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From Wikipedia, the free encyclopedia

The thorax or chest is a part of the anatomy of humans and various other animals located between the neck and the abdomen.

The thorax includes the thoracic cavity and the thoracic wall.

It contains organs including the heart, lungs, and thymus gland, as well as muscles and various other internal structures.

Many diseases may affect the chest, and one of the most common symptoms is chest pain.


In humans and other hominids, the thorax is the chest region of the body between the neck and the abdomen, along with its internal organs and other contents.

It is mostly protected and supported by the rib cage, spine, and shoulder girdle.


The contents of the thorax include the heart and lungs (and the thymus gland); the (major and minor pectoral muscles, trapezius muscles, and neck muscle); and internal structures such as the diaphragm, the esophagus, the trachea, and a part of the sternum known as the xiphoid process).

Arteries and veins are also contained – (aorta, superior vena cava, inferior vena cava and the pulmonary artery); bones (the shoulder socket containing the upper part of the humerus, the scapula, sternum, thoracic portion of the spine, collarbone, and the rib cage and floating ribs).

External structures are the skin and nipples.

The chest

In the human body, the region of the thorax between the neck and diaphragm in the front of the body is called the chest.

The corresponding area in an animal can also be referred to as the chest.

The shape of the chest does not correspond to that part of the thoracic skeleton that encloses the heart and lungs.

All the breadth of the shoulders is due to the shoulder girdle, and contains the axillae and the heads of the humeri.

In the middle line the suprasternal notch is seen above, while about three fingers' breadth below it a transverse ridge can be felt, which is known as the sternal angle and this marks the junction between the manubrium and body of the sternum.

Level with this line the second ribs join the sternum, and when these are found the lower ribs can often be counted.

At the lower part of the sternum, where the seventh or last true ribs join it, the ensiform cartilage begins, and above this there is often a depression known as the pit of the stomach.


The bones of the thorax, called the "thoracic skeleton" is a component of the axial skeleton.

It consists of the ribs and sternum.

The ribs of the thorax are numbered in ascending order from 1-12.

11 & 12 are known as floating ribs because they have no anterior attachment point in particular the cartilage attached to the sternum, as 1-7 are, and therefore are termed "floating".

Whereas ribs 8-10 are termed false ribs as their costal cartilage articulates with the costal cartilage of the rib above.

Anatomical landmarks

The anatomy of the chest can also be described through the use of anatomical landmarks.

The nipple in the male is situated in front of the fourth rib or a little below; vertically it lies a little external to a line drawn down from the middle of the clavicle; in the female it is not so constant.

A little below it the lower limit of the great pectoral muscle is seen running upward and outward to the axilla; in the female this is obscured by the breast, which extends from the second to the sixth rib vertically and from the edge of the sternum to the mid-axillary line laterally.

The female nipple is surrounded for half an inch by a more or less pigmented disc, the areola.

The apex of a normal heart is in the fifth left intercostal space, three and a half inches from the mid-line.

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Post by thelivyjr »

BC Open Textbooks

Anatomy and Physiology

Chapter 11. The Muscular System

76 11.4 Axial Muscles of the Abdominal Wall, and Thorax

Learning Objectives

By the end of this section, you will be able to:

• Identify the intrinsic skeletal muscles of the back and neck, and the skeletal muscles of the abdominal wall and thorax

• Identify the movement and function of the intrinsic skeletal muscles of the back and neck, and the skeletal muscles of the abdominal wall and thorax

It is a complex job to balance the body on two feet and walk upright.

The muscles of the vertebral column, thorax, and abdominal wall extend, flex, and stabilize different parts of the body’s trunk.

The deep muscles of the core of the body help maintain posture as well as carry out other functions.

The brain sends out electrical impulses to these various muscle groups to control posture by alternate contraction and relaxation.

This is necessary so that no single muscle group becomes fatigued too quickly.

If any one group fails to function, body posture will be compromised.

Muscles of the Abdomen

There are four pairs of abdominal muscles that cover the anterior and lateral abdominal region and meet at the anterior midline.

