THE THORACIC CAGE AND THE INTERCOSTAL SPACE
The bony thoracic cage is formed by the 12 thoracic vertebrae at the back, the sternum in front and 12 pairs of ribs in between.
The upper seven pairs of ribs articulate anteriorly direct with the sternum through their respective costal cartilages.
The costal cartilage of ribs 8, 9 and 10 articulates with that of the rib above.
These ribs with the xiphisternum form the lower costal margin.
The lowermost point of the thoracic cage is the tenth costal cartilage.
The space between two adjacent ribs is known as the intercostal space.
Thus there are 11 intercostal spaces on each side.
The junction between the manubrium and the body of the sternum is the sternal angle
The second costal cartilage articulates at the sternal angle .
The seventh costal cartilage articulates at the junction between the body and the xiphisternum.
Surface anatomy
The sternal angle is palpable on the surface as a transverse ridge.
This landmark is used to palpate the second costal cartilage and the second rib. It is possible to identify the other ribs as well as intercostal spaces by counting down from the second rib.
The first rib is not palpable as it is under the clavicle.
Ribs 11 and 12 are rudimentary, confined to the back covered by muscles and hence are not palpable.
The intercostal space
The intercostal space contains the external intercostal, the internal intercostal and the innermost intercostal muscles arranged in three layers.
The neurovascular bundle, consisting of the intercostal nerve and vessels, lies in between the internal and the innermost intercostals.
The external intercostal muscle fibres are directed downwards and forwards.
In the anterior part the muscle fibres are replaced by a membrane. The internal intercostal fibres lie in the opposite direction to those of the external.
The neurovascular bundle lies between the internal and the innermost intercostal muscles.
If it is necessary to insert a chest drain or a needle into the intercostal space it is always placed in the lower part of the space to avoid damage to the neurovascular bundle (which lies along the lower border of the rib along the upper part of the space).
The neurovascular bundle consists of, from above downwards, intercostal vein, artery and nerve.
The intercostal nerves are the anterior rami of the first eleven thoracic nerves.
The anterior intercostal arteries are branches of the internal thoracic artery or those of its musculophrenic branch. Most of the posterior intercostal arteries are derived from the descending thoracic aorta.
Anastomoses between the anterior and posterior intercostal arteries are important collateral channels for circulation in cases of obstruction to the blood flow in the aorta anywhere beyond the origin of the left subclavian artery.
THE THORACIC CAVITY, LUNGS AND PLEURA
The thoracic cavity contains on either side the right and left lungs surrounded by the pleural cavities and the mediastinum in between.
The lungs and pleural cavities
The right lung is subdivided into superior, middle and inferior lobes by an oblique fissure and a horizontal fissure.
The left lung usually has only two lobes, a superior and an inferior with an oblique fissure in between.
Each lung has an apex which extends about 3 cm above the clavicle into the neck, a costal surface, a mediastinal surface and a base or diaphragmatic surface.
The anterior border of the lung separates the costal and the mediastinal surfaces whereas the lower border is between the costal and the diaphragmatic surface.
The root of the lung connects the lung to the mediastinum and consists of, anterior to posterior, two pulmonary veins, the pulmonary artery and the bronchus.
The pulmonary veins are at a lower level compared to the pulmonary artery.
The right main bronchus gives off the superior lobar bronchus outside the lung.
All the branches of the left bronchus are given off inside the lung.
The root of the lung also contains the bronchial arteries supplying the bronchi and bronchioles as well as the lymph nodes draining the lung. The right bronchus is shorter, wider and more vertical than the left.
The angle between the two bronchi is about 70° in the adult; 25° to the right and 45° to the left from the midline.
Therefore foreign bodies getting into the trachea tend to go to the right bronchus rather than into the left. At birth the bifurcation angle is about 110° with both bronchi angulating equally from the midline (55° each way).
The lung is surrounded by the pleural cavity, the potential space between the two layers of pleura.
The outer parietal layer of pleura lines the thoracic cavity and the inner visceral or pulmonary layer closely fits on to the surface of the lung.
The two layers become continuous with each other at the root of the lung.
The parietal pleura lining the diaphragm is known as the diaphragmatic pleura and that lining the mediastinum as the mediastinal pleura.
Sur face anatomy
The apex of the lung and the surrounding pleural cavity extends about 3 cm above the medial part of the clavicle.
