The =common brachiocephalic trunk= or =anterior aorta= (Truncus
brachiocephalicus communis) is a very large vessel which arises from the
convexity of the arch of the aorta within the pericardium. It is
directed forward and upward. Its length in horses of medium size is
usually about two inches (ca. 5 to 6 cm.), but it is sometimes only half
an inch or less (ca. 1 cm.). It is crossed on the left by the left vagus
and cardiac nerves, and the left recurrent nerve runs between it and the
trachea. It divides opposite to the second intercostal space or third
rib into the brachiocephalic and left brachial arteries.
The =brachiocephalic artery= (A. brachiocephalica) is directed forward
and a little upward in the anterior mediastinum, beneath the trachea.
Opposite the first rib it gives off the bicarotid trunk and is continued
as the =right brachial artery=. The latter (A. subclavia dextra) turns
downward and bends around the anterior border of the first rib and the
insertion of the scalenus muscle above the brachial vein. Its course and
branches beyond this point will be described with the vessels of the
thoracic limb.
[Illustration:
FIG. 428.—TOPOGRAPHY OF THORACIC CAVITY OF HORSE, LEFT SIDE, AFTER
REMOVAL OF LEFT LUNG.
The ninth, fifteenth, and eighteenth ribs are retained. _a_,
Pericardium; _b_, thoracic aorta; _c_, brachiocephalic trunk; _d_,
_d″_, dorsal artery; _d′_, subcostal artery; _e_, _e′_, superior or
deep cervical artery; _f_, vertebral artery; _g_, _g′_, left
brachial artery; _h_, inferior cervical artery; _i_, internal
thoracic artery; _k_, common carotid artery; _l_, jugular vein; _m_,
brachial vein; _m′_, inferior cervical vein; _n_, anterior vena
cava; _o_, thoracic duct; _p_, œsophagus; _q_, costal part of
diaphragm; _q′_, tendinous center of same; _q″_, left crus of same;
_r_, anterior mediastinal lymph glands; _s_, trachea; _t_, bronchial
lymph glands; _u_, root of left lung; _v_, anterior mediastinum;
_v′_, posterior mediastinum; _w_, longus colli; _x_,
sterno-cephalicus; _y_, scalenus; _z_, transversalis costarum; _1_,
longissimus; _2_, splenius; _3_, spinalis et semispinalis; _4_,
_4′_, rhomboideus; _5_, left phrenic nerve; _6_, left vagus; _6′_,
_6″_, œsophageal continuations of vagi; _7_, dorsal branch of right
vagus; _8_, cardiac nerve; _9_, left recurrent nerve; _10_, cervical
trunk of sympathetic; _11_, first thoracic ganglion of sympathetic;
_12_, thoracic trunk of sympathetic; _13_, brachial plexus. (After
Ellenberger-Baum, Top. Anat. d. Pferdes.)
]
The =left brachial artery= (A. subclavia sinistra) is longer than the
right one and rises to a higher level. It forms an almost semicircular
curve, the concavity being ventral. It is related internally to the
œsophagus, trachea, and thoracic duct, and the left vagus, phrenic, and
cardiac nerves cross under its origin. It emerges from the thorax like
the artery of the right side. There is thus a difference at first
between the trunks of opposite sides, but beyond this their course and
distribution are similar.
The brachial and brachiocephalic arteries within the thorax give off the
dorsal, superior cervical, vertebral, and internal thoracic arteries. At
the first rib they give off the external thoracic and inferior cervical
arteries.
1. The left =dorsal= or =dorso-intercostal artery= (A. costo-cervicalis)
passes dorsally across the left face of the trachea and œsophagus toward
the second intercostal space. The right artery arises usually by a
common trunk with the superior cervical, crosses the right face of the
trachea and has no contact with the œsophagus. Both detach small
branches to the trachea, lymph glands, and pleura, and divide on
reaching the longus colli into two branches. Of these the =subcostal
artery= (A. intercostalis suprema) is the smaller. It passes backward
under the costo-vertebral joints with the sympathetic trunk. It gives
off the second, third, and fourth intercostal arteries, and ends at the
fifth space, where it anastomoses with the first aortic intercostal
artery, or constitutes the fifth intercostal artery, or dips into the
longissimus muscle. It also gives off spinal branches and twigs to the
longus colli and the pleura. The other branch (A. transversa colli) is
the direct continuation of the trunk. It emerges through the upper end
of the second intercostal space, passes across the transversalis
costarum and longissimus toward the withers, and divides into several
diverging branches. An anterior branch passes upward and forward between
the splenius and complexus and anastomoses with branches of the superior
cervical artery; the others are directed upward under the serratus
magnus and rhomboideus to the withers, supplying the muscles and skin of
this region.
[Illustration: