Thorax - Interior

THORAX (INTERIOR)

  • Note: The contents of the thorax should be studied by referring to the volume of lecture notes under the relevant headings. The student is warned not to damage the pericardium of heart while dissecting the mediastinum.

Dissection

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  • The lateral wall of the thorax shall be removed by cutting the first eight ribs. Clean and remove the eighth rib and proceed forwards. To remove the rib inserts the blade into the joint and cut across the joint to free the lower end of the rib. Then gently run the blade along the borders of the rib, cutting the attachements of the intercostal muscles without injuring the thoracic organs. By a handsaw cut the upper end of the rib cleanly and remove it. In this manner remove the anterior series of ribs. Examine the medial aspect of the ribs and note intercostal internal thoracic artery passing obliquely on the internal face of the first intercostal space. With the blunt end of scalpel gently raise the artery all along its course in the intercostal space seperating it from the artery. The dissector should exercise great care in removing the first rib, as numerous important structres are placed medial to it. Lower end of first rib from its articulation with the manubrium should be cut, which offers great difficulty and should be patiently done. The upper end should be cut clean by the saw.
  • Pleura: It is the serous membrane covering the walls of the thorax and reflected over the organs contained therein. It consits of two closed sacs right and left placed in the respective halves of the thoracic cavity. The lateral wall of the sac lines the internal faces of ribs and the internal intercostal muscles forming the costal pleura. The medial walls of the two sacs approximate on the median line to form a median septum the medistinum. The thoracic organs except the lungs are placed between the two laryers of the mediastinum.

Dissection

  • Note the extent of the cardiac notch of the left lung. Examine the lobes by gentle manipulatiuon. Carefully trace the mediastional line of pleural reflection and cut at the reflection to release the lung. The apical lobe of right lung should be gently pulled from in front of heart similarly gently pull out the mediastinal pleura wherever necessary to trace structures passing in the mediastinum. Finally cut the structures of the root of the lungs on both sides and remove the lungs and lay it on the table for study.
  • The heart, trachea and its bifurcation, oesophagus, vagi, left recurrent laryngcal neve, phrenic nerves, thoracic duct and venahemiazygos are placed in the middle mediastinum. The trachea, oesophagus, brachiocephalic trunk and its branches, anterior vena cava, thoracic duct, vagi, recurrent laryngeal nerves, phrenic nerves, dorsal, ventral and anterior mediastinal lymph glands, and the body of the thymus are placed in the anterior mediastinum. The thoracic aorta, terminal part of oesophagus, vagi, left phrenic nerve, thoracic duct, dorsal and posterior mediastinal lymph glands and vena hemiazygos are placed in the posterior mediastinum. Over the base of the heart the mediastinal pleura is reflected around and behind the hilus of the lung to form the pleura pulmonalis. The posterior vena cava and right phrenic nerve pass in a special fold of mediastinal pleura behind the heart between the mediastinal and diaphragmatic lobes of right lung.
  • Lungs: Each lung occupies a considerable part of one side of the thoracic cavity and is moulded upon all the other thoracic contents. Note the large quadrilateral cardiac notch of the left lung allowing the heart with its pericardium to contact the left chest wall.
  • Thymus: It is a ductless gland well developed only in the foetus and the newborn. It is made up of numerous lobules loosely connected by inter lobular tissue. The gland consits of a body and two horns. The body is thick and placed in the anterior mediastinum below the brachiocephalic trunk and applied against the anterior face of the heart. The horns extend from the body trhough the thoracic inlet along the ventrolateral aspect of the trachea.
  • Phrenic nerve: The formation of the nerve by the union of the root from the fifth with common cord of the sixth and seventh is behind the first rib. The left nerve passes in the anterior mediastinum below the left vagus, passes over the base of the heart and then passes through posterior mediastinum to reach the tendinous centre of the diaphragm. The right nerve passes on the lateral face of the anterior vena cava, runs over the base of the heart and passes on the lateral face of the posterior venacava in a special fold of pleura and reaches the tendinous centre of the diaphragm.
  • Vagus: The left vagus after seperating itself from the cervical sympathetic in front of the thoracic inlet passes posteriorly in the anterior mediastinum and crosses the arch of the aorta where it detaches left recurrent laryngeal nerve. The left recurrent laryngeal nerve winds round the arch pf aorta, passes on its right face and turns forwards to run betwen the brachiocephalic trunk and the trachea in the anterior mediastinum. It then leaves the thoracic inlet and ascends up the neck. The left vagus then reaches the terminal part of the oesophagus and divides into doral and ventral branches which unite with the corresponding branches of the right vagus to form the dorsal and ventral oesophageal continuations of the vagi that run on the respective faces of the oesophagus.
    • The right vagus crosses under the right brachial artery and detaches the right recurrent laryngeal nerve behind the first rib. The right recurrent laryngeal nerve winds round the common trunk arising from the right brachial artery and leave the thoracic inlet and ascend up the neck in the right carotid sheath.

