This section describes the thorax and its structures.

Thoracic Cavity 

The spaces within the body are called body cavities. The two principal body cavities are the dorsal and ventral cavity. The ventral cavity, located near the anterior surface is called thoracic, or chest, cavity. The thoracic cavity is separated from abdominopelvic cavity by a broad muscle, the diaphragm, which forms the floor of thoracic cavity. Divisions of the thoracic cavity are the pleural sacs, each containing a lung, and the mediastinum between them. Within the mediastinum lies the heart, the thymus gland, parts of esophagus, and the trachea, among various other structures. Structures of the thoracic cavity is protected by sternum and the rib cage.  

Sternum

The sternum is a flat, dagger shaped bone located in the middle of the chest. Along with the ribs, the sternum forms the rib cage that protects the heart, lungs, and major blood vessels from damage.

• The Sternum is composed of three parts:
  • Manubrium- located at the top of the sternum. connected to the first two ribs.
  • Body- located in the middle. Connects the third to seventh rib directly and eighth to tenth rib indirectly.
  • Xiphoid process- located at bottom

Sternum and Ribs

The Ribs

The ribs are thin, flat, curved bones that form a protective cage around the organs in the upper body. They are comprised of 24 bones arranged in 12 pairs.

• These bones are divided into 3 categories
  • True Ribs
    • The first seven bones are called the true ribs. These bones are connected to the spine (the backbone) in back. In the front, the true ribs are connected directly to the breastbone or sternum by a strips of cartilage called the costal cartilage.
  • False Ribs
    • The next three pairs of bones are called false ribs . These bones are slightly shorter than the true ribs and are connected to the spine in back. However, instead of being attached directly to the sternum in front, the false ribs are attached to the lowest true rib.
  • Floating Ribs
    • The last two sets of rib bones are called floating ribs . Floating ribs are smaller than both the true ribs and the false ribs. They are attached to the spine at the back, but are not connected to anything in the front

Trachea

• Also known as wind pipe
• Extends from 6 th cervical vertebra to 5 th thoracic vertebra
  • 11 cm long, diameter 2-2.5cm.
  • In children, the trachea is smaller, more deeply placed, and more movable than in the adult.

Major Muscles of thorax

• Diaphragm
• Intercostal muscles
  • Intercostales Externai
  • Intercostales Interni

Diaphragm

• The Diaphragm is a dome-shaped musculofibrous septum which separates the thoracic from the abdominal cavity, its convex upper surface forming the floor of the thorax, and its concave under surface the roof of the abdomen.
• Principal muscle of respiration

Nerve Supply

• The diaphragm is supplied by
  • the phrenic nerve
  • lower intercostal nerves

Intercostals

• Intercostals are two thin planes of muscular and tendinous fibers occupying each of the intercostal spaces
• They are named external and internal from their surface relations—the external being superficial to the internal. There are 11 pairs of external and internal intercostals
• The Intercostales externi ( External intercostals ) extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end in thin membranes, the anterior intercostal membranes, which are continued forward to the sternum. Each arises from the lower border of a rib, and is inserted into the upper border of the rib below. In the two lower spaces they extend to the ends of the cartilages, and in the upper two or three spaces they do not quite reach the ends of the ribs. They are thicker than the Intercostales interni, and their fibers are directed obliquely downward and lateralward on the back of the thorax, and downward, forward, and medialward on the front.
• Intercostales interni (Internal intercostals) commence anteriorly at the sternum, in the interspaces between the cartilages of the true ribs, and at the anterior extremities of the cartilages of the false ribs, and extend backward as far as the angles of the ribs, where they are continued to the vertebral column by thin aponeuroses, the posterior intercostal membranes. Each arises from the ridge on the inner surface of a rib, as well as from the corresponding costal cartilage, and is inserted into the upper border of the rib below. Their fibers are also directed obliquely, but pass in a direction opposite to those of the Intercostales externi.
• The intercostal muscles are supplied by the intercostal nerves.

Thoracic Nerve

There are twelve thoracic nerves. Eleven of them are situated between the ribs, and are therefore termed intercostal; the twelfth lies below the last rib. The intercostal nerves are distributed chiefly to the parietes of the thorax and abdomen, and differ from the anterior divisions of the other spinal nerves, i. e., there is no plexus formation. The first two nerves supply fibers to the upper limb in addition to their thoracic branches; the next four are limited in their distribution to the parietes of the thorax; the lower five supply the parietes of the thorax and abdomen. The twelfth thoracic is distributed to the abdominal wall and the skin of the buttock.

Mediastinum

The medial space between the two lungs. (Interpleural Space)

The mediastinum lies between the right and left pleuræ near the median sagittal plane of the chest. It extends from the sternum in front to the vertebral column behind, and contains all the thoracic viscera excepting the lungs. It may be divided for purposes of description into two parts: an upper portion, above the upper level of the pericardium, which is named the superior mediastinum; and a lower portion, below the upper level of the pericardium. This lower portion is again subdivided into three parts, viz., that in front of the pericardium, the anterior mediastinum; that containing the pericardium and its contents, the middle mediastinum; and that behind the pericardium, the posterior mediastinum.

Basic Study Questions

  1.  Which ribs connect to the manubrium?

  2.  What separates the thoracic and abdominal cavity?

  3.  How many ribs are there?  What are the 3 categories of ribs?

  4.  What is the nerve supply to the diaphragm?

Additional Study Questions