» The Great Vessels and Nerves Within the Thorax (see also pp. 229-230, 232, and 314-315)
The aorta is slightly expanded at its origin from the base of the heart between the pulmonary trunk to the left and the right atrium to the right, providing room for the aortic valve (Fig.
13.23/4). It first passes Craniodorsally before turning back to follow the vertebrae toward the diaphragm (see Fig. 13.14). Its arch, which is a prominent feature on lateral radiographs (Fig. 13.17/7), gives rise to the brachiocephalic trunk and, a short distance farther on at the level of the third intercostal space, to the left subclavian artery (Fig. 13.14/2). The brachiocephalic trunk lies ventral to the esophagus and trachea and detaches the two common carotid arteries that accompany these organs through the thoracic inlet before it continues as the right subclavian artery. The subclavian artery gradually shifts to the right before winding around the first rib to enter the forelimb. It is reported that the loss of a subclavian artery is compensated by the enlargement of collateral connections with the vertebral and other arteries.The pulmonary trunk arises from the craniosinistral aspect of the base of the heart to the left of the aorta. It passes dorsocaudally before dividing into divergent left and right pulmonary arteries (Fig. 13.23A/10 and 11). Shortly before its division, it is connected to the aorta by the ligamentum arteriosum. The right pulmonary artery, slightly larger than the left, passes across the base of the heart between the venae cavae. Each artery detaches a branch to the cranial lobe before entering the lung for further ramification.
The cranial vena cava passes ventral to the trachea, to the right of the brachiocephalic trunk, and in contact with the esophagus on the left side (see Fig. 13.15). It is the most ventral of the major structures that pass through the thoracic inlet and is formed cranial to the inlet by the union of the two brachiocephalic veins, each with tributaries corresponding to the branches of a subclavian artery (see Fig. 7.36), and is augmented by the addition of an external jugular vein.
The caudal vena cava spans the gap between the right atrium and the diaphragm and provides a very conspicuous feature of lateral radiographs of the chest. In the dog the right azygos vein receives the more cranial lumbar veins and, after entering the thorax, most intercostal veins; these vessels provide potentially significant connections with the internal vertebral venous plexus. The azygos vein ends by descending in front of the root of the right lung to join the cranial vena cava shortly before the latter opens into the right atrium opposite the third intercostal space.
There are no specific features of interest in the formation, course, or distribution of the phrenic, vagus, and sympathetic nerves.