Part 3: Vessels Cranial to the Heart

IMPORTANT NOTE: Reflect/push the sternum over to the left side to facilitate exposure and identification of the vessels for this part of the lab.

1. Using blunt dissection (scissor spreading technique), in the thoracic cavity, expose the following veins cranial to the heart:

   Related Learning Objective

   • D4.9

    

   • cranial vena cava (See DG Figs. 3-14A, 3-15A, 3-17, 3-20, and 3-35)

     • • The cranial vena cava is formed by the union of the left and right brachiocephalic veins at the thoracic inlet. It drains into the right atrium of the heart.

       • Dissection Note: You may see the left and right internal thoracic veins emptying into the cranial vena cava (they are found running alongside the internal thoracic arteries), but you need not identify them.

   • brachiocephalic v. (right and left) (See DG Figs. 3-15A, 3-17, and 3-35)

     • • Each brachiocephalic vein is formed by the union of the external jugular and subclavian veins. (Each brachiocephalic vein is typically very short.)

   • external jugular v. (right and left) (See DG Figs. 3-15A, 3-17, 3-20, 3-26, and 3-35) (Note that you previously dissected these in Chapter 3.)

     • • The external jugular veins are large and easily identifiable in most specimens. They will be seen in the neck, one on either side, draining the head. (Recall that these veins are a common venipuncture site.)

   • subclavian v. (right and left) (See DG Figs. 3-15A, 3-17, and 3-35)

     • • The subclavian veins drain the blood returning from the forelimbs. (Each of the subclavian veins is typically very short.)

   • azygos v. (See DG Fig. 3-14A)

     • • The azygos v. is usually the last branch entering the cranial vena cava (but in some cases it may enter the right atrium of the heart directly). It is best seen within the right side of the thoracic cavity, along the dorsal aspect of the ribs; it then curves around the root of the lung as it extends ventrally to join the cranial vena cava.

       • Comment: Only the right azygos v. develops in the dog and cat, but some species will have a left azygos instead of, or in addition to, the right azygos.

2. In the Application section of this course, you will learn about the thoracic duct. We will not spend time attempting to identify the thoracic duct during dissection because it is often difficult to see/identify unless it is congested with lymph. The thoracic duct can often be found between the azygos vein and the descending aorta (in the right side of the thoracic cavity) and will often appear ‘straw colored,’ or have a dark reddish-brown tint. It will also have a characteristic ‘segmented’ appearance if it contains lymph, due to the presence of valves. (If it is empty it will be quite difficult to see.) (See DG Fig. 3-14B)

   • Comment: The thoracic duct runs cranially and may also be observable after it crosses to the left side of the animal around the level of the fifth thoracic vertebra. On the left side, it may be seen extending cranially to the left brachiocephalic vein where it usually terminates. The thoracic duct may receive drainage from the tracheal lymphatic ducts.

   • If you wish, in demonstration animals, attempt to identify the thoracic duct and tracheal lymphatic ducts if they are visible.

     Related Learning Objective

     • D4.9

      

3. Still using blunt dissection (scissor spreading technique), in the thoracic cavity, expose the aorta (thoracic aorta). The aorta is a large, thick, unpaired vessel that emerges from the left ventricle of the heart. It has three parts: ascending aorta, aortic arch, and descending aorta. (See DG Figs. 3-16, 3-17, 3-18, and 3-20)

   • Attempt to identify the ascending aorta emerging from the heart and coursing cranially for a short distance before it curves to the left.

     • • Dissection Note: Be aware that the ascending aorta is quite short and is difficult to see without reflecting part of the pericardium surrounding the heart. If needed, come back to the ascending aorta after you have completed the heart dissection to re-identify it. (See DG Figs. 3-17 and 3-18)

   • Trace the ascending aorta and identify the aortic arch as the (ascending) aorta bends sharply to the left. (The aortic arch is also quite short.) (See DG Figs. 3-17 and 3-18)

   • Continue to trace the aorta and identify the descending aorta coursing caudally, just ventral to the vertebral column. (See DG Figs. 3-5, 3-14A, 3-16, and 3-20)

     • • Comment: The descending aorta has two parts: thoracic and abdominal. The portion of the descending aorta cranial to the diaphragm is the thoracic part and the portion of the descending aorta caudal to the diaphragm is the abdominal part. (Note: The abdominal part will not be seen until we enter the abdomen.)

