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1. Describe the osteological and muscular boundaries of the thorax, and identify the divisions and their major contents of the thoracic cavity.
1. Describe the osteological and muscular boundaries of the thorax, and identify the divisions and their major contents of the thoracic cavity.
The thorax is a region of the torso bounded:
Superiorly by the first ribs and thoracic inlet,
Anteriorly by the sternum,
Laterally by the costae (ribs, 12 pairs) and intercostal spaces,
Posteriorly by the vertebral column (12 thoracic vertebrae), and
Inferiorly by the diaphragm.
The sternum is the anterior-most osteological structure of the thorax, which overlays a region of the thoracic cavity called the mediastinum. The sternum is tripartite, consisting of a manubrium, body, and xiphoid process. The superior surface of the manubrium is the suprasternal (jugular) notch, and the clavicles articulate laterally to the suprasternal notch at the clavicular notches. The manubrium and body are orientated in slightly different planes, and the angle between them is known as the sternal angle (of Louis), and is an important anatomical landmark for establishing the thoracic plane. It is also at the sternal angle where the costal cartilages of the second ribs articulate with the sternum.
The thoracic cavity may be conceptualized as three compartments. Laterally, there are pleural spaces containing the pulmonary cavities, the pleurae, and the lungs. Medial to the pleural cavities is the mediastinum, which contains the pericardium & heart, great vessels, portions of the airway, esophagus, lymphatics, and various neurovascular elements.
The superior portion of the thorax, the root of the neck, is a transitional area that is of both the neck and thorax, which serves as a conduit for viscera and neurovasculature common to these regions, and as a means of articulation with, and conveyance of, elements to the upper limbs.
2. Diagram the major vascular (artery & vein) pathways to the head & neck and upper limbs.
2. Diagram the major vascular (artery & vein) pathways to the head & neck and upper limbs.
The arch of aorta (aortic arch) connects the ascending aorta to the descending aorta. Typically, three branches - brachiocephalic trunk (giving rise to the R. subclavian a. & R common carotid a.), L. common carotid a., & L. subclavian a. - originate from the arch of aorta to supply blood to the head, neck, upper limbs, and thorax.
The common carotid a. bifurcates into the internal and external carotid aa.
The internal carotid a. has no branches in the neck. It travels to the cranium, where it is transmitted through the carotid canal, and supplies blood to the brain, orbit, and forehead.
The external carotid a. is the primary source of blood to the face and superficial head. The external carotid has eight branches:
superior thyroid a.
ascending pharyngeal a.
occipital a.
lingual a.
facial a.
posterior auricular a.
maxillary a.
superficial temporal a.
The subclavian a. (SCA) supplies blood to the neck, cranial cavity (& brain), anterior wall of the thorax (and abdomen), and upper limbs. The subclavian a. is conceptually divided into three parts by the anterior scalene m.
The superior vena cava transmits blood from the head, neck, upper limbs, and thorax to the right atrium of the heart. The superior vena cava is formed by the confluence of the brachiocephalic vv., which are each formed by the confluence of the internal jugular (IJ) vv. and subclavian vv. The confluence of the internal jugular v. and the subclavian v. is known as the venous angle, and it is in this vicinity where lymph is returned to venous circulation. The L. brachiocephalic v. is located immediately anterosuperior to the aorta, and has a longer and more oblique course than the R. brachiocephalic v.
3. Explain how the parts of the subclavian artery (SCA) are delimited. Identify the major branches (arteries) from each part of the SCA, and what these branches supply. Identify the point where the SCA becomes the axillary artery.
3. Explain how the parts of the subclavian artery (SCA) are delimited. Identify the major branches (arteries) from each part of the SCA, and what these branches supply. Identify the point where the SCA becomes the axillary artery.
The subclavian a. is conceptually divided into three parts, with respect to the vessel’s relationship to the anterior scalene m. The first part of the subclavian a. is found medial to the anterior scalene, the second part of the subclavian a. is posterior to the anterior scalene m., and the third part is lateral to the anterior scalene m.
