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Blood vessels and other structures in the mediastinum
In this specimen we can see the arch of aorta and its branches, i.e., left subclavian, left common carotid and brachiocephalic trunk which will further divide into right common carotid and right subclavian arteries. We can also see the descending thoracic aorta which is piercing the diaphragm at T12 and continuing as abdominal aorta. The branches of the descending thoracic aorta such as posterior intercostal arteries can be seen in the costal groove along with the posterior intercostal vein and nerve. From above downwards the structures in the costal groove are VAN (vein, artery and nerve). We can also see the branches of abdominal aorta here. The ventral branches, i.e., - coeliac trunk which arise at T12 and supplies the foregut, Superior mesenteric artery which arise at the junction of T12 and L1 and supplies midgut and the Inferior mesenteric artery which arise at L3 and supplies hindgut. Then there are the terminal branches of abdominal aorta - the two right and left common iliac arteries. Other branches of abdominal aorta seen are the right and left renal arteries. Other structures seen in this specimen are the azygos vein which enters the thorax from the aortic opening of the diaphragm and drain into superior vena cava at T4. We can also see the thyroid gland, and the trachea. The trachea divides into right and left principal bronchi at T4 level. The thoracic duct which enters the thorax from the aortic opening of the diaphragm, and the oesophagus can also be seen. We can also see the thoracic sympathetic chain on either side of vertebral column and greater splanchnic nerve arising from 5-9 thoracic segments of it.
Arch of aorta:
The arch of aorta is the part of the aorta that distributes blood to the head and neck and upper extremities of the body via the brachiocephalic trunk, the left common carotid, and the left subclavian arteries.
Subclavian artery:
It is a direct branch of the arch of aorta on the left side and arises from the brachiocephalic trunk on the right side. It and mainly supplies the upper limbs (as it continues as the axillary artery). It also provides some branches supplying the head and thorax
Common carotid artery:
It is a direct branch of the arch of aorta on the left side and arises from the brachiocephalic trunk on the right side. The right and left common carotid arteries supply the head and neck with oxygenated blood. The common carotid artery divides in the neck at the level of the upper border of thyroid cartilage to form the external (supplies the outer regions of head and neck) and internal carotid arteries (supplies the brain)
Brachiocephalic trunk:
The brachiocephalic trunk is a branch of the arch of aorta arising from its right side. It is a common trunk that further divides into right common carotid and right subclavian arteries respectively.
Descending thoracic aorta:
The descending thoracic aorta begins as a continuation of the arch of aorta and is located in the posterior mediastinum. It extends from the lower border of the fourth thoracic vertebra at the level of the sternal angle to the lower border of the 12th thoracic vertebra. At its lower end it passes through the aortic opening of the diaphragm to become continuous with the abdominal aorta. The following are the branches of the descending thoracic aorta: pericardial, esophageal, mediastinal, left bronchial, posterior intercostal arteries (3rd to 11th spaces), subcostal, and superior phrenic arteries.
Diaphragm:
The diaphragm is a movable partition (partly muscular and partly tendinous) situated between the thoracic and abdominal cavities. It is a chief muscle of inspiration.
Abdominal aorta:
The abdominal aorta is the main blood vessel in the abdominal cavity. It is a direct continuation of the descending thoracic aorta at the lower border of the 12th thoracic vertebra. It extends into the abdominal cavity through the aortic opening of the diaphragm. The artery lies to the left of the inferior vena cava throughout its course. The artery gives rise to three unpaired arteries (ventral branches), i.e., coeliac trunk, superior mesenteric, inferior mesenteric arteries that supply the derivatives of the foregut, midgut and hindgut respectively. The lateral branches are the inferior phrenic, middle suprarenal, renal and gonadal arteries. The paired lumbar arteries (L1-L4) and an unpaired median sacral artery are its dorsal branches. The aorta ends at the level of L4 by bifurcating into common iliac arteries.
Costal groove:
The costal groove is present in the lower part of the inner surface of the shaft of the rib. It accomadates three important structures: posterior intercostal vein, posterior intercostal artery, and the intercostal nerve (VAN from above downwards).
Common iliac artery:
The right and left common iliac arteries are the two terminal branches of the abdominal aorta arising at the level of L4 vertebra. The common iliac arteries further branch to form the external and internal iliac arteries.
Renal artery:
The renal arteries are paired arteries that supply the kidneys. They arise as lateral branches from the abdominal aorta immediately below the superior mesenteric artery.
Azygos vein:
The azygos vein is a single vein (unpaired) present in the posterior mediastinum on the right side. It is formed by the union of the right ascending lumbar and right subcostal veins. It terminates by arching over the root of the right lung and finally drains into the superior vena cava. Its main tributaries are the right superior intercostal, posterior intercostal (5-11), hemiazygos and accessory hemiazygos veins.
Thyroid gland:
The thyroid gland is a highly vascular endocrine gland. It is situated in the lower part of the front of the neck in close relation to larynx and trachea. The follicular cells of the thyroid gland secrete T3 and T4 hormones. The thyroid hormones increase the metabolic activities of all the tissues of the body. They are also crucial for the growth and development of the fetal brain. The gland presents two lobes connected by a narrow isthmus. The gland is also covered by true (condensation of the glandular connective tissue) and false (pretracheal fascia) capsules.
Trachea:
The trachea (Latin, air vessel; also called the wind-pipe) is a 10-15 cm long tube situated in the lower part of the neck and superior mediastinum. Its upper under is continuous with the larynx at the level of C6 vertebra. It is made up of C-shaped cartilaginous rings (hyaline in nature and 16-20 in number) anteriorly. Posteriorly the gap is closed by a fibro-elastic membrane and contains trachealis muscle. At the level of the sternal angle (i,e, lower border of T4 vertebra) the trachea bifurcates into two principal bronchi.
Thoracic duct:
The thoracic duct is a large lymphatic vessel of the body. It has a beaded appearance and presents many valves. It begins as a continuation of the cisterna chyli near the lower border of T12 thoracic vertebra and enters the thorax through the aortic opening of the diaphragm. It extends upwards in the right side of the posterior mediastinum. At the level of T% vertebra it crosses from the right to the left side. It then courses through the superior mediastinum and finally terminates by opening into the angle of junction between the left subclavian and internal jugular veins. It drains the lymphatics from the entire body except the areas drained by the right lymphatic duct (right side of head and neck, right side of the chest wall and right upper limb).
Oesophagus:
The esophagus is a muscular tube which begins as a continuation of the pharynx at the level of C6 vertebra. Its course is divisible into three parts: cervical, thoracic and abdominal. It enters the abdominal cavity by passing through the esophageal opening of the diaphragm at the level of T8 vertebra and opens into the stomach. The esophagus presents a series of constrictions along its course. The constrictions are formed by the following structures: cricopharyngeus muscle (part of inferior constrictor of pharynx), arch of aorta, left bronchus, espophageal opening of the diaphragm.
Thoracic sympathetic chain:
The thoracic sympathetic chain is a paired structures present on either side of the vertebral column in the superior and posterior mediastinum. There are usually 11 ganglia on the thoracic sympathetic trunk.
Thoracic splanchnic nerves:
The thoracic splanchnic nerves are the preganglionic fibers arising from the lower eight thoracic sympathetic ganglia. They are paired visceral nerves carrying fibers from the autonomic nervous system (visceral efferent). They also carry sensory fibers from the organs (visceral afferent). The thoracic splanchnic nerves are of three types: Greater splanchnic nerves arise from the ganglia 5-9; lesser from the 10th and 11th ganglia and least from the 12th ganglia.
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