FOCUS QUESTIONS and ANSWERS:

1. Organize the innervation and blood supply of a complete intercostal space.

Nerves: An intercostal space is typically supplied by an intercostal nerve. Intercostal nerves are continuations of the ventral primary rami of spinal nerves that travel around the trunk in a groove under each rib. Just lateral to the costovertebral joint, the nerves (and arteries and veins) give off collateral branches that then travel on top of each rib. Other important branches of the intercostal nerves include the lateral cutaneous branches, coming off at about the midaxillary line, and the anterior cutaneous branches, emerging just lateral to the sternum.

Blood supply:

• The thoracic aorta gives off 9 pairs of posterior intercostal and 1 pair of subcostal arteries. (The first two intercostal spaces are supplied by branches of the costocervical trunk to be studied later.)
• The subclavian artery gives off the internal thoracic artery from which arise the anterior intercostal arteries
• The axillary arteries give rise to the highest and most lateral thoracic arteries.

Each intercostal space is supplied by three arteries, a large posterior intercostal artery and a small pair of anterior intercostal arteries.

2. Do you find any thymic nodules?  What are they?

This is an observation question. Remnants of the thymus, if present, should be in the anterior mediastinum. Although thymic nodules are rare, the remaining fat is in lobes that are shaped like the thymus that used to be there. (Greek, thymus = warty excrescence)

3. Define the boundaries of the superior mediastinum.

Boundary	Structure
Superior	superior thoracic aperture
Inferior	plane defined by sternal angle and T4/T5 IV disc
Lateral	mediastinal pleura
Anterior	manubrium of the sternum
Posterior	spinal column

4. Are there plexuses of nerves on the pulmonary arteries? 

The pulmonary plexuses lie around the roots of the lungs, and the pulmonary artery is one of the structures of the lung roots. Therefore, there are plexuses on the arteries.

5. Determine the relation of the left superior intercostal vein to the aortic arch, the phrenic nerve, and vagus nerves.

The left superior intercostal vein runs between the vagus and phrenic nerves. It crosses the aortic arch horizontally passing from the heads of the ribs anteriorly to end in the left brachiocephalic vein.

6. Observe the formation of the superior vena cava (confluence of both brachiocephalic veins), and relate its position to the manubrium and other great vessels.

The superior vena cava lies immediately to the right of the manubrium. It is overlapped on its left margin by the ascending aorta. Posteriorly, it lies against the right pulmonary artery and the right superior pulmonary vein.

7. Are there cardiac nerves arising from the left vagus? 

The left vagus normally gives parasympathetic branches to the cardiac plexus. They are most easily seen where the vagus crosses the aortic arch.

8. Do you find a vertebral artery from the arch of the aorta?

Sometimes the left vertebral artery will branch directly from the aortic arch.

9. Pull the aortic arch toward the left and observe the thoracic portion of the trachea. What innervates it and how?

The recurrent laryngeal nerves innervate the upper trachea and esophagus as they pass superiorly in the tracheoesophageal groove to reach the larynx, which they also innervate. Recurrent laryngeal nerves provide skeletal motor innervation to the voluntary muscle of upper esophagus and larynx and parasympathetic motor innervation to the smooth muscle of the trachea. They also provide parasympathetic motor innervation to the mucous glands of all of these structures.

10. What is the blood supply of the trachea? What structure does it lie upon?

For the blood supply, see above. The trachea lies upon the esophagus.

11. Locate the right and left bronchial arteries. What is their source? How do the two sides differ in number?

The two left bronchial arteries are branches from the highest part of descending aorta. The single right bronchial artery is either a branch of one of the left bronchial arteries or it may branch from the right 3rd posterior intercostal artery, in which case the common trunk is called the intercostobronchial trunk.

12. What is the difference between the "root" of the lung and the "hilum"?

The root of the lung is the collection of structures that connect the lung to the mediastinum. This includes the pulmonary arteries and veins, the primary bronchi and bronchial arteries, and the pulmonary nerve plexuses and lymphatics. The hilum is the place on the lung where these structures enter and leave the lung.