The apical pleura is covered by a fascia, the suprapleural membrane (Sibson’s fascia), which is attached to the inner border of the first rib.
This fascia prevents the lung and pleura expanding too much into the neck during deep inspiration.
From the apex, the anterior border of the pleural cavity descends behind the sternoclavicular joint to reach the midline at the level of the sternal angle. (Here the two pleural cavities are close to each other.)
The anterior limit of the right pleural cavity descends vertically downwards in the midline from the sternal angle to the level of the sixth costal cartilage.
From there the lower border extends laterally, crossing the eighth rib in the midclavicular line, the tenth rib in the midaxillary line and then ascends to the middle of the twelfth rib at the back.
The posterior border then ascends almost vertically upwards in the paravertebral region.
From the sternal angle the anterior border of the left pleural cavity deviates laterally to the lateral border of the sternum.
The extent of the lower and the posterior margins are similar to those on the right.
The surface marking of the lung is the same as that of the pleura except for the lower margin and the cardiac notch.
The lower margin of the lung is about two ribs higher than the lower margin of the pleura.
Because of the bulge of the heart and pericardium, the anterior border of the left lung deviates laterally from the sternal angle to the apex of the heart (usually in the fifth intercostal space a little inside the midclavicular line) producing the cardiac notch.
The oblique fissure lies along the sixth rib and the horizontal fissure on the right side extends from the midaxillary line along the fourth rib. Knowledge of the extent of the lung and pleura is clinically important.
Their lower parts overlap abdominal organs such as the liver, kidney and spleen.
On the apical pleura lie the subclavian vessels and the brachial plexus.
Procedures such as exposure of the kidney, liver biopsy and cannulation of the subclavian vein may inadvertently produce a pneumothorax (air in the pleural cavity) resulting in collapse of the lung. When the lung fields are markedly hyperinflated, the liver is pushed down by the diaphragm and may be palpable.
The trachea, bronchi and bronchioles
The trachea which is slightly to the right of the midline divides at the carina into right and left main bronchi.
The right main bronchus is more vertical than the left and, hence, inhaled material is more likely to pass into it.
The right main bronchus divides into three lobar bronchi (upper, middle and lower), whereas the left only into two (upper and lower).
Each lobar bronchus divides into segmental and subsegmental bronchi.
There are about 25 generations of bronchi and bronchioles between trachea and the alveoli; the first 10 are bronchi and the rest bronchioles.
The bronchi have walls consisting of cartilage and smooth muscle epithelial lining with cilia and goblet cells, submucosal mucous glands and endocrine cells containing 5-hydroxytryptamine.
The bronchioles are tubes less than 2 mm in diameter and are also known as small airways.
They have no cartilage or submucosal glands.
Their epithelium has a single layer of ciliated cells but only few goblet cells and Clara cells secreting a surfactant-like substance
The alveolar ducts and alveoli
Each respiratory bronchiole supplies approximately 200 alveoli via alveolar ducts.
There are about 300 million alveoli in each lung and their walls have type I and type II pneumocytes.
Type II pneumocytes are the source of surfactant.
The type I pneumocytes and the endothelial cells of adjoining capillaries constitute the blood–air barrier, the thickness of which is about 0.2–2 mm.
THE HEART
Borders and surfaces of the heart
The heart has an anterior or sternocostal surface. formed mostly by the right ventricle, an inferior or diaphragmatic surface, formed mostly by the left ventricle, a base or posterior surface, formed by the left atrium, and an apex, formed entirely by the left ventricle.
The borders of the heart are the right border, formed by the right atrium, the inferior border, formed by the right ventricle, the left or obtuse border, formed mostly by the left ventricle with the left auricle at its superior end.
The apex beat is defined as the lowermost and lateralmost cardiac pulsation in the precordium, normally felt inside the midclavicular line in the fifth left intercostal space (approximately 6 cm to the left of the midline).
However it is felt in the anterior axillary line when lying on the left side.
The right border of the heart extends from the third to the sixth right costal cartilage approximately 3 cm to the right of the midline, the inferior border from the lower end of the right border to the apex and the left border from the apex to the second left intercostal space approximately 3 cm from the midline.
Blood supply of the heart
The heart muscle is supplied by the right and left coronary arteries and is drained by the cardiac veins.
