< Thoracic wall >
The thorax is composed of the thoracic wall and the thoracic cavity. Equally, the abdomen is composed of the abdominal wall and the abdominal cavity (Fig. 6-3); the pelvis is composed of the pelvic wall and the pelvic cavity (Fig. 7-1). All three walls consist of bones and muscles; the thoracic wall consists of ribs (including costal cartilages) and intercostal muscles.
Other bones for the thoracic wall are the sternum and thoracic vertebrae (Fig. 2-5). The R2 is at the same level of the sternal angle.
R1, R10–R12 are each attached to only one thoracic vertebra (Fig. 2-5). Meanwhile, the other R2–R9 are attached to the two adjacent thoracic vertebrae.
Fig. 5-1. Intercostal muscles.
Along with the scalene muscles holding R1–R2 (Figs. 3-10,11), the intercostal muscles elevate the ribs. The elevation of the ribs results in an enlarged thoracic cavity, which is for inhalation. The mechanism between the rib elevation and thoracic cavity enlargement can be explained by two movements.
Fig. 5-2. Movement of thoracic wall for inhalation.
One is the bucket handle movement along the anteroposterior axis, which increases the mediolateral length of the thoracic cavity. When at rest, the lateral part of the ribs is inferior to the medial part; the bucket handle movement elevates and abducts the lateral part.
Another is the pump handle movement along the mediolateral axis, which increases the anteroposterior length of the thoracic cavity. When at rest, the anterior part of the ribs is inferior to the posterior part; the pump handle movement elevates and protracts the anterior part.
The mediolateral and anteroposterior elongations of thoracic cavity can be easily felt with your own body during the deep inhalation.
The other movement to increase the superoinferior length is achieved by the diaphragm. When this dome-shaped muscle contracts, the muscle lowers itself (Fig. 5-48). This increase of superoinferior length plays a bigger role in expanding the thoracic cavity than the previous two movements of the ribs.
If one fully inhales then relaxes the intercostal muscles and diaphragm, one will exhale naturally. This is because our lungs have a tendency to shrink due to the surface tension of the pulmonary alveoli. Surface tension is an object’s tendency to reduce its surface area. In other words, natural exhalation does not require energy.
However, energy is demanded for forced exhalation in some occasions (e.g., blowing a balloon). As surface tension is not enough for such tasks, the anterolateral abdominal wall muscles need to contract. The muscles are convex anteriorly, so their contraction can press down upon the abdominal cavity (Figs. 6-2,3), and subsequently push the diaphragm upward.
Fig. 5-3. Transversus thoracis.
Supportive muscles that depress the ribs are the transversus thoracis and the serratus posterior inferior. Meanwhile, a supportive muscle that elevates the ribs is the serratus posterior superior (Fig. 1-10).
Fig. 5-4. Intercostal muscles, vein, artery, and nerve.
The transversus thoracis is deep to the internal thoracic artery, while the innermost intercostal muscle is deep to the intercostal artery. Since the two arteries are connected (Fig. 5-7), we can logically say that these two muscles are at the same depth.
The intercostal artery is accompanied by the intercostal vein and nerve. If the troika did not exist, the internal and innermost intercostal muscles would not be distinguishable due to their identical muscle direction (Fig. 5-1).
Protected by ribs, the intercostal vein, artery, and nerve exist below the costal groove. When injecting a needle to the intercostal space, doctors should avoid injecting just below the rib so as to prevent injuring the three structures.
Although the rib protects these three structures, it can also cause damage to them when it is fractured. Therefore, the intercostal vein which is relatively less vital is located at a more dangerous (superior) place than the nerve and artery.
Intercostal nerves originate from TN1–TN11, while the subcostal nerve originates from TN12. More specifically, the intercostal nerves are the anterior rami and anterior cutaneous branches of TN1–TN11 (Fig. 1-2).
The intercostal nerves innervate both the intercostal muscles (somatic motor nerve) and the skin that covers the ribs (somatic sensory nerve). Sensory distribution of each spinal nerve to the skin is called a dermatome; the representative dermatomes of thorax and abdomen are explained below.
Fig. 5-5. Dermatomes of thorax, abdomen.
The nipple is located at the level of R4 (Fig. 2-7); therefore, the nipple must be innervated by TN3 or TN4 just above or below R4. It is indeed innervated by TN4.
Skin on the xiphoid process is innervated by TN7, since R7 is directly attached to the xiphoid process. As “xiphoid” is spelled with 7 letters, the xiphoid welcomes R7 and TN7.
Below R7, the anterior intercostal spaces gradually decrease in length, so TN7–TN11 pass not only the “thoracic” wall (as the intercostal nerves) but also the “abdominal” wall. This is the reason why TN7–TN11 are named “thoracoabdominal” nerves. Among them, TN10 is distributed to the umbilicus. This is why the umbilicus is drawn as X (10 in Roman numerals) in this book.
