BONES OF THE THORAX STERNUM The sternum is an elongated flat bone, which lies in the anterior median part of the chest wall. It is about 7 cm long. PARTS:The sternum consists of the following three parts:

1. Upper part, the manubrium sterni or episternum.

2. Middle part, the body or mesosternum.

3. Lower part, the xiphoid process or metasternum.

The sternum resembles a dagger or a small sword in shape. Its three parts— manubrium, body, and xiphoid process represent the handle, blade, and point of the sword, respectively. The upper part of sternum is broad and thick, whereas its lower part is thin and pointed. Its anterior surface is slightly rough and convex, while its posterior surface is smooth and slightly concave. The manubrium and body of sternum lie at an angle of 163° to each other, which increases slightly during inspiration and decreases during expiration.

The angle between long axis of manubrium and long axis of body of
sternum is about 17°. The angle between long axis of manubrium and long axis of body of sternum is about 17°.
Anatomical Position
In anatomical position, the sternum as a whole is directed downwards and inclined slightly forward with its rough convex surface facing anteriorly. Its broad end is directed upwards and lower pointed end is directed downwards.
Features and attachments
1. Manubrium It is roughly quadrilateral in shape. It lies opposite to the third and fourth thoracic vertebrae .

It is the thickest and strongest part of the sternum and presents the following features:

1. Two surfaces—anterior and posterior.

Four borders—superior, inferior, and lateral (right and left). Anterior surface: on each side provides attachment to the sternal head of sternocleidomastoid and pectoralis major muscles. Posterior surface: is smooth and forms anterior boundary of superior mediastinum. On each side, it provides attachment to two muscles: (a) sternohyoid at the level of clavicular notch, and (b)sternothyroid at the level of facet for 1st costal cartilage.

Lower half is related to arch of aorta
Upper half is related to three branches of the arch of aorta, viz. brachiocephalic artery, left common carotid artery, left subclavian artery, and left brachiocephalic vein.
• Superior or Upper border is thick, rounded, and concave. It presents a
notch called suprasternal notch or jugular notch. It provides
attachment to the interclavicular ligament. Clavicular notch on either
side of suprasternal notch articulates with the clavicle to form
sternoclavicular joint.
• Lateral border presents two articular facets: Upper facet articulates
with the 1st costal cartilage to form primary cartilaginous joint.
Lower demifacet along with other demifacet in the body of sternum
articulates with the 2nd costal cartilage.

• Inferior or Lower border articulates with the upper end of the body of sternum to form secondary cartilaginous joint called manubriosternal joint. The manubrium makes a slight angle with the body at this junction called sternal angle or angle of Louis. It is recognized by the presence of a transverse ridge on the anterior aspect of the sternum. 2. Body (Mesosternum; Figs 1 & 2) The features of the body are as follows:

1. It is longer, narrower, and thinner than the manubrium.

2. It is broadest at its lower end.

Its upper end articulates with the manubrium at the sternal angle to form manubriosternal joint.

• 4. Its lower end articulates with the xiphoid process to form primary
cartilaginous xiphisternal joint.
• 5. Its anterior surface presents three faint transverse ridges indicating
the lines of fusion of four small segments called sternebrae. The
anterior surface on each side gives origin to the pectoralis major
muscle.
• 6. Its posterior surface is smooth and slightly concave. (a) Lower part
of posterior surface gives origin to sternocostalis muscle. (b) On the
right side of median plane, posterior surface is related to pleura,
which separates it from the lung. (c) On the left side of median plane,
upper half of the body is related to the pleura and lower half to the
pericardium (bare area of the pericardium)
• 7. Its lateral border articulates with the 2nd–7th costal cartilages (to
form synovial joints.

Strictly speaking, 2nd costal cartilage articulates at the side of manubriosternal junction and 7th costal cartilage articulates at the xiphisternal junction). 3. Xiphoid Process (Metasternum; Figs 1 & 2)

1. It is the lowest and smallest part of the sternum.

2. It varies greatly in size and shape.

3. It may be bifid or perforated.

4. Its anterior surface provides insertion to the medial fibres of the rectus abdominis.

5. Its posterior surface gives origin to the sternal fibres of the diaphragm

N.B. Features of interest at the sternal angle:
Sternal angle can be felt as a transverse ridge on the sternum about 5 cm below the suprasternal notch. The sternal angle is an important surface bony landmark for many anatomical events accurate this level.
These are:
• Second costal cartilage articulates, on either side, with the sternum at this level, hence this level is used for counting the ribs.
• It lies at the level of intervertebral disc between T4 and T5 vertebrae.
• Horizontal plane passing through this level separates superior mediastinum from inferior mediastinum.
• Ascending aorta ends at this level. • Arch of aorta begins and ends at this level
• Descending aorta begins at this level.
• Trachea bifurcates into right and left principal bronchi at this level.
• Pulmonary trunk divides into right and left pulmonary arteries at this
level.
• Upper border of heart lies at this level.
• Azygos vein arches over the root of right lung to end in the superior
vena cava

• OSSIFICATION The sternum develops from two vertical cartilaginous plates (sternal plates), which fuse in the midline. The sternum ossifies from six double centres, viz.

1. One for manubrium.

2. Four for body.

3. One for xiphoid process. Appearance: The centers appear in descending order for different parts of sternum as follows:

   1. Manubrium: 5th month

   2. Body (a) First sternebra: 6th month (b) Second sternebra: 7th month (c) Third sternebra: (d) Fourth sternebra: 9th month

   3. Xiphoid process: 3rd year

Fusion The fusion occurs as follows:

1. Fusion between sternal plates takes place from below upwards. It begins at puberty and completed by 25 years.

2. The xiphoid process fuses with the body at the age of 40 years.

3. Manubrium does not fuse with the body. As a result, the secondary cartilaginous manubriosternal joint usually persists throughout life. In about 10% individuals, fusion occurs in old age.

Clinical correlation:

Sternal foramen and cleft sternum: The two sternal plates fuse in caudocranial direction. Sometimes sternebrae fail to fuse in the midline, as a result defect occurs in the body of sternum in the form of sternal foramen or cleft sternum. The cleft sternum is often associated with ectopia cordis.

RIBS
The ribs are flat, ribbon-like, elastic bony arches, which extend fromthoracic vertebrae posteriorly to the lateral borders of the sternum anteriorly.
Their anterior ends are connected to the costal cartilage. The ribs along with its costal cartilage constitute the costa. The ribs and their costal
cartilages form greater part of the thoracic skeleton.
Number:
Normally there are 12 pairs of ribs (but occurrence of accessory
cervical or lumbar rib may increase them to 13 pairs or absence of
12th rib may reduce them to 11 pairs)