» The Great Vessels and Nerves Within the Thorax
The pattern of arterial branching of the aortic arch is shown in Fig. 7.37 and need not be further described because details are of no clinical significance. Rupture of the aortic wall in the sinus region or at the origin of the brachiocephalic trunk may occur and with fatal consequences in horses.
It appears to indicate inherent weakness at these sites because pathologic change is rarely evident.The presence of a single right azygous vein may be used to distinguish the equine from the bovine heart.
The formations, the courses, and the ramifications of the phrenic, sympathetic, and vagus nerves conform to the usual patterns. The relationship of the left recurrent laryngeal nerve to the aortic arch, though not specific to the horse, deserves emphasis because intermittent stretching of the nerve with the pulsation of the vessel has been postulated as a factor in the etiology of laryngeal hemiplegia ("roaring"). The closer association of the left nerve to the tracheobronchial lymph nodes is a second factor of alleged but unproven significance.