       • As you trace the descending aorta caudally, note the (dorsal) intercostal arteries leaving the thoracic (descending) aorta. The (dorsal) intercostal arteries come off of the descending aorta in pairs and then continue along the caudal aspect of each rib (where they were previously identified ). (See DG Figs. 3-5, 3-16, 3-19, and 3-20)

         • • Comment: There are eight to nine pairs of dorsal intercostal arteries that leave the aorta, beginning around the fourth or fifth intercostal space. (Recall that the costocervical trunk supplies the first three or four intercostal spaces.)

4. The first branches coming from the (ascending) aorta are the right and left coronary arteries. These will not be dissected now, but instead will be identified when we study the heart later in this lab.

5. Return to the aortic arch; identify the brachiocephalic trunk, which is the first branch coming off of it. The brachiocephalic trunk courses obliquely to the right, crossing the ventral surface of the trachea. It is a large artery that then gives rise to the left common carotid, right common carotid, and right subclavian arteries. (Note that the continuations of the carotid aa. were previously identified in Chapter 3, along with other structures within the carotid sheath of the neck.) (See DG Figs. 3-14A, 3-15B, 3-16, 3-17, 3-18, and 3-20)

   • Identify the left common carotid a. The left common carotid passes to the left side of the neck, coursing cranially, within the carotid sheath, alongside the trachea. (See DG Figs. 3-15B, 3-16, 3-17, and 3-18)

   • Identify the right common carotid a. The right common carotid will course cranially in the neck on the right side, within the carotid sheath, alongside the trachea. (See DG Figs. 3-15B, and 3-17)

   • Identify the right subclavian a. given off of the brachiocephalic trunk after the left and right common carotid aa. (The subclavian artery gives off several branches that will be dissected after identification of the left subclavian a.) (See DG Figs. 3-14A, 3-17, 3-18, and 3-19)

6. Return to the aortic arch and trace it as it curves to the left side. Identify the left subclavian a. coming off of the aortic arch after the origin of the brachiocephalic trunk.

7. The left and right subclavian arteries give off several named branches, but, in the interest of time, we will not dissect these – with the exception of the internal thoracic a., which you already identified running alongside the sternum.

   • Comment: The branches given off the subclavian arteries are the vertebral a., costocervical trunk, superficial cervical a., and internal thoracic a. These branches all arise from the subclavian medial to the first rib (before leaving the thoracic cavity). If you wish to identify these arteries, please view the demonstration specimens. (See DG Figs. 3-14A, 3-16, 3-17, 3-18, 3-19, and 3-20)

     • • The vertebral a. leaves the subclavian and dives dorsally (usually in between the longus colli and scalenus mm.). This is often the first branch from the subclavian a.

         • • Comment: The vertebral a. then enters the transverse foramen of the sixth cervical vertebra and passes through the transverse foramina of the first six cervical vertebrae. (See DG Figs. 3-14A, and 3-16)

       • The costocervical trunk extends dorsally from the subclavian. Almost immediately, the trunk branches again to supply the first three (or four) intercostal spaces as well as muscles of the neck and muscles dorsal to the first few thoracic vertebrae. Often the costocervical trunk is the second branch from the subclavian. (See DG Figs. 3-14A, 3-16, and 3-19)

       • Re-identify the internal thoracic a. (previously identified at the beginning of this lab). The internal thoracic a. leaves the subclavian and courses caudally alongside the sternum (within the thorax). (See DG Figs. 3-14A, 3-16, 3-19, and 3-20)

       • The superficial cervical a. arises from the subclavian opposite the origin of the internal thoracic a., just medial to the first rib. The superficial cervical a. then emerges from the thoracic inlet to supply the base of the neck and scapular region. (See DG Figs. 3-14A, 3-16, and 3-19)

         • • Comment: The superficial cervical a. will course near the superficial cervical lymph nodes.

8. After giving off the four branches just described, the subclavian a. will exit the thorax and change names to the axillary a. (both left and right sides) as described in Chapter 1. (See DG Figs. 3-14A, 3-16, 3-17, 3-18 and 3-19)

• • Trace the subclavian a. and locate the region where this name change occurs.