The first part of the subclavian a. typically hosts three major branches:
Vertebral a. (supplies the brain & spinal cord)
Thyrocervical trunk,
Inferior thyroid a. (supplies the thyroid, parathyroid glands, and larynx)
Ascending cervical a. (supplies deep muscles of the neck)
Suprascapular a. (supplies supra- and infraspinatus mm.)
Transverse cervical a. (supplies trapezius m. & often gives rise to the dorsal scapular a.)
Internal thoracic a. (supplies anterior thoracic & abdominal walls & the diaphragm).
The second part of the subclavian a. typically hosts one major branch, the:
Costocervical trunk
Deep cervical a. (supplies deep neck muscles)
Highest (supreme) intercostal a. (supplies 1st & 2nd intercostal spaces).
The third part of the subclavian a. typically hosts one major branch, the:
Dorsal scapular a. (supplies rhomboid mm. and levator scapulae m.). The dorsal scapular a. may also be a branch of the transverse cervical a.
Beyond the lateral border of the first rib, the subclavian a. transitions into the axillary a.
4. Define the Root of Neck (RON). Explain how structures (e.g. neurovasculature, muscles, & bones) are associated with the RON, and their classic anatomical relationships of these structures to one another.
4. Define the Root of Neck (RON). Explain how structures (e.g. neurovasculature, muscles, & bones) are associated with the RON, and their classic anatomical relationships of these structures to one another.
The root of the neck (RON) is the nexus between the neck, thorax, and upper limbs. The RON is the proximal attachment site for many neck muscles and transmits important neurovasculature (e.g. common carotid aa., jugular vv., subclavian aa. & vv., vagus nn., and trunks of the brachial plexus). The RON rests upon the 1st ribs, and has indistinct boundaries with the neck and upper limbs.
Anterior scalene m.
The anterior scalene m. is an important anatomical landmark for understanding the neurovasculature of the root of the neck. There are four classic anatomical relationships to understand:
the anterior scalene m. is sandwiched between the subclavian v. (anterior to) and subclavian a. (posterior to),
the anterior scalene m. establishes the borders for the divisions of the subclavian a.,
the anterior scalene m. is immediately posterior to (and closely associated with) the phrenic n. as the phrenic n. descends the neck and the root of the neck, and
the anterior scalene m. is separated from the middle scalene m. by the roots (ventral primary rami of spinal nn.) and roots and trunks of the brachial plexus and subclavian a.
Phrenic n.
The phrenic n. (C3,4,5) descends from the cervical plexus through the root of the neck (just anterior to, and upon the anterior scalene m.) before entering the thorax between the subclavian a. & v. The phrenic n. is efferent (motor) and afferent (sensory) to the diaphragm and afferent (sensory) to the pericardium and diaphragmatic pleura.
Vagus n. (CN X)
The vagus n. (CN X) is the major parasympathetic supply and conduit to the thorax and abdomen, and is the major innervation to muscles of the larynx and pharynx, and aspects of the head.
The R. and L. vagus nn. take different routes through the thorax. Both nerves descend the neck within the carotid sheaths, and cross anteriorly over the subclavian aa., deep to the first ribs. The R. vagus n. then sends a major branch (the R. recurrent laryngeal n.) inferiorly and then posteriorly around the R. subclavian a., lateral to the trachea, on a course for the larynx. The L. vagus n. sends the L. recurrent laryngeal n. inferiorly and then posteriorly around the concavity of the arch of the aorta, lateral to the trachea, also on a course for the larynx.
Lymphatics
Lymphatic ducts and trunks are often overlooked, yet important features associated with the root of the neck. Lymphatic vessels transmit lymph, filtered blood plasma, from interstitial tissues to venous circulation in the root of the neck. Lymphatic trunks are larger, regional lymphatic vessels that may coalesce into ducts.
The thoracic duct is the largest of all lymphatic vessels. The thoracic duct typically conveys all the lymph from inferior to the diaphragm, the left thorax (via the left bronchomediastinal trunk), the left upper limb (via the left subclavian trunk), and the left head & neck (via the left jugular trunk). The thoracic duct anastomoses with the venous system in the vicinity of the left venous angle, the point at which the subclavian v. meets the internal jugular v.