13. How do the cardiac and pulmonary plexuses differ? Where do they distribute?

The cardiac and pulmonary plexuses are continuous at the tracheal bifurcation. The heart receives the cardiac plexus and lungs the pulmonary plexus.

14. Where and what is the cardiac notch? 

The cardiac notch is a lateral deviation in the anterior border of the left lung, caused by the position of the heart. The lingula is an anterior projection of the left superior lobe below the cardiac notch.

15. What is the average projection of each lung and its fissures to the rib cage?

The anterior borders of each lung generally follow the sternal margin, although the cardiac notch causes a lateral deviation to the left anterior border between the 4th and 6th ribs. The apex of each lung projects to the level of T1, which means, due to the declination of the first rib anteriorly, that the apex rises above the first rib. The inferior border of each lung is located at rib 6 in the midclavicular line, rib 8 at midaxillary line, and rib 10 in the paravertebral line. (Recall that the pleura goes 8, 10, 12.) Both oblique fissures begin posteriorly at the spine of T3, passing obliquely (like the name implies) to reach the 6th costal cartilage near the midclavicular line. The horizontal fissure of the right lung begins at rib 5 in the midaxillary line, then reaches rib 4 to follow it anteriorly to the sternal margin.

16. What is the lingular bronchus?

The left superior lobe bronchus has two divisions: superior and lingular. The lingular division or lingular bronchus has two segmental bronchi: superior lingular and inferior lingular.

17. What is its significance of the superior segmental branch of the right inferior lobar bronchus?

Due to its larger caliber and more vertical orientation, the right primary bronchus is more likely to receive foreign bodies that enter the airway. The superior segmental bronchus branches posteriorly off of the intermediate bronchus or the inferior lobe bronchus, so it is the most likely segmental bronchus to receive those foreign bodies that enter the right bronchus. This is especially true if the patient is in a supine position.

18. Is the artery to the atrioventricular node a branch of the right, left, or both coronary arteries?

In 80-85% of people, the artery to the AV node is a branch of the right coronary artery. It arises just before the RCA gives off the posterior interventricular branch.

19. Define anterior cardiac and small cardiac veins.

Anterior cardiac veins: 3 or 4 small vessels arising on the anterior surface of the right ventricle. They cross the RCA (and the coronary sulcus) and end by penetrating the wall of the right atrium. Notably, most other veins drain into the coronary sinus rather than directly into the right atrium.

Small cardiac vein: usually begins as the marginal vein along the heart's acute margin. It receives tributaries from the right atrium and turns to the back of the heart, running with the RCA in the coronary sulcus. It ends by dumping into the right end of the coronary sinus.

20. Review the cardiac plexus and its distribution along the coronary vessels. (Latin, plectere = to braid)

The cardiac plexus is a network of sympathetic and parasympathetic nerve fibers located in the concavity of the arch of the aorta and behind it on the trachial bifurcation (the superficial and deep plexuses respectively). The sympathetic nerves, which increase the rate and force of the heartbeat and dilate the arteries, arise from cervical and thoracic ganglia. The vagus nerve supplies the parasympathetic fibers, which act in opposition to the sympathetics.

21. Define endocardium.

The endocardium is the thin internal layer or lining membrane of the heart. It also covers the heart's valves.

22. Examine the right atrioventricular (tricuspid) valve. Define the cusps.

There are three cusps. The two larger ones are the anterior and the septal cusps. The smaller one is the posterior cusp.

23. Define chamber walls: interventricular (or interatrial), anterior and posterior.

Right atrium: thin-walled sinus venarum posteriorly, more muscular anteriorly (including a muscular auricle), with interatrial septum between it and the left atrium

Left atrium: thin-walled portion posteriorly, muscular auricle more anteriorly, with interatrial septum between it and the right atrium

Right ventricle: lateral and anterior walls are muscular, while the posterior portion is mostly composed of interventricular septum

Left ventricle: interventricular septum is more anterior, rest of wall is muscular

24. In the right ventricle, define papillary muscles, chordae tendineae, and tricuspid valve cusps. (Latin, papilla = nipple)

The right ventricle usually has only two papillary muscles (anterior and posterior); occasionally there is one or more small septal papillary muscles. They are muscular projections from the ventricular wall which have tendinous cords (chordae tendineae) extending to the tricuspid valve cusps. The anterior muscle is the largest and has tendinous cords attaching to the anterior and posterior cusps. The smaller posterior muscle, which may consist of several parts, has cords attaching to the posterior and septal cusps. The septal muscle, when there is one, has cords attaching to the anterior and septal cusps. If there is no septal muscle, chordae tendineae arise directly from ventricle's septal wall.

25. Do you find a septomarginal trabecula?

The septomarginal trabecula (moderator band), when present, is a muscular ridge of myocardium extending from the right ventricle's septal wall to the base of the anterior papillary muscle. (Latin, trabecula = little beam)

26. Name the semilunar cusps of the pulmonary valve.

The pulmonary valve's three semilunar cusps are named according to their orientations: right, left and anterior.

27. Does the left atrioventricular (mitral, bicuspid) valve have any commissural cusps? Note relation of anterior cusp to aortic wall.

Commissural cusps are small accessory cusps occurring between the named cusps (anterior and posterior). The anterior cusp of the mitral valve abuts and curves around the base of the aorta.

28. What is the aortic vestibule?

The aortic vestibule is the superior portion of the left ventricle which leads into the root of the aorta.

29. Name the semilunar cusps of the aortic valve.

The aortic valve's three cusps, like the pulmonary valve's, are named according to position: right, left and posterior. The right and left cusps are associated with the right and left coronary arteries. The posterior cusp is referred to as the "non-coronary cusp".

30. Where are the sounds associated with each heart valve best heard with a stethoscope?

Mitral: 5th left intercostal space
Tricuspid: 4th left intercostal space
Pulmonary: left upper sternal border
Aortic: right upper sternal border

31. What are the fibrous rings (annulus fibrosus)?

There are four fibrous rings, one surrounding each valve. They are made of dense collagen and are part of the heart's fibrous skeleton. This skeleton is the point of attachment/insertion for cardiac muscle - the atrial muscle from above and the ventricular muscle from below.

32. What is the relationship of the heart's fibrous skeleton to its conduction system? Why is this important?

The fibrous skeleton provides attachment for heart's myocardium and valves. Additionally, it serves as an electrical insulator between the atria and ventricles penetrated only by the conduction system. This ensures that there is a pause between the contraction of the atria (leading to ventricular filling) and the contraction of the ventricles (ventricular ejection or emptying).

33. Are there sympathetic branches to the lung? Along what do they distribute?

Sympathetic fibers reach the lungs via the pulmonary plexuses, which are located along the roots of the lungs. Pulmonary plexuses are continuous with the cardiac plexus at the tracheal bifurcation. Additional sympathetics reach the pulmonary plexuses via the thoracic visceral nerves, which are branches from T1-T4/T5 sympathetic chain ganglia.

34. Where does the esophagus begin? Where does it pass into the abdomen? Where does it terminate?

The laryngopharynx "becomes" the esophagus at the C6 level. The esophagus passes through the esophageal hiatus of the diaphragm, which is at the T10 level, to end in the cardiac portion of the stomach.

35. Consider the course, relations, constrictions of the esophagus. 

For course and relations, see above. Constrictions of the esophagus are found at its beginning, at the tracheal bifurcation, and at the esophageal hiatus.

36. Describe the blood supply and venous drainage of the esophagus. Are there venous collaterals to stomach?

Two or three esophageal arteries branch from the descending aorta. Esophageal veins drain into the azygos system, which eventually reaches the superior vena cava. The esophageal tributaries of the left gastric vein drain the terminal esophagus. Since the gastric veins first drain into the portal system before going to the heart, this part of the esophagus is an important site of portal-caval (portal-systemic) anastomosis in cases of portal hypertension.

37. Do the right and left mediastinal pleurae come together?

Very low in the posterior mediastinum the esophagus sweeps forward, so there is potential for the right and left mediastinal parietal pleurae to touch one another posterior to esophagus and anterior to aorta. However, typically the anterior deviation of the esophagus is not sufficient to allow enough space for this contact of the pleurae.

