THE THORAX

 THE THORAX

    The Thorax is a region of the human body that houses the visceral organs with the aid of the thoracic cage.

BOUNDARIES OF THE THORACIC CAGE.

Anterior Boundary - Sternum

Posterior Boundary - The 12 Thoracic Vertebrae

Lateral Boundary - The 12 Ribs

 

OSTEOLOGY OF THE BONES OF THE THORAX

Osteology of The Sternum

    The Sternum consists of 3 parts namely, the Manubrium, Body of the sternum and the Xiphoid process.

The Manubrium

It lies on the level of the T3 - T4 vertebrae. Superiorly the manubrium of the sternum has a notch called the jugular or suprasternal notch, superolaterally, it consists of clavicular notch for articulation with the sternal ends of the clavicle, inferiorly to the clavicular notch is a concavity for the attachment of the first rib.

Joints of the Manubrium

1. Sternoclavicular joint - A saddle joint that acts like a ball and socket joint.

2. Sternocoastal joint of the first rib

3. Manubriosternal joint - Secondary cartilaginous joint

    The manubrium forms an angle with the body of the sternum at the manubriosternal joint called the sternal angle or angle of louis. a lot of events occur at this angle, few of which are mentioned below:

1. The imaginary line that divides the mediastinum into superior and inferior mediastinum begins at this angle.

2. It is a point of attachment for the second rib.

3. The aortic arch begins

4. The aortic arch ends

5. The trachea bifurcates etc.

The body of the Sternum

    It lies in the level of the T5-T9 vertebrae and serves as the point of attachment for the 2nd rib through to the 7th ribs. It has 3 main joints, namely, the manubriosternal joint, Xiphiosternal joint, and sternocoastal joints.

The Xiphoid

    It is the little cartilaginous projection inferior to the body of the sternum, Its inferior end lies at the level of T1 vertebrae.

The Ribs.

    They are 12 in number. They are classified based on:

1. Attachment to the sternum

2. Features

1. Based on attachment to the sternum.

    Although the ribs are not directly attached to the sternum, the joint is aided by coastal cartilages of each ribs.

The ribs are classified as true, false and floating in this segment.

- The first 7 ribs which are also known as the vertebrosternal ribs are considered true ribs because their coastal cartilages attach to the sternum directly to form sternocoastal joints with the sternum.

- The last 5 ribs (8th - 12th ribs) which are also known as vertebrochondral ribs
are considered as False ribs because their coastal cartilages do not attach directly to the sternum, or do not even attach at all (in the case of the 11th and 12th ribs). Instead the attach to the coastal cartilage of the rib superior to them with the aid of interchondral joints.

- The last 2 ribs are considered as floating ribs because they do not have any attachment to the sternum.

2. Based on Features.

    In this section, ribs are classified into Typical and Atypical ribs.

A. Typical Ribs.

    The 3rd - 9th ribs are typical ribs. Criteria to be a typical rib is the possession of a head(with two articular facets), neck, tubercle, body/shaft, hence the above mentioned ribs are considered typical due to possession of these features.

B. Atypical Ribs.

    These are the 1st, 2nd, 10th, 11th, and 12th ribs. They are classified as atypical due to their lack of one or two typical features or possession of extra feature.

    The 1st rib is the broadest, shortest, and most sharply curved of the 7 true ribs. It has a single facet on its head for articulation with T1 vertebrae only and two transversely directed grooves crossing its superior surface for the subclavian vessels. The grooves are separated by a scalene tubercle and ridge to which the anterior scalene muscle is attached.

    The 2nd rib has a thinner, less curved body and is substantially longer than the 1st rib. Its main atypical feature is a tuberosity for serratus anterior.

    The 10th - 12th ribs, also only have a facet on theirs heads and articulate with a single vertebrae like the 1st rib, meanwhile the 11th and 12th ribs are short and have no neck or tubercle.

The Thoracic Vertebrae.

    There are 12 of them. They are typical in that they have bodies, vertebrae arches, and 7 processes for muscular and articular connections other features include presence of demi-facets usually on the vertebrae bodies, usually occurring in pairs superiorly and inferiorly for articulation with the heads of ribs, long inferiorly slanting spinous process(protecting the spinal cord), coastal facets on the transverse processes for the articulation with the tubercles of the ribs except for the inferior 2 or 3 thoracic vertebrae.

    T2 - T9 vertebrae are considered typical because of the possession of the typical features while the others are not.

    The superior coastal facets of T1 are not demi-facets because C7 does not have a coastal facet, hence the 1st rib articulates only to the T1 vertebrae, although it has inferior coastal demi-facets.

    T10 only has a bilateral pair of whole coastal facets, located partly on its body and pedicle.

    T11 and T12 also has only one pair of whole coastal facets located on their pedicles.