ernum, mark the level of the aortic arch,
on the back of the chest its level is to be taken from the vertebral
ends of the third or fourth ribs. This difference is caused by the
oblique descent of the ribs from the spine to the sternum. The first and
second dorsal vertebrae, with which the first and second ribs
articulate, are considerably above the level of the first and second
pieces of the sternum.

In a practical point of view, the pulmonary artery possesses but small
interest for us; and in truth the trunk of the systemic aorta itself may
be regarded in the same disheartening consideration, forasmuch as when
serious disease attacks either vessel, the "tree of life" may be said to
be lopped at its root.

When an aneurism arises from the aortic arch it implicates those
important organs which are gathered together in contact with itself. The
aneurismal tumour may press upon and obstruct the bronchi, H H*; the
thoracic duct, L; the oesophagus, I; the superior vena cava, H, Plate
26, or wholly obliterate either of the vagi nerves. The aneurism of the
arch of the aorta may cause suffocation in two ways--viz., either by
pressing directly on the tracheal tube, or by compressing and irritating
the vagus nerve, whose recurrent branch will convey the stimulus to the
laryngeal muscles, and cause spasmodic closure of the glottis. This
anatomical fact also fully accounts for the constant cough which attends
some forms of aortic aneurism. The pulmonary arteries and veins are also
liable to obstruction from the tumour. This will occur the more
certainly if the aneurism spring from the right or the inferior side of
the arch, and if the tumour should not break at an early period, slow
absorption, caused by pressure of the tumour, may destroy even the
vertebral column, and endanger the spinal nervous centre. If the tumour
spring from the left side or the fore part of the arch, it may in time
force a passage through the anterior wall of the thorax.

The principal branches of the thoracic aorta spring from the upper part
of its arch. The innominate artery, 2, is the first to arise from it;
the left common carotid, 6, and the left subclavian artery, 5, spring in
succession. These vessels being destined for the head and upper limbs,
we find that the remaining branches of the thoracic aorta are
comparatively diminutive, and of little surgical interest. The
intercostal arteries occasionally, when wounded, call for the aid of the
surgeon