RAD 251 Cross-Sectional Anatomy
Module F
The thorax (also called the chest) is the focus of this module. This module will begin the
identification and review of the structures located in the region between the neck and the
abdomen. Module G will continue that process. A CD study guide accompanies this module,
but is designed for study with both Modules F and G.
Many authors identify the thorax as having an inlet and outlet. The thoracic inlet is said to be
formed by the first thoracic vertebra, the first pair of ribs, and the upper margin of the
manubrium of the sternum. The inlet separates the thoracic structures from those in the lower
portion, or root, of the neck. The so-called thoracic outlet in found inferiorly, and covered by the
muscular diaphragm. It is bounded posteriorly by the twelfth vertebrae, and anteriorly by the
xiphisternal junction and the sloping cartilages of the costal margins.
The thorax can be divided into two distinct descriptive portions: bony and internal structures.
The bony thorax consists of the structures that provide protection for the internal structures and
assist in respiration. These structures include the twelve thoracic vertebrae, sternum, clavicles,
ribs and the costal cartilages. The bony thorax is bounded posteriorly by the twelve thoracic
vertebrae and anteriorly by the sternum. Superiorly, the bony thorax is bounded by the clavicles
and their articulations with the sternum and the scapulae. While the scapulae are located on the
posteriolateral aspects of the bony thorax, they will not be discussed in detail in this module, but
will be subsequently covered in a later module dealing with the upper and lower extremities.
THE THORACIC VERTEBRAE
The twelve thoracic vertebrae are considered typical. The only defined differences are that these
vertebrae contain costal facets for articulation with the ribs, have long, slender, and inferiorly
projecting spinous processes. The heads of the ribs articulate posteriorly with the ribs at what
are known as costovertebral joints, while the tubercles of the ribs articulate with the transverse
processes of the vertebrae at the costotransverse joints.
THE STERNUM
Located in midline on the anterior aspect of the bony thorax, the sternum is divided into three
parts: manubrium, body and xiphoid. The most superior part of the sternum is the manubrium.
It is said to be somewhat triangular in shape and articulates directly with the first pair of ribs and
the clavicles. The superior aspect of the manubrium contains a notch or depression, called the
jugular notch. This depression is a commonly used landmark in medical imaging. Inferior to the
manubrium is located the body of the sternum. The body and manubrium of the sternum
articulate at an angle called the sternal angle or Angle of Louis. The sternal angle corresponds to
the level of the disk space between thoracic vertebrae 4-5. Considered a flat bone, the thin
elongated sternal body contains several indentations along its lateral aspects for articulation of
the third through seventh pairs of ribs (true ribs). The sternum ends inferiorly with the xiphoid
process. Before the age of fourteen years the xiphoid process is usually composed of hyaline
cartilage, but ossifies after that time to become more bony in nature. It fuses with the body of
the sternum at the xiphisternal junction. The xiphisternal junction is located at the level of
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thoracic vertebra # 9 in most individuals. The xiphoid process serves as an attachment for
muscles and is frequently palpated to center the median sagittal plane of the body for medical
imaging purposes.
THE CLAVICLES
The clavicles are mentioned in this module only because they are sometimes considered to be
bones associated with the thorax and because they articulate with the sternum at the
stenoclavicular joints. These two long, slender bones are considered a part of the shoulder joint.
They extend transversely from their articulations with the sternum to their articulations with the
acromion processes of the scapulae, forming the acromioclavicular joints.
THE RIBS
As mentioned earlier in this module, twelve pairs of ribs are contained within the thorax. The
ribs form the lateral boundaries of the thorax. The first seven pairs of ribs are called “true ribs”
because of their direct articulations with the sternum by way of their costal cartilages. The last
five pairs of ribs are called “false” or “floating” ribs because of they do not have a direct
connection with the sternum. The 8th, 9th, and 10th pairs of ribs attach to the cartilages of the
preceding ribs (7th, 8th, and 9th, respectively). The last two pairs of ribs are considered “floating
ribs”, because they have no attachment to the sternum whatsoever. The ribs bound the thorax
anteriorly, posteriorly, and laterally, forming a protective “cage” which protects the internal
structures of the thorax.
The schematic below will help you to identify the bony anatomy of the thorax. This schematic is
located on page 138 of the Sectional Anatomy for Imaging Professionals textbook.
