Topographic position
In topographic anatomy, the location of the thymus (thymus) is defined quite precisely. The organ is localized within the superior interpleural space. This space is formed between the mediastinal leaves of the parietal pleura of the right and left pleural cavities.
Anteriorly, the gland directly adheres to the retrosternal fascia, which separates it from the posterior surface of the sternum and the muscles attached to it. Understanding this localization is critically important because during access through the sternum (sternotomy), the surgeon first encounters the retrosternal fascia and the thymus lying behind it. It is a misconception to think that the gland is located in the costodiaphragmatic, costomediastinal, or inferior interpleural recesses — its true place is precisely in the superior part of the interpleural space.
Syntopy: proximity to critical structures
The syntopy of the thymus is of particular interest to operative surgery due to its proximity to vital structures of the mediastinum.
- Posterior to the gland lie major great vessels: the brachiocephalic veins (venae brachiocephalicae) and the aortic arch (arcus aortae). This close proximity requires extreme caution when dissecting the organ.
- Inferiorly and posteriorly, the gland borders the pericardium (pericardium), the fibrous sac protecting the heart.
Thus, the thymus seems to "rest" on major venous and arterial collectors, descending with its lower pole onto the anterior surface of the pericardium.
Features of the capsule structure
The proper capsule of the thymus is represented by a thin fascial sheath. This sheath is not exclusively an outer capsule: it sends multiple connective tissue extensions into the substance of the gland, dividing its parenchyma and creating an internal framework.
The most important surgical feature is that externally, this thin fascial sheath is not isolated. It is firmly connected (fused) with surrounding structures:
- With the fascial sheaths of the brachiocephalic veins.
- With the fascial sheath of the aortic arch.
- With the pericardium.
- With the transitional costomediastinal pleural folds.
Due to such robust fascial attachments, traction (pulling) on the thymus during surgery inevitably transmits tension to the walls of the brachiocephalic veins, aorta, and pleura, creating a risk of iatrogenic injury.
Briefly: answers to topical questions
Which arteries supply blood to the thymus?
Blood supply to the thymus is provided by thymic branches (rr. thymici). These branches originate from the following arteries:
- Internal thoracic artery (a. thoracica interna)
- Inferior thyroid artery (a. thyroidea inferior)
- Pericardiacophrenic artery (a. pericardiacophrenica)
- Superior phrenic artery (a. phrenica superior)
An abundance of blood supply sources is particularly characteristic of childhood.
Into which veins does venous blood drain from the thymus?
Venous drainage from the thymus is carried out via thymic veins (vv. thymicae). Blood from the organ collects into the unpaired thyroid plexus (plexus thyroideus impar), which is located at the lower edge of the thyroid isthmus and anterior to the trachea. From this venous plexus, blood flows into the brachiocephalic veins (vv. brachiocephalicae).
What are the sources of innervation of the thymus?
Innervation of the thymus is provided by the following sources:
- Vagus nerves (nn. vagi)
- Phrenic nerves (nn. phrenici)
- Branches of the cervical sympathetic ganglia.
Which specific muscles lie anterior to the retrosternal fascia in the projection of the thymus?
Anterior to the thymus in the region of its upper parts lie the sternothyroid muscles. The anterior surface of the upper parts of the thymus directly serves as the attachment site for these muscles. The thymus itself is located in the superior interpleural space and closely adheres to the retrosternal fascia.
To remember the posterior syntopy, use the mnemonic: Posterior — Veins (brachiocephalic) and Aortic Arch.
Class and exam questions
In which specific interpleural space is the thymus located?
It is located strictly in the superior interpleural space. The inferior interpleural space, as well as the costodiaphragmatic and costomediastinal recesses, are not related to the location of the gland.
To which structures is the fascial sheath of the gland externally connected?
Externally, the sheath is tightly connected to the fascial sheaths of the brachiocephalic veins, the aortic arch, the pericardium, as well as the transitional costomediastinal pleural folds.
What is located directly posterior to the thymus?
Posterior to the gland lie the brachiocephalic veins and the aortic arch. Inferiorly and posteriorly, the pericardium is adjacent to it.
What else to study on this topic
- Blood supply and venous drainage of the thymus
- Innervation of the thymus and its connections with the vagus nerve
- Lymphatic drainage from structures of the superior mediastinum
- Topography of the brachiocephalic veins and aortic arch
- Surgical approaches to organs of the anterior mediastinum
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