Next to the subcostal nerve, LN1 (ilioinguinal nerve) (Fig. 6-55) innervates the skin on the inguinal ligament.
In summary, dermatomes of the nipple (TN4), xiphoid process (TN7), umbilicus (TN10), and inguinal ligament (LN1) have same intervals between them. Skin distances that separate the four landmarks are also almost identical.
Intercostal arteries are composed of the anterior and posterior intercostal arteries. Even though memorizing the two sets of arteries is cumbersome, we should be thankful because the two sets make anastomosis to supply the thoracic wall with sufficient blood.
Fig. 5-6. Posterior intercostal arteries.
The 3rd–11th posterior intercostal arteries branch from the thoracic aorta. The 1st and 2nd posterior intercostal arteries are located too high to branch off from the thoracic aorta, of which the upper border is on the level between TV4 and TV5 (Figs. 5-39,41). Instead, the 1st and 2nd posterior intercostal arteries come from the costocervical trunk of the subclavian artery (Fig. 3-26).
Fig. 5-7. Anterior intercostal arteries.
The 1st–6th anterior intercostal arteries are branches of the internal thoracic artery (Fig. 5-4) of the subclavian artery (Fig. 3-26). The internal thoracic artery ends at the level of R7 (Fig. 5-5) to divide into the musculophrenic artery and superior epigastric artery. The 7th–9th anterior intercostal arteries are branches of the musculophrenic artery.
In the “musculophrenic” artery, “musculo” refers to the intercostal muscles (Fig. 5-4); “phrenic” refers to the diaphragm (Fig. 5-48). The diaphragm is fed by the musculophrenic artery, the superior phrenic artery from the thoracic aorta, and the inferior phrenic artery from the abdominal aorta (Fig. 6-48).
There are no 10th and 11th anterior intercostal arteries, because there are no 10th and 11th anterior intercostal spaces. R11 and R12 are floating ribs.
< Heart >
The heart is a vital organ which is covered by the pericardium.
Fig. 5-8. Development of serous pericardium, pleura, peritonea.
In the case of crowded tendons of the wrist, the synovial sheath containing synovial fluid relieves friction (Fig. 2-52). Likewise, in the case of heart, lungs, and gastrointestinal tract that move constantly, the serous pericardium, pleurae, and peritoneum are responsible for the lubrication.
Embryologically, a single balloon called the intraembryonic celom is separated to become the five balloons with the same property. The five balloons are the serous pericardium, two pleurae (Fig. 5-28), and two peritonea (Fig. 6-15). The serous pericardium develops to be situated ventral to the heart and pleurae after head fold takes place (Fig. 6-16).
Cavities of the five balloons are the pericardial, pleural, and peritoneal cavities. All the cavities contain serous fluid which is slippery unlike mucous fluid (e.g., viscous sputum). As a matter of fact, serous fluid is small in amount, so that the cavities have close to zero volume, but they can be expanded by the influx of air or blood. Thus, the cavities are potential spaces like the subdural spaces of the spinal cord (Fig. 1-16) and the brain (Fig. 4-7).
Fig. 5-9. Serous and fibrous pericardia.
The serous pericardium consists of visceral and parietal layers. The visceral layer is in close contact with the cardiac muscle, so the visceral layer is regarded as a part (epicardium) of the heart wall.
On the other hand, the parietal layer is attached to the fibrous pericardium which also sticks to the underlying diaphragm (to be exact, central tendon) (Fig. 5-47). During dissection, the parietal layer, fibrous pericardium, and diaphragm cannot be detached from one another.
Fig. 5-10. Development of transverse pericardial sinus.
The serous pericardium undergoes the following developmental process. After the head fold (Fig. 6-16), the anterior serous pericardium grows to surround the tube-like heart with the exception of the posterior portion. The posterior portion is the dorsal mesentery made of double layers of the serous pericardium. From this stage, visceral and parietal layers are apparent.
Successively, the dorsal mesentery disappears and the serous pericardium completely encircles the heart. The part that formerly used to be the dorsal mesentery is called the transverse pericardial sinus now, a part of the pericardial cavity (Fig. 5-9).
Fig. 5-11. Two pericardial sinuses after development.
As the heart gets bent and starts to take shape of the heart as we know it, the serous pericardium is reformed. In the longitudinal plane, the transverse pericardial sinus also appears where the dorsal mesentery has disappeared. Another sinus (another part of the pericardial cavity) is the oblique pericardial sinus between the heart and the vein. It is a dead end pouch, where visceral and parietal layers meet. The two sinuses can be touched after exposing the pericardial cavity of a cadaver. It is important remember that the transverse one is opened, but the oblique one is closed.