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MUSCLES ASSOCIATED WITH THE THORAX
As with all areas of the human body, the thorax contains muscles. Most of the muscles
associated with the thorax are either associated with the functional aspects of the shoulder joint
or movement of the so-called pectoral girdle. Of course, some of the major muscles of the
thorax assist with increasing the intrathoracic volume as in respiration,
The pectoral muscles are the most familiar of the muscles covering the thorax. Pectoralis major
and pectoralis minor are located on the anterior aspect of the thorax. Both of these two muscles
assist in movement of the upper extremities, but pectoralis major has the additional responsibility
of expanding the thoracic cavity upon deep inspiration,
The serratus muscles, anterior and posterior, also assist in respiration, but may also have other
functions. Serratus anterior may be found on the lateral borders of the thorax, extending from
the medial aspect of the scapula to the lateral aspect of the first rib to the eighth rib. The serratus
anterior muscle along with the major and minor rhomboid muscles function to keep the scapula
positioned against the thorax. Serratus posterior is generally divided into two portions; inferior
and superior. The inferior portion originates from thoracic vertebra number twelve and lumbar
vertebras one thru three. It extends laterally to ribs eight or nine through twelve. It primarily
functions in respiration. The superior portion of serratus posterior originates from the spinous
processes of cervical vertebra number seven, through thoracic vertebra number three. It inserts
on the on the superior aspects of ribs number two thru four.
The levator costarum muscles are twelve fan-shaped muscles that have their origins on the
transverse processes of the twelve thoracic vertebrae. They insert on the rib immediately inferior
to their origins. On cross-sectional images of the thoracic, the levator costarums can be
visualized extending from the vertebrae to the ribs. The levator costarums, along with the
intercostal muscles allow increases in the anteroposterior and transverse dimensions of the
thoracic cavity.
The intercostal muscles are three layers of muscles that occupy the spaces between the ribs. The
layers consist of the external, internal, and innermost. The layers act in unison to elevate the ribs
and to assist in expansion of the thoracic cavity
The diaphragm is a muscular, movable partition, located between the thoracic and abdominal
cavities. It is the primary muscle of respiration and covers the thoracic outlet entirely. It is
attached to the lumbar vertebrae by two structures called crura. The crura are composed of
tendons. Contraction of the diaphragm enlarges the thoracic cavity in the vertical dimension on
inspiration. This is accomplished as its two most superior portions (domes) move inferiorly and
flatten on inspiration.
Although the diaphragm covers the thoracic outlet, there are openings with it to allow for
passage of anatomical structures from the thorax into the abdomen. There are three major
openings, called hiatuses, in the diaphragm for this purpose; aortic, caval, and esophageal.
The largest of the three openings in the diaphragm is the aortic hiatus. It is not actually a true
opening in the diaphragm itself, but is formed by a space located where the left and right
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diaphragmatic crura come together. It allow for passage of the descending aorta, azygos vein,
and the thoracic duct. The caval hiatus allows for passage of the inferior vena cava and the right
phrenic nerve. The esophageal hiatus, as its name implies, allows for passage of the esophagus
as well as the vagus nerve.
Please see figures 5.63 and 5.64 of the Kelley/Petersen textbook for a visual schematic of the
muscles associated with the thorax.
THE THORACIC CAVITY
The area enclosed by the bony thorax and the associated musculature is called the thoracic
cavity. There are three major divisions defining the cavity; right and left pleural and the
mediastinum. The right and left lungs occupy the plural cavities while the heart, great vessels,
trachea, esophagus, and thymus gland, occupy the mediastinum. The mediastinum is the area
between the two pleural cavities.
The two pleural cavities are closed, separate cavities. Each one is lined with a serous membrane
called pleura. Pleura is a continuous sheet of membrane consisting of two layers; visceral, the
innermost layer, and parietal, the outermost layer. The lungs are the primary organs enclosed
within the pleura.
THE LUNGS
The chief organs of respiration, the lungs are elongated cone-shaped structures which contain all
of the components of the bronchial tree, with the exception of the primary bronchi. The lungs
are separated by the mediastinum. They extend superiorly to just above the mid-point of each
clavicle and rest on the diaphragm inferiorly. On the medial surface of each lung is an opening
for structures to enter and exit the lungs, called the hilum. Blood vessels, bronchi, lymph
vessels, and nerves are the structures found in the hilum of each lung and are the structures
collectively called the “root” of the lung.