The trunks associated with the right thorax (the right bronchomediastinal trunk), the right upper limb (the right subclavian trunk), and the right head & neck (the right jugular trunk), may either coalesce into a right lymphatic trunk that anastomoses in the vicinity of the right venous angle, or the trunks may each independently anastomose in the vicinity of the right venous angle.
5. Describe a typical intercostal space.
5. Describe a typical intercostal space.
The thorax is more kinetic than people realize. The ribs articulate with thoracic vertebrae to form costovertebral joints, with heads of ribs attaching to demifacets on vertebral bodies and tubercles attaching to transverse processes. Costovertebral joints allow for the elevation of ribs, and thus the expansion of the thoracic cavity in volume. This action controls pressure in the thoracic cavity. Boyle’s law (p*v = k) demonstrates that pressure and volume are inversely proportional, so as the volume of the thoracic cavity increases, pressure decreases, which is the basis for the movement of air into the lungs during inhalation.
The intercostal spaces (spaces between the ribs) are occupied by layers of muscle and the neurovasculature which serves the muscle and overlying skin. Generally, the intercostal muscle is divided into three layers:
External intercostal mm. - pull ribs upwards and outwards, to slightly increase the volume of the thoracic cavity (inspiratory)
11 pairs
Distributed from tubercles of ribs to costal cartilages (only present anteriorly as aponeuroses)
Typically more robust than internal intercostal mm.
Internal intercostal mm. - pull ribs downwards and inwards, to slightly decrease the volume of the thoracic cavity (expiratory)
11 pairs
Distributed from sternum to costal angle (only present posteriorly as aponeuroses)
Innermost intercostal mm. - function akin to internal intercostal mm.
Neurovascular supply of the intercostal spaces
Intercostal spaces are typically served by branches of three arteries:
Highest (supreme) intercostal a., via posterior intercostal aa. (1st-2nd IC spaces)
From costocervical trunk (2nd part of subclavian a.)
Variably absent, especially on the left
Thoracic part of descending aorta, via posterior intercostal aa. (3rd-11th IC spaces & subcostal space)
Dominant blood supply to IC spaces
Internal thoracic a., via anterior intercostal brs.
From 1st part of subclavian a.
Anterior intercostal brs. 1-6 typically arise directly from internal thoracic
Anterior intercostal brs. 7-9 typically arise from musculophrenic br.
Important anastomoses for IC spaces
Posterior intercostal brs. are typically the dominant source of blood to the intercostal spaces. Many typically openly anastomose (physically connect) to their smaller anterior intercostal br. counterparts, and these vessels are referred to as intercostal aa.
Intercostal aa. are found in the subcostal groove of the rib forming the superior boundary of the intercostal space, and are accompanied by an intercostal v. and n. The arrangement of the intercostal neurovasculature is often conceptualized by the mnemonic ‘VAN,’ which places the Vein in closest (most superior) contact with the subcostal groove, then the Artery, and finally the Nerve. Often, intercostal neurovasculature has associated collateral brs. which are 1) much smaller, 2) found along the superior margin of the rib at the inferior boundary of the intercostal space, and 3) have the inverse spatial relationships to the lower rib (i.e. NAV - Nerve is most superior, then Artery, then the Vein is in closest contact with the rib).
Blood is returned from the intercostal spaces through similar routes as it is supplied, chiefly via:
Posterior intercostal vv.
Supreme intercostal vv.
1st intercostal spaces
Drain into brachiocephalic vv. (variable)
Superior intercostal vv.
Intercostal spaces 2-4
Drain into azygos system (right), or brachiocephalic v. (left)
Azygos system (detailed in Posterior Mediastinum Session)
Ultimately drains into superior vena cava
Anterior intercostal vv.
Internal thoracic vv.
Intercostal spaces are innervated by intercostal nn., which are typically ventral primary rami (VPR) of spinal nn. T1-T11. The VPR of T1 ramifies, with the bulk of the VPR joining superiorly with the brachial plexus (C5-T1) and the remaining fibers becoming the first intercostal n. The VPR of T12 is known as the subcostal n.
For the greater part of their conveyance, neurovasculature serving the intercostal spaces may be found between the internal intercostal mm. and the innermost intercostal mm.