Fig. 5-12. Removal of heart, visceral layer of serous pericardium (left) to examine arteries, veins, parietal layer (right).
After the heart and the visceral layer are removed from the left of the figure above, the arteries, veins, and parietal layer can be depicted as in the right. After the development, two arteries (ascending aorta and pulmonary trunk) and six veins (superior vena cava, inferior vena cava, and four pulmonary veins) are formed. The transverse and oblique pericardial sinuses are represented just as one would find in a cadaver.
Fig. 5-13. Heart with four chambers.
In the heart, the apex is the inferior left vertex, while the base (not labeled in figure) is the superior border. A noteworthy structure above the base is the ligamentum arteriosum (formerly, ductus arteriosus (Fig. 6-36)) connecting the left pulmonary artery and the aortic arch. Around the ligamentum arteriosum, left X and left recurrent laryngeal nerve (Fig. 3-23) can be traced.
The coronary sulcus exists between the atria and ventricles. “Coronary” means crown; another instance where coronary is used in anatomical nomenclature is the coronary ligament of the liver (Fig. 6-21). Extending its meaning, “coronal” refers to the queen’s tiara that fits at the coronal suture and the coronal plane.
Other sulci are the anterior and posterior interventricular sulci; there is no interatrial sulcus.
Contrary to the name “sulcus,” the name “groove” is commonly employed for bone structures (e.g., groove for radial nerve (Fig. 2-28), costal groove).
Fig. 5-14. Heart’s rotation position.
In an imaginary heart, the coronary sulcus is horizontal, and the interventricular sulci are sagittal. This heart is rotated along two axes. Suppose that the heart is a person in a thong (three sulci). He/she leans back in a sofa and turns left. Because of the action of leaning back, the ventricles are anterior to the atria; due to the action of turning left, the right ventricle is anterior to the left ventricle. This rotation is a tragedy for medical students and doctors who have to identify a patient’s heart chambers using computed tomographs and ultrasonographs.
Fig. 5-15. Inside of right atrium with right auricle reflected.
Coherently, since the heart is leaning back, the valves between atria and ventricles are obliquely placed. Inside right atrium, the tricuspid valve toward the right ventricle is situated inferiorly and anteriorly.
The inside of the right atrium is divided into rough and smooth areas. The rough area possesses pectinate muscles. The exterior of the rough area is called the right auricle which is observable in the anterior view of the heart (Fig. 5-13). The boundary between the rough and smooth areas is termed the terminal crest (inside) and the terminal sulcus (outside) (Fig. 5-13).
In all heart chambers, the rough area pumps blood, while the smooth area receives and conveys blood. The smooth area of the right atrium is where the superior vena cava, inferior vena cava, and coronary sinus open.
The superior vena cava does not need a valve at all because of gravity. The inferior vena cava has a nonfunctional valve, but the coronary sinus has a functional valve. The two valves of the inferior vena cava and coronary sinus are continuous within the terminal crest, since the three structures have been derived from a single in an embryo.
The smooth area of the right atrium also contains an oval fossa and its prominent superior border (limbus of oval fossa).
Fig. 5-16. Partitioning of atrium.
The partitioning of one atrium into two must be explained. The heart used to be a tube just like a blood vessel (Fig. 5-11). The atrium was a single pouch. To divide an atrium, septum primum grows from the top. Since septum primum does not fully reach the endocardial cushion, foramen primum remains. Endocardial cushion is the anteroposterior column between the four heart chambers.
When the foramen primum is closed, foramen secundum (not secondum) is formed. In the coronal plane, septum primum looks as if it drops down due to gravity.
The problem of the incomplete septum primum is solved by the septum secundum that sprouts from the top again. The gap between the two septa is called the oval foramen.
Before birth, it is difficult for the blood in the right atrium to depart, due to unexpanded, nonelastic lungs. Sequentially, the blood pressure of prenatal right atrium is high. Reversely, it is easy for the blood in the left atrium to leave, because of the umbilical arteries (Fig. 6-36). Thus, the blood pressure of prenatal left atrium is low. Consequently, blood in the right atrium passes to the left atrium through the oval foramen.
After birth, such situation is reversed. Lungs expand while the umbilical artery is obliterated. Blood pressure in the left atrium becomes higher than that in the right atrium. Then, the blood passage stops between the atria because the thin septum primum plays the function of a valve.
Since the septum secundum is thicker than and more right than the septum primum, one can clearly identify the oval fossa and its limbus inside the right atrium, and not inside the left atrium.
Fig. 5-17. Interior of right ventricle.
The inside of the right ventricle is also divided into the conveying smooth area and the pumping rough area. The smooth area between the tricuspid valve and the pulmonary valve is called conus arteriosus because the shape resembles a cone.