The coronary arterial supply is of great clinical importance.
Its occlusion is the chief cause of death in the western world.
The right coronary artery arises from the anterior aortic sinus.
It passes between the pulmonary trunk and the right atrium to lie in the atrioventricular groove.
It winds round the inferior border to reach the diaphragmatic surface where it anastomoses with the terminal part of the left coronary artery.
It gives off an artery to the sinoatrial node, the right (acute) marginal artery and the posterior interventricular artery which is also known as the posterior descending artery.
The left coronary artery arises from the left posterior aortic sinus.
It passes behind the pulmonary trunk and the left auricle to reach the atrioventricular groove where it divides into the circumflex and the anterior interventricular (anterior descending) arteries, both of equal size.
The circumflex artery winds round the left margin where it gives off the left (obtuse) marginal artery and reaches the diaphragmatic surface to anastomose with the right coronary artery.
The anterior descending artery (LAD), also known as the ‘widow maker’ because many men die of blockage of this artery, descends in the inter ventricular septum and gives off ventricular branches, septal branches as well as the diagonal artery.
It then winds round the apex to reaching the diaphragmatic surface to anastomose with the posterior descending artery.
The main stem of the left coronary artery varies in length between 4 and 10 mm.
In 10% of the population in whom the left coronary is larger and longer than usual ‘left dominance’ the posterior descending artery arises from it instead of from the right coronary.
Another 10% have ‘codominant’ coronary circulation where both left and right coronaries contribute equally to the posterior interventricular artery.
In a third of the population the left main stem divides into three branches instead of two, the third being a branch lying between the circumflex and the anterior descending the lateral aspect of the left ventricle.
The blood supply of the conducting system is of clinical importance.
In about 60% of the population the sinoatrial node is supplied by the right coronary and in the rest by the circumflex branch of the left coronary.
However occasionally (3%) it can have a dual supply.
The atrioventricular node is supplied by the right coronary in 90% and the circumflex in 10%.
Cardiac veins accompany the arteries. Most of them are tributaries of the coronary sinus, a sizable vein lying in the posterior part of the atrioventricular groove and opening into the right atrium.
The great cardiac vein accompanies the anterior interventricular artery; the middle cardiac vein accompanies the posterior interventricular artery and the small cardiac vein accompanies the marginal artery.
Right coronary artery occlusion leads to inferior myocardial infarction (necrosis of cardiac muscle), often associated with dysrhythmia (abnormal heart beats) due to ischaemia of SA node and/or AV node.
Occlusion of the left coronary artery or its branches leads to anterior and/or lateral myocardial infarction, often with substantial ventricular damage and very poor prognosis.
The pericardium
The heart lies within the pericardial cavity, in the middle mediastinum.
The pericardial cavity is similar in structure and function to the pleural cavity.
The pericardium provides for the heart a friction-free surface to accommodate its sliding movements.
Components of the pericardium are the fibrous pericardium which is a collagenous outer layer fused with the central tendon of the diaphragm, the serous pericardium consisting of a parietal layer which lines the fibrous pericardium and a visceral layer which lines the outer surface of the heart and the commencement of the great vessels.
The pericardial cavity is the space between the parietal and the visceral layers.
Two regions of the pericardial cavity have special names.
The transverse sinus of the pericardial cavity lies between the ascending aorta and the pulmonary trunk in front and the venae cavae and the atria behind.
The pericardial space behind the left atrium is the oblique sinus.
The oblique sinus separates the left atrium from the oesophagus.
Anteriorly the pericardium is related to the sternum, third to sixth costal cartilages, lungs and the pleura.
Posterior relations are oesophagus, descending aorta and T5–T8 vertebrae. Laterally on either side lie the root of the lung, mediastinal pleura and the phrenic nerve.
Innervation of the fibrous and the parietal layer of serous pericardium is by the phrenic nerves.
Pericardial pain originates in the parietal layer and is transmitted by the phrenic nerves.
The pericardial cavity is closest to the surface at the level of the xiphoid process of sternum and the sixth costal cartilages.
Percardiocentesis
To remove fluid from the pericardial cavity a needle is inserted into the angle between the xiphoid process and the left seventh costal cartilage and is directed upwards at an angle of 45° towards the left shoulder.
The needle passes through the central tendon of the diaphragm before entering the pericardial cavity.