Dissection

  • Clean the vessels and nerves in front of the heart and the stallate ganglion on the longus colli at the vertebral end of the first rib. Carefully trace the branches of stellate ganglion.
  • Stellate ganglion: It is a large synpathetic ganglion placed medial to the vertebral end of the first rib on the longus colli. It results by the fusion of the posterior cervical ganglion with the first thoracic ganglion. The cervical sympathetic cord after leaving the vagus joins the ganglion at its anterior extremity. From the anterior part of the ganglion the nervus transversarius arises and accompanies the vertebral vessels in the canalis transversarius. Behind is a large sympathetic ganglion placed medial to the vertebral end of the first rib on the longus colli. It results by the fusion of the posterior cervical ganglion with the first thoracic ganglion. The cervical synpathetic cord after leaving the vagus joins the ganglion at its anterior extremity. From the anterior part of the ganglion the nervus transversarius arises and accompanies the vertebral vessels in the canalis transversarius. Behind the nervus transversarius the ganglion detaches gray rami to the eighth cervical, first and second dorsal spinal nerves. It also detaches two or three cardiac and pulmonary nerves from its ventral border, and these pass toweards the base of heart. The posterior extremity of the ganglion is connected with the dorsal sympathetic trunk placed in the ventrolateral aspects of the dorsal vertebrae.
  • Thoracic duct: This is the largest lymph duct in the body. It is visible in the anterior mediastinum crossing the left face of oesophagus. Its further dissection is beyond the scope of the dissector. He should study its formation and course and note its position in the anterior mediastinum. It may not be visible in some specimens.
  • Vena hemiazygos: It is a large vein of the left side descending from the root of the thoracic cavity. It arises from the first lumbar vein, passes towards receiving intercostal veins, runs downwards on the thoracic aorta passing over the arch and the vagus, then passes over the base of the heart downward and backward and joins the great cardiac vein.
  • Oesophagus: From the thoracic inlet it passes on the dorsal face of the trachea in the anterior mediastinum inclines to the right and crosses the right face of the aortic arch passes between the lungs, inclining to the left in the posterior mediastinum, passes through the hiatus oesophagi of the diaphragm, enters the abdomen and immediately opens into the atrium ventriculi.
  • Trachea: From the thoriacic inlet it passes in the anterior mediastinum ventral to the oesophagus related on the left to the brachiocephalic trunk and on the right to anterior vena vaca and the right vagus. At the level of the third rib it detaches the apical branch on the right side inclines to the right to cross the right face of aortic arch and terminates by dividing into two branchi at the level of the fifth rib.
  • Anterior vena cava: It is formed at the thoracic inlet below the trachea by the union of the brachial veins with the jugular confluence. It passes in the anterior mediastinum below the trachea to the right of the median line with the right phrenic nerve on its lateral face.
  • Longus colli: The thoracic parts of the muscle covering the ventral aspects of the bodies of the first six dorsal vertebras are available for examination. The stellate ganglion and dorsal sympathetic trunks are placed on the muscle covered by the mediastinal pleura. Oesophagus runs under the muscle.
  • Aorta: It is the main systemic arterial trunk arising from the left ventricle. It passes upward and backward describing the aortic arch with its covexity forwards. It reaches the body of the seventh or eighth dorsal vertebra and runs backwards as the thoracic aorta between the lungs under the bodies of the dorsal vertebrae. It passes through the hiatus aorticus of the diaphragm and runs under the bodies of the lumbar vertebrae as the abdominal aorta.
