expiration. The first part of the
aortic arch, as it springs from the left ventricle of the heart, is the
most superficial, being almost immediately sub-sternal, and on a level
with the sternal junctions of the fourth ribs. By applying the ear at
this locality, the play of the aortic valves may be distinctly heard.
From this point the aorta, G, rises and arches from before, backwards,
to the left side of the spine, G*. The arch of the vessel lies more
deeply between the two lungs than does its ventricular origin. The
descending thoracic aorta lies still more deeply situated at the left
side of the dorsal spine. At this latter situation it is in immediate
contact with the posterior thick part of the left lung; whilst on its
right are placed, L, the thoracic duct; I, the oesophagus; K, the vena
azygos, and the vertebral column. In Plate 26 may be seen the relation
which the superior vena cava, H, bears to the aortic arch, A.

In the span of the aortic arch will be found, H*, the left bronchus,
together with the right branch of the pulmonary artery, and the right
pulmonary veins. The pneumo-gastric and phrenic nerves descend on either
side of the arch. The left pneumo-gastric nerve winds round beneath the
arch at the point where the obliterated ductus arteriosus joins it. See
Plates 12 & 26.

The pulmonary artery, B, Plates 1 & 2, lies close upon the fore part,
and conceals the origin, of the systemic aorta. Whenever, therefore, the
semilunar valves of either the pulmonary artery or the systemic aorta
become diseased, it must be extremely difficult to distinguish by the
sounds alone, during life, in which of the two the derangement exists.
The origins of both vessels being at the fore part of the chest, it is
in this situation, of course, that the state of their valves is to be
examined. The descending part of the thoracic aorta, G*, being at the
posterior part of the chest, and lying on the vertebral ends of the left
thoracic ribs, will therefore require that we should examine its
condition in the living body at the dorsal aspect of the thorax. As the
arch of the aorta is directed from before backwards--that is, from the
sternum to the spine, it follows that when an aneurism implicates this
region of the vessel, the exact situation of the tumour must be
determined by antero-posterior examination; and we should recollect,
that though on the fore part of the chest the cartilages of the second
ribs, where these join the st