There are twelve pairs of the rib in the human being and these ribs attached posteriorly to thoracic vertebrae.

The ribs can be classified by their attachment to the sternum.



Classification of ribs

There are three classes which are

1. True

These are seven pairs of ribs attached directly to the sternum through their costal cartilages.

2. False

These ribs do not attach directly to the sternum through their costal cartilages but the attached anteriorly to each other and to the 7th ribby mean of their costal cartilages and small synovial joints. These are three pairs of ribs.

3. Floating

Floating ribs are two pairs of ribs that do not attach to the sternum.

Part of the ribs

Ribs are flat bones and attached to many structures in different parts. Therefore anatomists divide the ribs into different parts that are head, neck, shaft, tubercle, and angle.

Head lies next to vertebrae. The head has two articulating regions separated by a crest. The articulating regions are named as superior and inferior articulating regions. The head articulates with the superior and inferior costal facets on the connecting vertebrae. Crest if the head provides an attachment site for various ligaments that connect vertebrae and ribs.

The neck of the rib is a flattened part about three cm long. The neck extends from the head laterally. The anterior surface of the neck is flat and smooth while the posterior part of the neck is rough. The posterior part of the rib provides an attachment site to the ligament of the neck.  In the upper border of the neck, a crest is present that provides an attachment site to the costotransverse ligament.

The shaft of the ribs has a flat and curved structure. on the internal of the shaft, there is a groove present. This groove allows the neurovascular supply of the thorax. The groove also protects the blood vessels and nerves from damage.

The tubercle is present on the posterior side of the rib. Tubercle has two faces one is articulating and the other is non-articulating. The articulating face of the tubercle is small and oval and articulates with the costal facets of on thoracic vertebrae. The non-articulating part is rough and provides an attachment site for the ligament of tubercles. In the upper ribs, the tubercle is more prominent than the lower ribs.

Angle of rib
The angle of ribs is the bending part of the ribs. A prominent line is also present in this area. The line provides an attachment site to the tendon of iliocostalis muscle. The line is directed downward and laterally. The distance between the tubercle and angle varies in different ribs.

For example, the distance between the angle of ribs and tubercle of ribs is progressively greater from 2nd to 10th ribs.

Rib cage

The first seven ribs also are known as true ribs attached to the sternum by costal cartilages. The first rib is unique, short, flat, and C shaped bone. 2 to 7 ribs are less curved. 3 pairs of ribs also known as false ribs are attached to the sternum through costal cartilages. While the last 2 pairs of ribs do not attach to the sternum are known as floating ribs. The length of ribs from 1 to 7 increases and from 7 to 12 decrease. With the change in size, the ribs also become slanted. Due to the above attachment and changes, the ribs make a cage-like structure that is known as the rib cage.

Cage is separated from the abdomen area by a structure known as the diaphragm.

Development of ribs

In embryonic life, somites are formed which then divides into three components that are myotome, dermatome, and sclerotome. The ribs developed from the sclerotome. During the fourth week of development, the costal process is formed. These processes are small.

During the fifth week of development, the costal process becomes longer to form the ribs.  In the sixth week of development, the costovertebral joints start developing and separate the ribs from the vertebrae.

Here is one important point that you have to remember that ribs begin as cartilage and later it ossifies. The ossification of ribs occurs through a type of ossification known as endochondral ossification.

The primary ossification center of the ribs is located near the angle of ribs.

Rib excision

Ribs excision is performed by the surgeons to gain entrance to the thoracic cavity.
In this process, the surgeons first make a longitudinal incision through the periosteum on the outer surface of the rib and a segment of a rib is removed.

Then the surgeons make the second incision through the bed of the rib and thus the surgeons gain entrance to the thoracic cavity.
After the operation, the rib regenerates from the periosteum.

Typical ribs

Typical ribs are the ribs that have the same features. A typical rib is a long twisted bone having a smooth superior and sharp inferior border.
The inferior border forms a coastal groove through which intercostal vessels and nerves pass.
A rib has a
and angle
The head articulates with the corresponding vertebral body and that of the vertebra above it. The neck is located between the head and the tubercle and is constricted. The tubercle articulates with the transverse process of the corresponding vertebrae.
The shaft is thin and flattened.
The angle is where the shaft of the ribs bend sharply forward.

Atypical ribs

Atypical ribs are the ribs that are different in features from the others. Among the Atypical ribs, the first rib is very important clinically.
The Ist rib is small and flattened from above to downward.
Subclavian vein, subclavian artery, and lower trunk of the brachial plexus crosses the ribs and in contact with them.

Cervical Rib

It arises from the transverse process of the 7th cervical vertebrae which occurs in about 0.5% of the humans.
The importance of the cervical rib is that it can cause pressure on the lower trunk of the brachial plexus which produces pain in the medial side of forearm and hand.
The cervical rib can also put pressure on the subclavian artery and thus interfere with the circulation.