  • Common brachiocephalic trunk: It is a large vessel arising from the anterior face of the convexity of the arch. It passes upward and forward in the anterior mediastinum. At the third rib or the second intercostal space on the left it detaches the left brachial artery and continues forwards as the brachiocephalic artery. Opposite to the first rib on the right the brachicephalic artery detaches the right brachial artery and is continued as the bicarotid trunk which at the thoracic inlet below the trachea divides into right and left common carotid arteries.
  • Brachial arteries: After its origin, each artery winds round the anterior border of the first rib below the insertion of the scalenus ventralis and enter the axilla. The left brachial is related to the oesophagus and trachea. The right brachial is related to the trachea above and the anterior vena cava below. Both the arteries are related below to the vagus, phrenic and recurent laryngeal nerves.
  • Collateral branches: A common trunk arises from the dorsal face of the artery close to its origin and divides into the subcostal, dorsal, deep cervical and vertebral arteries.
    • The subcostal artery pases upward and backward below the bodies of the dorsal vertebrae detaching the second to fifth intercostal arteries at the corresponding intercostal spaces.
    • The dorsal artery leaves the thoracic cavity by passing in front of the first costotransverse joint and ascends in the serratus cervicis to supply the muscles and skin on the lateral aspect of the neck. It also gives the first intercostal artery.
    • The deep cervical artery usually arises from the dorsal face of the vertebral artery outside the thorax behind the transverse process of the seventh cervical vertebra and passes up deeply on the deep face of the serratus cervicis to supply the muscles and skin on the dorsal aspect of the cervical spine.
    • The vertebral artery passes out of the thorax under the transverse process of the seventh cervical vertebra and enters the foramen transversarium of, the sixth cervcical and passes in the canalis transversarius.
    • At the first rib the brachial artery detaches from its ventral face, the interal thoracic artery which passes downward and backward on the medial face of the first rib, crosses the lower part of the first intercostal space, passes back under the transverses thoracis over the chondrosternal joint.
  • Intercostal arteries: They are paired. Of the thirteen pairs first five arise from the dorsal and sub-costal and the remaining from the thoracic aorta. The aortic intercostal arteries arise in pairs from the dorsal face of the aorta at regular intervals. From their origin they diverge and pass upwards across the bodies of dorsal vertebrae and the sympathetic trunk.
  • Transversus thoracis: It is a triangular muscle on the superior face of the stenum. It arises from the sternal ligament and is inserted to the costal cartilages from the second to the eighth and adjacent parts of ribs. Its fibres run across the sternum.
  • Diaphragm: It is a large dome shaped unpaired muscle separating the thoracic and abdominal cavities. It consists of a muscular rim and a tendinous centre. The muscular rim consits of costal, sternal and lumbar parts. Examine details of attachment etc., by reference to the lecture notes.
  • Posterior vena cava: It passes through the foramen venacavae of the diaphragm and runs forward in the groove between the mediastinal and diaphragmatic lobes of the right enclosed in a special fold of pleura with the right phrenic nerve on its lateral face. It opens into the right atrium.
  • Posterior mediastinum: It extends from the posterior face of the heart and pericardium to the anterior face of the diaphragm. Behind the heart two layers of the posterior mediastinum diverge greatly enclosing a large mediasinal space and reach the diaphragm. In the dissection subject this mediastinal space and the two layers of the posterior mediastinum are easily demonstrable. In the living subject the space is obliterated by the distension of the lungs.
Last modified: Thursday, 3 May 2012, 5:07 AM