =lobular bronchioles=. The latter enter a lobule and
branch within it, forming the =respiratory bronchioles=; these give off
the =alveolar ducts=, the walls of which are pouched out to form
hemispherical diverticula, the =alveoli= or =air-cells=.
A =lobule= of the lung, the unit of lung structure, is made up of a
lobular bronchiole with its branches, and their air-cells, blood- and
lymph vessels, and nerves. Between the lobules is the =interlobular
tissue=, which forms the supporting framework of the lung.
=Vessels of the Lungs.=—The branches of the =pulmonary artery= carry
venous blood to the lungs. They accompany the bronchi, and form rich
capillary plexuses on the walls of the alveoli. Here the blood is
arterialized, and is returned to the heart by the =pulmonary veins=. The
=bronchial arteries= are relatively small vessels which carry arterial
blood for the nutrition of the lungs. The branches of these arteries
accompany the bronchial ramifications as far as the alveolar ducts, but
do not extend to the alveoli. The œsophageal artery also supplies
pulmonary branches (in the horse) which reach the lung by way of the
ligament of the latter. These branches vary much in size, and ramify
chiefly in the subpleural tissue of the basal part of the lung.[131] The
=lymph vessels= are numerous, and are arranged in two sets. The
superficial set forms close networks in and under the pleura, while the
deep set accompanies the bronchi and pulmonary vessels. All converge to
the root of the lung and enter the =bronchial lymph glands=.
=Nerve-supply.=—The pulmonary nerves come from the vagus and sympathetic
nerves. They enter at the hilus and supply branches to the bronchial
arteries and the air-tubes.