The pulmonary valve is structurally complete like the aortic valve (Fig. 5-24) and the venous valve of cutaneous vein. However, the tricuspid valve is not; thus, it is possible that its cusps move into the right atrium when the right ventricle contracts. In order to prevent the blood from regurgitating, these cusps are held in place by the tendinous cords which arise from the papillary muscles.
The three (anterior, posterior, septal) cusps of tricuspid valve (Fig. 5-21) determine the three papillary muscles. Theoretically, only three tendinous cords are required, but in reality six tendinous cords are present for stabilization; each cusp is connected to two papillary muscles, while each papillary muscle is connected to two cusps. Everything is double dealing!
The rough area of the right ventricle contains not only papillary muscles but also the trabeculae carneae. Trabeculae carneae, formed by cardiac muscles, are equivalent to pectinate muscles in the right atrium (Fig. 5-15). Among the trabeculae carneae, the most prominent one is the septomarginal trabecula which connects the interventricular septum (muscular part) (Fig. 5-27) to the anterior papillary muscle.
In clinics, the left chambers (left atrium and ventricle) of the heart are more meaningful than the right chambers. But the left chambers will be dealt with briefly, because there is little difference from the right chambers.
Fig. 5-18. Inside of left atrium with left auricle reflected.
The left atrium receives blood from four pulmonary veins. Unlike the right atrium (Fig. 5-15), the left atrium has neither a terminal crest nor a terminal sulcus between the smooth and rough areas. The rough area, equipped with the pectinate muscles, is the left auricle when seen from the outside (Fig. 5-13). Just as there are two auricles attached to the head (Fig. 4-50), there are also two auricles in the heart.
Fig. 5-19. Inside of left ventricle.
In the left ventricle, the smooth area between the mitral and aortic valves is named the aortic vestibule. The mitral valve consists of two cusps; accordingly, the rough area includes two papillary muscles. Perhaps, since the word “left” has one less letter than the word “right,” the left mitral valve has one less cusp than the right tricuspid valve (Fig. 5-17).
The connecting tendinous cords in the left ventricle are more significant than those in the right ventricle because the left ventricle has the strongest force of pumping blood. Oftentimes, the mitral valve fails to function, and this causes pathological regurgitation of the blood.
Fig. 5-20. Walls of four heart chambers.
The role of the atria is simply to send blood to the ventricles, so their walls (cardiac muscles) do not have to be of great thickness. In contrast, the right ventricle to pump blood to the lungs requires a thicker wall. The left ventricle, responsible for pumping blood to the entire body, has an even thicker wall than the right ventricle. Supplying blood to the brain against gravity especially is an arduous job.
Regardless of the wall thicknesses of four chambers, they must pump the same amounts per unit time.
The atria, which serve as reservoirs, do not require major valves. On the other hand, the valves of ventricles are crucial at both the entrances and exits.
Fig. 5-21. Heart valves.
Locations of the four valves do matter. As mentioned, the atria are posterior to ventricles (Fig. 5-14), and thus the tricuspid and mitral valves from the atria are posterior to aortic and pulmonary valves (Figs. 5-17,19). The tricuspid valve from the right atrium is on the right, while the mitral valve is on the left. As explained, the right ventricle is situated anterior to the left ventricle (Fig. 5-14), and thus the pulmonary valve from the right ventricle is situated anterior to the aortic valve (Fig. 5-13). One cannot understand the morphology of the heart, unless one understands the rotation of the heart.
Fig. 5-22. Fibrous skeleton of heart, coronary arteries.
The circumferences of the tricuspid and mitral valves are the right and left fibrous rings. The rings hold not only the cusps but also the cardiac muscles spreading to atria and ventricles. The fibrous rings, which function as the origins of the cardiac muscles, are called the fibrous skeleton of the heart.
The fibrous skeleton also encompasses two fibrous trigones. The fibrous trigones are shifted toward the left to reinforce the left fibrous ring that is for the left ventricle (Fig. 5-19). The importance of the left ventricle cannot be emphasized enough.
In anatomy, the adjective “fibrous” means durable due to the fibers inside. The fibrous cartilage (Fig. 1-6), fibrous membrane (Fig. 2-59), and fibrous pericardium are more durable than the hyaline cartilage, synovial membrane, and serous pericardium. The fibrous joint (Figs. 4-14,29) and fibrous skeleton of heart (Fig. 5-22) have durable fibrous tissues as well.
Fig. 5-21 may be misleading as it depicts four valves to exist at the same height. One must note that the pulmonary valve is superior to the tricuspid valve (Fig. 5-17); the aortic valve is superior to the mitral valve (Fig. 5-19).
Fig. 5-23. Shift of heart valve sounds.
Circles in the figure represent the positions of the “Aortic, Pulmonary, Tricuspid, and Mitral” valves, projected on the left side of the sternum. Some clinicians remind the locations by a word “AParTMent.”
The sounds of valves closing can be heard with a stethoscope. As shown in Fig. 5-23, the sounds shift along with the blood stream. Sounds of the tricuspid and the mitral valves shift toward the apex of the heart. Sounds of the pulmonary valve shift upward along with the pulmonary trunk, while that of the aortic valve shifts upward and to the right along with the ascending aorta (Fig. 5-13).
Fig. 5-24. Aortic sinuses, coronary arteries.
Just above the aortic valve, ascending aorta bulges outward due to gravity. These bulges are the right and left aortic sinuses which are named after the right and left cusps of aortic valve (Fig. 5-21). The right and left coronary arteries start from the right and left aortic sinuses.
Fig. 5-25. Coronary arteries.
The right coronary artery passes the coronary sulcus. However, the left coronary artery is too short to fill the rest of the sulcus (Fig. 5-13); instead, its circumflex branch fills the sulcus. On the posterior side of heart, the right coronary artery anastomoses with the circumflex branch (Fig. 5-22).
Right coronary artery gives off four branches: the sinuatrial nodal branch, the right marginal branch, the atrioventricular nodal branch, and the posterior interventricular branch.
On the other hand, left coronary artery divides into the circumflex branch and the anterior interventricular branch (Fig. 5-22). The circumflex branch gives off the left marginal branch.
The anterior interventricular branch is easily obstructed and may easily lead to myocardial infarction. This is critical because this branch feeds the left ventricle.
Clinicians often refer to the anterior interventricular branch as the left anterior descending artery. Such nomenclature is not desirable, because this term does not specify the branch in the heart.
Fig. 5-26. Cardiac veins.
Blood supplied by the coronary arteries drains into the cardiac veins. Cardiac veins have different names from coronary arteries because the structures do not match.
The great cardiac vein accompanies the anterior interventricular branch, then the circumflex branch; the middle cardiac vein accompanies the posterior interventricular branch; the small cardiac vein accompanies the right marginal branch, then the right coronary artery (Fig. 5-25).
The destination of all three cardiac veins is the coronary sinus. Cardiac veins receive blood from the right or left ventricles that exhaust blood intensively. Therefore, the coronary sinus contains the most deoxygenated blood throughout the body.
Finally, coronary sinus drains into the right atrium (Fig. 5-15). The word “sinus” in “coronary sinus” and “dural venous sinuses” (Fig. 4-10) means thick vein.
Fig. 5-27. Conducting system of heart.
If one extracts a heart from a living animal then puts it into a solution containing oxygen and nutrient, the heart will continue to beat regularly. This is because the heart has its own nerves that keep it beating.
The first nerve, the sinuatrial node, is located at the top of the terminal crest, beside the opening of the superior vena cava (Fig. 5-15). Embryologically, this node used to be located between the “sinus” venosus (smooth area) and primordial “atrium” (rough area), so it is called the “sinuatrial” node.
The sinuatrial node makes an impulse by itself. The impulse induces the contraction of the two atria, and then travels to the atrioventricular node.
The second nerve, the atrioventricular node relays the impulse to the atrioventricular bundle, and successively to the right and left bundles, so as to make the two ventricles contract. The ventricles have more cardiac muscles than the atria, and thus require more nerves.
The atrioventricular node resides in the inferior, posterior part of the interatrial septum, beside the opening of coronary sinus (Fig. 5-15).
The atrioventricular bundle is in the membranous part of the interventricular septum; the right and left bundles are in its muscular part that belongs to the rough area of ventricles. Inside the two ventricles of a cadaver, right and left bundles are slightly visible underneath the thin, semitransparent endocardium. The right bundle then enters the septomarginal trabecula (Fig. 5-17).
The sinuatrial node, the headquarter of the all nerves in heart, is influenced by the sympathetic nerve (TN1–TN4) (Fig. 5-43) and the parasympathetic nerve (X) (Fig. 5-44). Both sympathetic and parasympathetic nerve signals make the heart beat faster and slower.
< Lung >
The pleura of the lung is structurally simpler than the serous pericardium of the heart and the peritoneum of the abdomen.
Fig. 5-28. Pleura covering lung.
Let us think about an analogy: The fist, wrist, and forearm are the lung, hilum of lung, and root of lung, respectively. A squashed balloon surrounding the fist is the pleura, composed of the visceral pleura, parietal pleura, and pleural cavity.
Fig. 5-29. Structures passing hilum of right lung.
Another point to note from the fist analogy is the inferiorly extended thumb. Therefore, the hilum of the lung shows an inferior extension which is the pulmonary ligament.
Fig. 5-30. Subdivision of pleura.
A lung is the shape of a half cone, so it has three surfaces. The lateral surface is covered by the costal pleura; the medial surface by the mediastinal pleura; the inferior surface by the diaphragmatic pleura. Additionally, the cervical portion above R1 is covered by the cervical pleura.
Fig. 5-31. Recesses of pleural cavity.
We must note two recesses that are spreads of the pleural cavity. Due to the presence of the costodiaphragmatic recess, the inferior border of the parietal pleura is located lower than that of the visceral pleura. Likewise, due to the costomediastinal recess, the anteromedial border of the parietal pleura is more medial than that of the visceral pleura.
As one can assume, the visceral pleura is in contact with the lung, while the parietal pleura is in contact with the thoracic wall.
If a needle is mistakenly injected into the two recesses, it will result in the fatal aspiration of air into the pleural cavity. This is because the lung has a tendency to reduce itself in volume. Such iatrogenic accident may occur because one cannot detect any sound of the lung on the recesses during the auscultation.
Fig. 5-32. Lobes of lung.
The right lung consists of three (superior, middle, inferior) lobes, whereas the left lung consists of two (superior, inferior) lobes. One easy way to remember this is that there is one more letter in the word “right” than in “left.” The actual reason is that the left-deviated heart (Fig. 5-9) hinders the left lung’s growth in development. Boundaries between the lobes are oblique and horizontal fissures (Fig. 5-29).
Fig. 5-33. Fissure of lung.
A fissure including the visceral pleura prevents diseases from spreading to neighboring lobes. Within the fissure, the pleural cavity contains serous fluid which lubricates neighboring lobes during the respiration.
Fig. 5-34. Segments of lung.
Each lobe is further divided into segments. The five segments in the left inferior lobe are not drawn since they are same as those in the right. Small tips to enhance memorization of the segments are as follows.
First, the lung is a half cone that is vertically cut (Fig. 5-30), so only the superior lobe is qualified to possess the apical segment. If the inferior lobe possessed it, the apical segment would awkwardly be in direct contact with the basal segments.
Second, in the lateral view of the right lung, the lateral segment and lateral basal segment are situated relatively in the center because the lung is a half cone. In the lateral view, medial basal segment is hidden.
Lastly, the MiddLe lobe of the right lung includes the Medial, Lateral segments. The middle lobe is homologous with the superior and inferior lingular segments of the left lung. The lingular segments look like a tongue because the left-shifted heart presses them. “Lingular” segments have nothing to do with the actual tongue, so they are not called “lingual” segments.
Let us further examine Fig. 5-29. In the hilum of the lung, the main bronchus is situated posteriorly. A single pulmonary artery and two pulmonary veins are localized superiorly and inferiorly, respectively. The reason is as follows.
Fig. 5-35. Structures passing hilum of lung (anterior view).
The main bronchus occupies the posterior hilum because the trachea descends backward in order to keep its distance from the heart. The pulmonary artery is superior because it starts from the pulmonary trunk and goes above the heart (Fig. 5-13). The pulmonary veins are inferior because they horizontally enter the left atrium (Fig. 5-18).
Fig. 5-36. Trachea, bronchi.
One can feel not only the laryngeal cartilages (Fig. 3-38), but also the tracheal cartilages at the front of one’s neck. Likewise, bronchi have cartilages which ensure structural integrity and maintain airway. In bronchioles, these cartilages decrease in size, and then finally disappear.
The right main bronchus is thicker, shorter, and more vertical in shape than the left main bronchus. Therefore, when food is mistakenly aspired (Fig. 3-43), it tends to descend to the right lung, and may result in pneumonia.
The right bronchus is thicker than the left one, so as to bring more air into the bigger right lung (Fig. 5-40). Hilum of the right lung is more medial than that of the left lung. The right main bronchus is thus short and more vertical (Fig. 5-36).
If a lung were the United States, then the lobe would be the states, and the segment would be the cities. In succession, the main bronchus, lobar bronchus, and segmental bronchus would be the president, governor, and mayor.
Mucus in the bronchi catches the dust and microscopic cilia in the bronchi force the mucus out toward the mouth. Mucus is then expelled by coughing, and this is called sputum.
After successive branching of the bronchi, their final destinations are the pulmonary alveoli. The size of each alveolus is 0.2 mm; the alveoli of a cadaver can be observed macroscopically. The total surface area of the numerous alveoli in both lungs is as large as 70 square meters.
The cross sectional area of the entire alveoli is larger than that of the trachea or the entire bronchi (Fig. 5-36). Therefore, airflow in the alveoli is slow, which enables gas to be fluently exchanged. Such phenomenon happens in the capillaries too.
Like the segmental bronchus, a segmental artery occupies the center of a segment. However, its continuing vein is located between the segments, rather than at the center of the segment. Therefore, it is called an intersegmental vein, not a segmental vein (Fig. 5-36).