As learned in introductory human anatomy courses, the right lung has three lobes and the left one
has two. The right one is broader, shorter, and has greater volume than the left. The three lobes
of the right lung (superior, middle, and inferior) are divided by two fissures; one between the
superior and middle lobes, and one between the middle and inferior lobes. The left lung has only
two lobes, and is longer and narrower than the right. On the medial surface of the left lung there
is an indention called the cardiac notch for the apex of the heart. The left lung is divided into the
two lobes (superior and inferior) by a single lobar fissure. The right lung is shorter than the left
due to space occupied by the liver.
At the hilum, the primary bronchi given off by the division of the trachea at the carina (T-5)
enter each lung. The primary bronchi then divide into secondary bronchi. The secondary
bronchi further divide into tertiary segmental bronchi that supply oxygen to specific regions of
each lung. As the bronchi further divide into specific segments to supply specific regions of
each lobe of the lungs, they are termed bronchopulmonary segments. This division of the
bronchial tree continues until it ends in the functional units of the respiratory system called
alveoli.
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The following illustration demonstrates the internal structures of the thorax.
THE MEDIASTINUM
The midline area between the lungs is called the mediastinum. For anatomical location purposes,
the mediastinum may be divided into superior and inferior portions. The superior mediastinum
is situated above the fibrous pericardium and separated from the inferior mediastinum by an
imaginary line from the angle of the sternum to the intervertebral disc spaces between T-4 and T5. The contents of the superior mediastinum include the arch of the aorta and its branches, the
thymus gland, and all or the other structures passing between the neck and the thorax.
The inferior mediastinum is divided into three portions; anterior, middle, and posterior. The
anterior portion of the inferior mediastinum is a rather small area located anterior to the
pericardium, posterior to the sternum, inferior to the sternal angle, and superior to the diaphragm.
This area contains only connective tissue, fat, and lymph nodes. The middle portion of the
inferior mediastinum is a very important area since it is the location of the ascending aorta, heart,
pulmonary artery, inferior and superior vena cavae, and the four pulmonary veins. This area is
centrally located within the mediastinum and limited by the fibrous pericardium. The posterior
portion of the inferior mediastinum is located inferior to T-4 and posterior to the pericardium. It
contains the descending aorta, thoracic duct, esophagus, and the azygos and hemiazygos veins.
The posterior portion of the inferior mediastinum is bounded inferiorly by the diaphragm.
Most anatomy courses present information on the major structures of the thorax, but may not
have placed emphasis on several mediastinal structures presented above; the thymus gland,
thoracic duct and the azygos and hemiazygos veins.
The thymus gland produces lymphocytes and plays a significant role in the development and
maintenance of the body’s immune system. It is located in the superior mediastinum, posterior
to the manubrium of the sternum. It is somewhat triangular in shape and has two lobes (bilobed).
In children it is rather prominent on medical images. It should gradually decrease in size with
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increasing age, until by adulthood it has been replaced by mediastinal fat. However, if the
immune system becomes compromised or damaged by disease or illness, the thymus gland may
once again become prominent as it tries to assist with the maintenance of the immune system.
The thoracic duct is considered the main vessel of the immune system. It is responsible for most
of the lymphatic drainage from the body. The thoracic duct, as mentioned earlier, passes through
the aortic hiatus along with the descending aorta and the azygos vein. In the thorax it is
positioned between the azygos vein and the descending aorta. It empties into the subclavian vein
at the level of the clavicles.
The azygos and hemiazygos veins form what is known as the azygos venous system. The system
provides collateral circulation between the inferior and superior vena cavae. The azygos vein
ascends the thorax along the right side of the thoracic vertebrae, while the hemiazygos vein
travels along the left side and crosses to the right to anastamose with the azygos at about thoracic
vertebral level 7-9. The azygos then empties into the posterior portion of the superior vena cava
(SVC). The azygos venous system drains blood from the bronchi, esophagus, pericardium, and
most of the posterior thoracic wall.
This module contains quite a bit of information on the anatomy of the thorax. The
accompanying CD will help to facilitate a better understanding of the information and allow for
association of the provided information with the cross-sectional imaging location of the
structures mentioned in this module. Module G will continue the study of the anatomy of the
thorax, with focus on the remaining thoracic anatomical structures.
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