These muscles of the anterolateral abdominal wall can be divided into four groups: the external obliques, the internal obliques, the transversus abdominis, and the rectus abdominis (Figure 1 and Table 6).

The top panel shows the lateral view of the superficial and deep abdominal muscles.

The bottom panel shows the anterior view of the posterior abdominal muscles.

Figure 1. Muscles of the Abdomen.

(a) The anterior abdominal muscles include the medially located rectus abdominis, which is covered by a sheet of connective tissue called the rectus sheath.

On the flanks of the body, medial to the rectus abdominis, the abdominal wall is composed of three layers.

The external oblique muscles form the superficial layer, while the internal oblique muscles form the middle layer, and the transverses abdominus forms the deepest layer.

(b) The muscles of the lower back move the lumbar spine but also assist in femur movements.

There are three flat skeletal muscles in the antero-lateral wall of the abdomen.

The external oblique, closest to the surface, extend inferiorly and medially, in the direction of sliding one’s four fingers into pants pockets.

Perpendicular to it is the intermediate internal oblique, extending superiorly and medially, the direction the thumbs usually go when the other fingers are in the pants pocket.

The deep muscle, the transversus abdominis, is arranged transversely around the abdomen, similar to the front of a belt on a pair of pants.

This arrangement of three bands of muscles in different orientations allows various movements and rotations of the trunk.

The three layers of muscle also help to protect the internal abdominal organs in an area where there is no bone.

The linea alba is a white, fibrous band that is made of the bilateral rectus sheaths that join at the anterior midline of the body.

These enclose the rectus abdominis muscles (a pair of long, linear muscles, commonly called the “sit-up” muscles) that originate at the pubic crest and symphysis, and extend the length of the body’s trunk.

Each muscle is segmented by three transverse bands of collagen fibers called the tendinous intersections.

This results in the look of “six-pack abs,” as each segment hypertrophies on individuals at the gym who do many sit-ups.

The posterior abdominal wall is formed by the lumbar vertebrae, parts of the ilia of the hip bones, psoas major and iliacus muscles, and quadratus lumborum muscle.

This part of the core plays a key role in stabilizing the rest of the body and maintaining posture.

Muscles of the Thorax

The muscles of the chest serve to facilitate breathing by changing the size of the thoracic cavity (Table 7).

When you inhale, your chest rises because the cavity expands.

Alternately, when you exhale, your chest falls because the thoracic cavity decreases in size.

The Diaphragm

The change in volume of the thoracic cavity during breathing is due to the alternate contraction and relaxation of the diaphragm (Figure 2).

It separates the thoracic and abdominal cavities, and is dome-shaped at rest.

The superior surface of the diaphragm is convex, creating the elevated floor of the thoracic cavity.

The inferior surface is concave, creating the curved roof of the abdominal cavity.

Defecating, urination, and even childbirth involve cooperation between the diaphragm and abdominal muscles (this cooperation is referred to as the “Valsalva maneuver”).

You hold your breath by a steady contraction of the diaphragm; this stabilizes the volume and pressure of the peritoneal cavity.

When the abdominal muscles contract, the pressure cannot push the diaphragm up, so it increases pressure on the intestinal tract (defecation), urinary tract (urination), or reproductive tract (childbirth).

The inferior surface of the pericardial sac and the inferior surfaces of the pleural membranes (parietal pleura) fuse onto the central tendon of the diaphragm.

To the sides of the tendon are the skeletal muscle portions of the diaphragm, which insert into the tendon while having a number of origins including the xiphoid process of the sternum anteriorly, the inferior six ribs and their cartilages laterally, and the lumbar vertebrae and 12th ribs posteriorly.

The diaphragm also includes three openings for the passage of structures between the thorax and the abdomen.

The inferior vena cava passes through the caval opening, and the esophagus and attached nerves pass through the esophageal hiatus.

The aorta, thoracic duct, and azygous vein pass through the aortic hiatus of the posterior diaphragm.

