Role in the thoracic cage
The sternum is a critical part of the bony framework of the trunk. The entire thoracic cage (Skeleton thoracis) is formed by three main components:
- Sternum (Sternum);
- Ribs (Costae);
- Thoracic vertebrae (Vertebrae thoracicae).
This complex performs several critical tasks simultaneously. First, it creates a reliable shield to protect the organs of the thoracic cavity (heart, lungs, and major blood vessels). In addition, the thoracic cage takes direct part in the mechanics of breathing and serves as a firm support for the attachment of the shoulder girdle muscles.
Anatomical parts of the sternum
Topographically, this spongy bone is divided into three distinct parts:
- Manubrium of sternum (Manubrium sterni) — the upper, widest part.
- Body of sternum (Corpus sterni) — the middle, longest section.
- Xiphoid process (Processus xiphoideus) — the lower segment of the bone.
Interestingly, the shape of the xiphoid process is highly variable. It depends directly on how ossification centers are established during development. As a result, the process may be pointed, spade-shaped, bifurcated at the end, or even have apertures.
A characteristic curve is formed between the manubrium and the body — the sternal angle (Angulus sterni). It is worth noting that in some individuals, ossification of the cartilage between the manubrium and body never occurs, which can cause these elements to remain separate bones throughout life.
Notches and rib articulations
Along the margins of the sternum are a series of depressions (notches) designed for articulation with other bones:
- On the upper margin of the manubrium lies the unpaired jugular notch (Incisura jugularis). This is an important clinical landmark that is easily palpable subcutaneously in the neck.
- Flanking the jugular notch are the paired clavicular notches (Incisurae claviculares), which serve for articulation with the clavicles.
- Further down, along the lateral borders of the bone, run the costal notches (Incisurae costales).
The distribution of costal notches strictly corresponds to the parts of the sternum. The manubrium features a notch for the cartilage of the 1st rib. At the level of the sternal angle (the junction between the manubrium and body) lies the notch for the 2nd rib. The remaining costal notches, intended for the 3rd–7th ribs, are located along the margins of the sternal body and on the xiphoid process.
Spatial orientation of the specimen
To correctly orient an anatomical specimen of the sternum relative to one's own body, three simple rules must be followed:
- The wide manubrium must point strictly upward.
- The variable xiphoid process must point downward.
- The sternal angle must project forward — this landmark reliably determines the anterior surface of the bone.
Briefly: answers to topical questions
Which muscles originate from the anterior surface of the sternum?
The pectoralis major muscle originates from the anterior surface of the sternum. This muscle consists of several parts, and its sternocostal portion arises from the aforementioned bone.
- Pectoralis major muscle (m. pectoralis major) — originates from the anterior surface of the sternum and the cartilages of ribs II–VII, forming the sternocostal part (pars sternocostalis).
At the level of which thoracic vertebrae is the jugular notch located?
The jugular notch of the sternum is located at the level of the lower border of the second thoracic vertebra.
- Jugular notch (Incisura jugularis) — projects onto the lower border of the II thoracic vertebra (Th2).
The superior thoracic aperture is located at this same level, and the cervical trachea transitions into the thoracic part, descending 4–5 centimeters deep.
What joints does the sternum form with the clavicles?
The sternum forms the sternoclavicular joints with the clavicles.
- Sternoclavicular joint (articulatio sternoclavicularis) — formed by the articulation of the saddle-shaped sternal articular surface of the clavicle with the sternum.
Stabilization of this articulation is provided by the sternoclavicular and costoclavicular ligaments. Mobility in this joint occurs during movements of the shoulder and shoulder girdle.
To easily remember the distribution of ribs: "The manubrium holds the first, the angle meets the second, and the body takes all the rest up to the seventh."
Class and exam questions
How to quickly identify the anterior surface of the sternum?
The anterior surface is identified by the sternal angle (Angulus sterni). This angle is formed at the junction of the manubrium and body, and it always projects forward.
Why does the xiphoid process look different in different specimens?
The shape of the xiphoid process is highly variable and depends on individual patterns of ossification center formation. It can be pointed, bifurcated, spade-shaped, or have apertures.
Where does the second rib attach?
The notch for the cartilage of the second rib is located precisely at the level of the sternal angle — at the border between the manubrium and the body of the bone.
Is the sternum always a single monolithic bone?
Not always. In some cases, ossification between the manubrium and body does not occur, and they may remain separate bones throughout a person's life.
What else to study on this topic
- Peculiarities of ossification center formation in the xiphoid process
- Ligamentous apparatus of the clavicular notches
- Biomechanics of sternal participation in the act of breathing
- Anatomy of the shoulder girdle muscles originating from the sternum
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