6. Differentiate between the subdivisions of the mediastinum, both the 4 part schema (superior, anterior, middle, and posterior mediastinal spaces) as well as the 3 part schema (prevascular, visceral, and paravertebral compartments).
6. Differentiate between the subdivisions of the mediastinum, both the 4 part schema (superior, anterior, middle, and posterior mediastinal spaces) as well as the 3 part schema (prevascular, visceral, and paravertebral compartments).
The mediastinum may be classically described as four contiguous spaces, or (more recently) clinically described as three contiguous compartments. Both classifications may be encountered, but anatomists typically use the four-space model.
The mediastinum is the region bounded:
Superiorly by the thoracic inlet (and first ribs),
Inferiorly by the diaphragm,
Laterally by the pleura,
Anteriorly by the sternum and pleurae, &
Posteriorly by the vertebral column.
Classically, the mediastinum may be subdivided into four regions: superior, anterior, middle, and posterior mediastina.
The superior mediastinum is distinguished from all other regions by the plane of the sternal angle, i.e. a transverse plane running from the sternal angle (where the manubrium meets the body of the sternum; a landmark for locating the level of the 2nd costal cartilage) to the T4/5 intervertebral disc.
The anterior mediastinum is delimited as the space:
Inferior to the plane of the sternal angle,
Superior to the diaphragm,
Posterior to the sternum and thoracic wall,
Anterior to the pericardium, &
Medial to the pleurae.
The posterior mediastinum is the area:
Inferior to the plane of the sternal angle,
Superior to the diaphragm,
Posterior to the pericardium,
Anterior to the vertebral column, &
Medial to the pleurae.
The middle mediastinum is the area:
Inferior to the plane of the sternal angle,
Superior to the diaphragm,
Posterior to the anterior-most extent of the pericardium,
Anterior to the posterior-most extent of the pericardium, &
Medial to the pleurae.
The mediastinum may also be described as three compartments: the prevascular, visceral, and paravertebral compartments. As with the four-regions model, the pericardium is a pivotal element for delimiting the compartments, with prevascular anterior to the pericardium, the visceral compartment bounded by the pericardium anteriorly. The boundary between the visceral and paravertebral compartments is a line that courses approximately 1 centimeter posterior to the anterior edges of the vertebral bodies.
The prevascular compartment is bounded:
Superiorly by the thoracic inlet and first ribs,
Inferiorly by the diaphragm,
Anteriorly by the sternum,
Posteriorly by the anterior-most extent of the pericardium, &
Laterally by the pleurae.
The visceral compartment is bounded:
Superiorly by the thoracic inlet and first ribs,
Inferiorly by the diaphragm,
Anteriorly by the anterior-most extent of the pericardium,
Posteriorly by the a line including the anterior 1 cm of each thoracic vertebral body, &
Laterally by the pleurae.
The paravertebral compartment is bounded:
Superiorly by the thoracic inlet and first ribs,
Inferiorly by the diaphragm,
Anteriorly by a line including the anterior 1 cm of each thoracic vertebral body,
Posteriorly by the area adjacent to the thoracic vertebral bodies, &
Laterally by the pleurae.
It is important to note that the classic (4-part) schema of the mediastinum ends posteriorly at the vertebral bodies, whereas the (3-part) schema includes areas beyond and lateral to the bodies.
7. Identify the contents of the superior mediastinum.
7. Identify the contents of the superior mediastinum.
The major contents of the superior mediastinum include the:
Arch of the aorta
Brachiocephalic trunk & bifurcation
Left common carotid a.
Left subclavian a.
Pulmonary trunk bifurcation & pulmonary arteries
Ligamentum arteriosum (remnant of the ductus arteriosus)
Pulmonary veins
Superior vena cava & tributaries
Right & left brachiocephalic vv.
Azygos v.
Thoracic duct
Lymph nodes & Bronchomedistinal trunk
Vagus nn. (CN X) & Left recurrent laryngeal n.
Cardiac plexuses
Cardiac nn. (sympathetics)
Phrenic nn. (from cervical plexus)
Trachea, to the carina (division of the primary bronchi)
Esophagus
8. Describe the basic structure of the autonomic innervation of the thoracic viscera with a special emphasis on the cardiac plexuses.