The blood flow from segmental artery to intersegmental vein is one-way; this architecture is made for efficient use of the space in the already crowded lung.
The root of the lung (Fig. 5-28) includes not only the bronchus, pulmonary artery, and pulmonary vein (Figs. 5-29,35), but also the bronchial artery, lymphatics, and autonomic nerve.
Fig. 5-37. Bronchial arteries.
The lung which oxygenates old blood also needs to be oxygenated by fresh blood itself. However, the pulmonary artery does not supply the lung with fresh blood. Therefore, there are extra bronchial arteries accompanying the bronchi to serve this purpose.
The left bronchial artery directly comes from the thoracic aorta which is shifted to the left (Fig. 5-46), while the right bronchial artery comes from the left bronchial artery or from the right posterior intercostal artery (Fig. 5-6).
Fig. 5-38. Lymph drainage of thoracic cavity.
During dissection, students can easily come across black lymph nodes in the hilum of lung (Fig. 5-29). Aspirated dust particles in pulmonary alveoli enter the lymphatics and eventually gather at local lymph nodes, which turn black in color. Lymph from the “bronchi” (lung) and the “mediastinum” (Fig. 5-40) meet and ascend through the “bronchomediastinal” lymphatic trunk in each side (Fig. 6-47).
< Mediastinum >
The volume of the lungs is considerable in the thorax, so the concept of mediastinum (thoracic cavity excluding the bilateral lungs) is rather easy to grasp. This is analogous to talking about Russia excluding Siberia or talking about a hamburger excluding the buns.
Fig. 5-39. Subdivision of mediastinum.
The inferior border of the mediastinum (diaphragm) is drawn horizontally in the figure above. In reality, the diaphragm is convex and declining (Fig. 5-48).
The mediastinum is subdivided into the superior, anterior, middle, and posterior mediastina. The border between the superior mediastinum and the others is an imaginary plane that horizontally passes the sternal angle (Fig. 2-7) and the intervertebral disc between TV4 and TV5.
Fig. 5-40. Contents of mediastina (horizontal plane).
The middle mediastinum is occupied by the heart. Additionally, the middle mediastinum includes big blood vessels such as the ascending aorta, pulmonary trunk, superior vena cava (Fig. 5-13), and the roots of lungs (Figs. 5-28,35).
This horizontal plane shows the phrenic nerve (Fig. 3-16) in the middle mediastinum. The phrenic nerve which intervenes between the pericardium and the pleura descends just anterior to the root of lung.
The anterior mediastinum between the heart and the sternum is very small and thus is usually neglected.
Fig. 5-41. Contents of mediastina (right view).
The thymus belongs to the superior mediastinum (Fig. 3-37), hidden by the manubrium (Figs. 2-7, 5-39). Hence, students can see the thymus only after opening the thoracic wall. Disappointingly, the thymus of an adult cadaver appears like fat tissue. Such degenerated thymus is a result of decreased immunity of the aged. Nevertheless, the thymus is famous for T (Thymus) cell which is the symbol of immunity.
The trachea and main bronchi (Fig. 5-36) exist in the superior and middle mediastina, respectively; this arrangement of the airway is incidental. Ascending aorta, aortic arch (Fig. 5-13), and thoracic aorta (Fig. 5-46) exist in the middle, superior, and posterior mediastina in that order; this nomenclature of the aorta is intentional. Note that the aortic arch is situated left to the trachea and the esophagus (Fig. 5-41,42).
Fig. 5-42. Narrowed sites of esophagus (anterior left view).
The esophagus normally has three sites where its diameter is narrowed: first, a site pressed by the aortic arch in the superior mediastinum (Fig. 5-41); second, a site pressed by the left main bronchus between the middle and posterior mediastina; and third, a site pressed by the diaphragm (esophageal hiatus) (Figs. 5-47,48).
Additionally, just above the esophagus, the inferior constrictor causes narrowing of the laryngopharynx (Fig. 3-45). If a coin is accidentally swallowed, it usually stops at one of these four sites.
The upper 1/3 of esophagus in contact with the laryngopharynx (Fig. 3-45) is composed of skeletal muscle, while the lower 1/3 in contact with the stomach (Fig. 6-31) is composed of smooth muscle. The middle 1/3 is composed of both skeletal and smooth muscles.
If one carefully dissect the wall of the esophagus, one can observe the external longitudinal muscle and the internal circular muscle.
Fig. 5-43. Sympathetic nerve of thorax.
The superior and posterior mediastina contain autonomic nerves as follows. The 1st to 12th thoracic ganglia (paravertebral ganglia) receive abundant sympathetic preganglionic fibers from TN1–TN12 (Fig. 3-18). These ganglia are responsible not only for the cardiac and smooth muscles in the thoracic cavity but also for the smooth muscle in the abdominal cavity. The latter is more dominant when the abundant amount of smooth muscles in the gastrointestinal tract is considered. The sympathetic nerve only from LN1 and LN2 is absolutely insufficient to cover the vast abdominal cavity.