38. Through what and at what level does the aorta enter the abdominal cavity?

The descending thoracic aorta passes through the diaphragm at the aortic hiatus, a passageway between the two diaphragmatic crura, located at the T12 level.

39. What is the subcostal artery?

Below the 12th rib, there is no intercostal space or intercostal artery, so we call the segmental neurovascular structures subcostal.

40. Completely review the blood supply to an intercostal space.

Intercostal spaces in general are supplied by posterior and anterior intercostal arteries. Posterior intercostal arteries 3 through 11 are branches of the descending thoracic aorta. The first two posterior intercostal arteries are branches of the highest intercostal artery, which is a branch of the costocervical trunk from subclavian artery. Anterior intercostals are branches of the internal thoracic or the musculophrenic arteries.

41. What are the posterior branches of the posterior intercostal (segmental) arteries? What do they supply?

The posterior branches of posterior intercostal arteries supply the deep and superficial back muscles, skin of the back, and the vertebral column. They have radicular branches that reach the spinal cord along the dorsal and ventral rootlets.

42. Observe the azygos venous system. If you have two primary veins, do they communicate with one another? How? Where? What is the pattern of venous drainage in your specimen? Are all the veins present? If not, where does the drainage go?

Hemiazygos and accessory hemiazygos usually cross the midline at T7, 8, or 9 to empty into the azygos vein. There are also connections between the various left-sided venous channels.

43. What vein drains the first intercostal space? Into what does it drain? What veins drain into the azygos system?

Both first posterior intercostal veins drain directly into their respective brachiocephalic veins. Bronchial and esophageal veins drain into the azygos system, the latter being a significant site of portal-caval anastomosis in cases of portal hypertension. (This is the second mention... could be important!)

44. How does the thoracic duct get into the thorax? At what level does it deviate to the left side?

The thoracic duct enters the chest through the aortic hiatus along the right side of the aorta. It deviates to the left at the level of the sternal angle. (The trachea pushes the esophagus against the vertebral bodies which pushes thoracic duct to the left.)

45. Do you find posterior mediastinal lymph nodes or afferent lymph channels?

The thoracic duct is usually paralleled by posterior mediastinal nodes.

46. What are bronchomediastinal lymph trunks?

The lymph from the lungs and chest passes through the paratracheal nodes to form the bronchomediastinal lymph trunks. These drain to the thoracic duct on the left and the right lymphatic duct on the right.

47. Is the sympathetic trunk located within the posterior mediastinum? Does it change positions in different regions of the chest?

The sympathetic trunk lies on the heads of the ribs through most of the chest, so it is almost, but not quite, within the posterior mediastinum. It deviates anteromedially as it travels inferiorly to its exit through the diaphragm.

48. How many thoracic ganglia do you find?

There are typically 12 thoracic ganglia, although the first may be fused with the inferior cervical ganglion to form a cervicothoracic (stellate) ganglion.

49. Identify white and gray rami communicans. What is their significance and distribution? What do they contain?

Since the preganglionic sympathetic neurons live within the spinal cord at T1 to L2 levels, these are the only levels where white rami communicans are found. Gray rami, on the other hand, are found at every level at which there are spinal nerves. Gray rami carry postganglionic sympathetic fibers back to the ventral primary rami, to be distributed along their branches (and also the branches of the dorsal primary rami).

50. Do you see thoracic visceral nerves to the aorta, esophagus, and trachea? What about to the cardiac and pulmonary plexuses?

The first 4 or 5 thoracic ganglia (T1-4) give small visceral branches that pass anteroinferiorly to reach the cardiac, pulmonary, esophageal, and aortic plexuses, as well as the trachea.

51. Expose the greater (thoracic) splanchnic nerve. From what does it come? At what level? What types of fibers does it contain? To what does it distribute?

The greater thoracic splanchnic nerve is made by contributions from sympathetic chain ganglia at T5 to T9 (or T10) levels. These are preganglionic fibers that leave the chest to enter the abdomen. They synapse in the celiac ganglion and innervate the abdominal viscera that is supplied by the celiac trunk.