The Intercostal Muscles

There are three sets of muscles, called intercostal muscles, which span each of the intercostal spaces.

The principal role of the intercostal muscles is to assist in breathing by changing the dimensions of the rib cage (Figure 3).

Figure 3.

Intercostal Muscles.

The external intercostals are located laterally on the sides of the body.

The internal intercostals are located medially near the sternum.

The innermost intercostals are located deep to both the internal and external intercostals.

The 11 pairs of superficial external intercostal muscles aid in inspiration of air during breathing because when they contract, they raise the rib cage, which expands it.

The 11 pairs of internal intercostal muscles, just under the externals, are used for expiration because they draw the ribs together to constrict the rib cage.

The innermost intercostal muscles are the deepest, and they act as synergists for the action of the internal intercostals.

Muscles of the Pelvic Floor and Perineum

The pelvic floor is a muscular sheet that defines the inferior portion of the pelvic cavity.

The pelvic diaphragm, spanning anteriorly to posteriorly from the pubis to the coccyx, comprises the levator ani and the ischiococcygeus.

Its openings include the anal canal and urethra, and the vagina in women.

The large levator ani consists of two skeletal muscles, the pubococcygeus and the iliococcygeus (Figure 4).

The levator ani is considered the most important muscle of the pelvic floor because it supports the pelvic viscera.

It resists the pressure produced by contraction of the abdominal muscles so that the pressure is applied to the colon to aid in defecation and to the uterus to aid in childbirth (assisted by the ischiococcygeus, which pulls the coccyx anteriorly).

This muscle also creates skeletal muscle sphincters at the urethra and anus.

Muscles of the Pelvic Floor.

The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina.

The perineum is the diamond-shaped space between the pubic symphysis (anteriorly), the coccyx (posteriorly), and the ischial tuberosities (laterally), lying just inferior to the pelvic diaphragm (levator ani and coccygeus).

Divided transversely into triangles, the anterior is the urogenital triangle, which includes the external genitals.

The posterior is the anal triangle, which contains the anus (Figure 5).

The perineum is also divided into superficial and deep layers with some of the muscles common to men and women (Figure 6).

Women also have the compressor urethrae and the sphincter urethrovaginalis, which function to close the vagina.

In men, there is the deep transverse perineal muscle that plays a role in ejaculation.

Muscles of the Perineum.

The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in women.

The levator ani pubococcygeus and levator ani iliococcygeus control movements during defaction, urination, coughing, and giving birth.

They originate in the pubis and ischium.

The superficial transverse perineal supports the perineal body maintaining the anus at the center of the perineum.

It originates in the ischium.

The bulbospongiosus is a superficial muscle that causes an involuntary response that compresses the urethra when excreting urine in both sexes or while ejaculating in males; it also aids in erection of the penis in males.

It originates in the perineal body.

The ischiocavernosus is a superficial muscle that compresses veins to maintain erection of the penis in males and erection of the clitoris in females.

It originates in the ischium, ischial rami, and pubic rami.

The external uretral sphincter is a deep muscle that voluntarily compresses the urethra during urination.

It originates in the ischial rami and pubic rami.

The external anal sphincter is a deep muscle that closes the anus.

It originates in the anoccoccygeal ligament.

Chapter Review

Made of skin, fascia, and four pairs of muscle, the anterior abdominal wall protects the organs located in the abdomen and moves the vertebral column.

These muscles include the rectus abdominis, which extends through the entire length of the trunk, the external oblique, the internal oblique, and the transversus abdominus.

The quadratus lumborum forms the posterior abdominal wall.

The muscles of the thorax play a large role in breathing, especially the dome-shaped diaphragm.

When it contracts and flattens, the volume inside the pleural cavities increases, which decreases the pressure within them.

As a result, air will flow into the lungs.

The external and internal intercostal muscles span the space between the ribs and help change the shape of the rib cage and the volume-pressure ratio inside the pleural cavities during inspiration and expiration.

The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in women.

The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina.

https://opentextbc.ca/anatomyandphysiol ... nd-thorax/
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