As a result, the 1st–4th thoracic ganglia (including cervicothoracic ganglion) in the superior mediastinum send postganglionic fibers to the heart (Fig. 5-27), lungs, and esophagus (Route A in Fig. 3-18).
The remaining thoracic ganglia (5th–12th) in the posterior mediastinum send preganglionic fibers to the abdominal cavity. Preganglionic fibers from the 5th–9th and 10th–12th thoracic ganglia form the greater and lesser splanchnic nerves, respectively (Route B in Fig. 3-18; Fig. 6-49). The two thoracic splanchnic nerves as well as the sympathetic trunk pierce the crus of diaphragm (Fig. 5-47) to enter the abdominal cavity.
Fig. 5-44. Parasympathetic nerve of thorax.
Related to the parasympathetic nerve, X is responsible for thoracic and abdominal organs. A branch in the neck is for innervating the heart (Fig. 5-27) and another branch in the thorax is for innervating the lung (Fig. 3-23). The main trunk of X accompanies the esophagus and penetrates the esophageal hiatus (Figs. 5-47,48). This is an efficient route for X because it contributes to the esophagus, stomach, and other abdominal organs by way of the celiac plexus, etc (Fig. 6-49).
Prior to arriving at the prevertebral ganglia, the right X and left X become the posterior and anterior vagal trunks, respectively. This results from the 90 degrees counterclockwise rotation of the stomach in the inferior view (Figs. 6-18,23,39).
Here, we introduce another tip for memorization of X.
To understand intercostal veins, recall the intercostal arteries. The anterior intercostal veins have main streams (internal thoracic vein and musculophrenic vein) similar to those of the anterior intercostal arteries (Fig. 5-7). So far, so easy.
Fig. 5-45. Azygos veins.
However, posterior intercostal veins follow a different route from posterior intercostal arteries, since there are no corresponding veins to the thoracic aorta and costocervical trunk (Fig. 3-26). Alternative routes are the azygos, hemiazygos, and accessory hemiazygos veins which eventually empty blood into the superior vena cava.
These three veins do not pair with arteries, nor with themselves on the right and left sides. Thus, the name is azygos (a = without, zygos = pair). “Azygos” is a noun, while “azygous” is an adjective. It is rare to use the noun form such as “azygos” in front of “vein.” Like other veins, these veins are prone to variations.
Fig. 5-46. Vessels passing through the aortic hiatus.
The azygos vein runs right to the thoracic aorta, while the (accessory) hemiazygos vein runs posterior to the thoracic aorta. Between the two veins, the thoracic duct also runs upward. If the thoracic duct of cadaver is identified, one can say that the dissection of the thorax is thoroughly done (Fig. 6-47).
The thoracic aorta is situated posteriorly (Fig. 5-41) and thus is in contact with the thoracic vertebrae. Pulsation of the thoracic aorta hence causes slight depression of the bodies of thoracic vertebrae. All of those four vessels pass the aortic hiatus that is at the level of TV12 (Fig. 5-48).
A further description on the comics is given with the mnemonic: Let us think of the posterior mediastinum has “TASTE” which stands for the structures in there: Thoracic aorta (Fig. 5-41), Azygos vein (Fig. 5-45), Sympathetic trunk (Fig. 5-43), Thoracic duct (Fig. 6-47), and Esophagus (Fig. 5-44).
< Diaphragm >
Fig. 5-47. Diaphragm (inferior view).
The diaphragm partitions the thoracic and abdominal cavities, and consists of a peripheral muscle belly and a central tendon. The central tendon itself can be considered as the insertion of the diaphragm since the muscle contraction descends down the central tendon (Figs. 5-2,48).
Because the fibrous pericardium is attached to the central tendon (Fig. 5-9), the caval opening for inferior vena cava (Fig. 5-13) is in the central tendon. The thoracic aorta (Figs. 5-41,46) becomes the abdominal aorta (Fig. 6-48) immediately after it passes through the aortic hiatus (Fig. 5-42). The aortic hiatus is a tear between the two muscle bellies attached to LV2, which are right and left crura (Figs. 5-48, 6-32). The intervening esophagus (Fig. 5-41) requires the esophageal hiatus (Fig. 5-42) as it also descends through the muscle bellies.
Fig. 5-48. Diaphragm (sagittal plane).
The diaphragm, when viewed from a sagittal plane, is convex and declines posteriorly and inferiorly. The aortic hiatus is located most posteriorly and inferiorly (TV12 level). The esophageal hiatus and caval opening are on the TV10 and TV8 levels, respectively. The vertebral levels are in an arithmetic progression.