8. Describe the basic structure of the autonomic innervation of the thoracic viscera with a special emphasis on the cardiac plexuses.
There are three major types of autonomic plexuses within the thorax, including:
Cardiac plexuses,
Pulmonary plexuses, &
Esophageal plexus.
These plexuses contain sympathetic and parasympathetic fibers, largely sourced from either the cervical and thoracic sympathetic trunks, or the vagus nn. (CN X). While larger contributions to an autonomic plexus (e.g. nerves and branches) may be visible, the smaller fibers which comprise the plexus are typically not visible, and are frequently removed from structures with investing fascia. Understanding the sources (inputs) for plexuses will allow you to in lab possibly locate source nerves or branches, whereas understanding the locations of the plexuses will allow you to understand the relationships of the plexuses to their target organs.
Cardiac Plexus:
Control of the cardiac cycle is intrinsic, or myogenic, and arises in the sinu-atrial (SA) node of the heart as will be discussed in later sessions. Extrinsic control of the SA node occurs through autonomic stimulation. Sympathetic stimulation increases heart rate and dilates the coronary arteries, whereas parasympathetic stimulation decreases heart rate and maintains resting tone of the walls of the coronary aa.
The cardiac autonomic plexus is typically conceptualized in two parts: a superficial part which nestles in the inferior concavity of the arch of the aorta, and a deep part which is anterior to the tracheal bifurcation (and contiguous with the anterior pulmonary plexuses).
The superficial cardiac plexus receives fibers from the left superior cervical sympathetic ganglion and the left vagus n. (CN X).
The deep cardiac plexus receives fibers from cervical and thoracic sympathetic trunks, vagus nn. (CN X), and recurrent laryngeal nn.
Fibers from the superficial and deep parts of the cardiac plexus coalesce to form sub-plexuses that serve specific parts of the heart and its major arteries, chiefly the:
Left coronary plexus, serving the left coronary a. (LCA),
Right coronary plexus, serving the right coronary a. (RCA), & the
Atrial plexuses, serving the left and right atria.
The cardiac plexuses are contiguous with one another and with the pulmonary plexuses. Given the coordinated functions of the cardiovascular and respiratory systems, well-orchestrated responses from and between these systems is advantageous.
Pulmonary Plexuses:
There are two anterior and two posterior autonomic pulmonary plexuses which nestle on either surface of the hila of the lungs.
Anterior pulmonary plexuses receive fibers from the vagus nn. (CN X) and the cervical sympathetic trunks via the superficial cardiac plexus.
Posterior pulmonary plexuses receive fibers from the vagal trunks and the thoracic sympathetic trunks. The left posterior plexus often will receive fibers from the left recurrent laryngeal n. As well. Sympathetic responses cause bronchodilation through relaxing smooth muscle of the conductive airways, whereas parasympathetic stimulation maintains resting tone of the smooth muscle.
Esophageal Plexus:
The esophageal plexus is predominantly formed from fibers originating from the vagal trunks, which are loosely homologous to the left (becomes the bulk of the anterior trunk) and right (becomes the bulk of the posterior trunk) vagus nn. (CN X). Some vagus fibers (especially for those of the proximal esophagus) arise from the recurrent laryngeal nn. The sympathetic portions of the esophageal plexus are derived from the thoracic and cervical sympathetic trunks.
Visceral pain affecting the esophagus may be difficult to distinguish from visceral cardiac pain, because the visceral afferents run with sympathetic fibers, and both organs are served from fibers associated with the thoracic spinal cord.
9. Describe the typical auscultation points for the heart valves.
9. Describe the typical auscultation points for the heart valves.
As the actual heart valves are deep to bony landmarks, direct auscultation of valves may be difficult. As sound conducts in the direction of blood flow, auscultating superficial to the flow of blood from valves offers relatively unobstructed opportunities for auscultating the valves. These locations are the auscultation points for listening to heart valve closure and blood flow. They are as follows:
Aortic valve: Right 2nd intercostal space at R. sternal border
Pulmonary valve: Left 2nd-3rd intercostal space at L. sternal border
Right AV (tricuspid) valve: L. 4th-5th intercostal space at L. sternal border
Left AV (mitral) valve: L. 5th intercostal space, midclavicular line
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