The sympathetic trunk and the dorsal intercostal vessels show through the mediastinum and pleura. Puncture the mediastinum between these structures and bluntly remove fat until you see the bodies of the thoracic vertebrae. It is possible to reach behind a set of dorsal intercostal vessels with a finger and to help remove the fat. Observe that there are pairs of dorsal intercostal arteries rising from the dorsal side of the aorta to supply the intercostal muscles along the ribs 6 (5) and up in the horse and 4 and up in the ruminant. The intercostal muscles of first few intercostal spaces are supplied by branches from the supreme intercostal a branch of the costocervical trunk, which arises from the brachiocephalic trunk. The intercostal veins drain into the left and/or right azygous veins (species dependent). Also note that the intercostal vessels and nerves course caudal to the rib.
The arteries arising from the aortic arch are primarily distributed to the thoracic limb, neck and head. In the case of the horse and ruminant, there is only one main artery arising directly from the aortic arch and this is the brachiocephalic trunk. From the brachiocephalic trunk of these species both the right and left subclavian arteries will branch as will the common carotid arteries. In the pig and carnivore, the left subclavian artery branches directly from the arch followed by the brachiocephalic artery.
The naming of vessels branching off of the subclavian arteries (right and left) and brachiocephalic trunk is primarily based on where they are going. Their can be variations where these arteries arise between the right and left sides (ie on one side they branch from the subclavian on the other from the brachocephalic trunk), between individuals of the same species, and between species. That is why it is important name to the arteries based on where they are going. The following is a general discussion of typical arterial branching on the left side only.
Generally speaking, the following arteries are found branching from the subclavian artery: Supreme intercostal, dorsal scapular, deep cervical, vertebral, superficial cervical, and internal thoracic. In some species, 2-4 of these arteries arise via a common origin referred to as the costocervical trunk.
The supreme intercostal and dorsal scapular are two arteries that commonly arise via the costocervical trunk. The supreme intercostal gives rise to the first 4 to 6 dorsal intercostal arteries and can be seen branching from the costocervical trunk heading in a caudal direction. The dorsal scapular artery can be seen coursing towards the scapula and withers region where it supplies many of the muscles medial to the scapula and some of the skin in the region of the withers.
The vertebral and deep cervical arteries arise via the costocervical trunk in the ruminant but in the horse independently from the subclavian. The vertebral artery branches from its parent and runs in a dorsocranial direction towards the vertebral canal. The deep cervical artery has a similar course but it is more dorsally directed than the vertebral. The deep cervical artery supplies the epiaxial muscles of the neck from the withers cranially.
The last two arteries arising from the subclavian are the superficial cervical and internal thoracic. After these two branches the subclavian artery becomes the axillary. The superficial cervical artery provides for many structures located in the ventral portion of the neck and some of the muscles located lateral to the scapula; its general course reflects this distribution. The internal thoracic artery can be found coursing caudally on the floor of the thoracic cavity where it gives off many important branches. The ventral intercostal arteries arise from the internal thoracic and they anastomose with the dorsal intercostal arteries. In animals with a mammary glands located in the thoracic region, mammary branches are seen arising from the internal thoracic. The terminal braches of the internal thoracic are the musculophrenic and cranial epigastric arteries.