Layer-by-layer structure of the region
The anatomy of the sternocleidomastoid region is characterized by a distinct layer-by-layer arrangement of tissues. The most superficial layer is the skin, which exhibits varying physical characteristics depending on the level. In the upper third, it is quite dense and relatively fixed, whereas in the lower parts, it becomes elastic and is easily gathered into a fold along with the subcutaneous adipose tissue and superficial fascia.
In the middle third of the region, directly within the sheath formed by the superficial fascia, lies the platysma muscle (m. platysma). The next layer is the investing (second) fascia of the neck. It plays a key role as it splits to form an isolated fascial sheath for the sternocleidomastoid muscle (m. sternocleidomastoideus).
Superficial vessels and nerves
On the superficial surface of the muscular sheath, directly beneath the m. platysma, lies a complex of important structures: superficial cervical lymph nodes, cutaneous branches of the cervical plexus, and large veins.
The main venous collector here is the external jugular vein (v. jugularis externa). It crosses the muscle diagonally from the angle of the mandible toward the midpoint of the clavicle. Upon reaching the lower parts, the vein pierces the second and third cervical fasciae, passing deep to drain into the subclavian or internal jugular vein.
Clinical significance: the adventitia of the external jugular vein is firmly fused with the edges of the openings in the fasciae. When this area is injured, the vessel lumen does not collapse but remains patent, creating a critical risk of air embolism.
Nerve trunks on the surface of the muscle:
- Greater auricular nerve (n. auricularis magnus): accompanies the external jugular vein, lying posterior to it. It provides cutaneous innervation to the retromandibular fossa and the angle of the mandible.
- Transverse cervical nerve (n. transversus colli): crosses the superficial surface of the muscle approximately at its midpoint and divides into superior and inferior branches near the anterior border of the muscle.
- Lesser occipital nerve (n. occipitalis minor): ascends within the fascial sheath along the posterior border of the muscle, heading toward the skin in the region of the mastoid process.
Deep structures and topography beneath the muscle
The deep space posterior to the muscle is bounded anteriorly by the investing layer of the cervical fascia (second fascia) and posteriorly by the prevertebral layer (fifth fascia). The primary content of this zone is the medial neurovascular bundle (common carotid artery, internal jugular vein, and vagus nerve). The bundle is surrounded by a common fascial sheath, the carotid sheath (v. carotica), which anteriorly fuses tightly with the muscle sheath and the omohyoid-clavicular fascia.
Topographically, the deep structures are divided by levels:
- Upper third of the region: beneath the muscular sheath, between the second and fifth fasciae, runs the external branch of the accessory nerve (n. accessorius). It emerges from beneath the posterior border of the muscle and travels into the tissue of the omotrapezoid triangle.
- Beneath the upper half of the muscle (medially): here lie the longus capitis and longus colli muscles, as well as the anterior and middle scalene muscles, covered by the fifth fascia. Between them, from the anterior rami of the spinal nerves, the cervical plexus (plexus cervicalis) is formed. The phrenic nerve (n. phrenicus) descends along the anterior scalene muscle from top to bottom (from its lateral to medial margin). Here also lies the inferior root of the ansa cervicalis, which loops around the internal jugular vein and, joining the superior root from the hypoglossal nerve, forms the ansa cervicalis.
- Beneath the lower half of the muscle: lies the blind pouch (saccus caecus retrosternocleidomastoideus). In the deepest part, posterior to the third fascia and the carotid sheath, the scalenovertebral triangle is concealed.
Briefly: answers to topical questions
What are the contents of the blind pouch (saccus caecus retrosternocleidomastoideus)?
The blind pouch (saccus caecus retrosternocleidomastoideus) is located posterior to the inferior part of the sternocleidomastoid muscle and communicates with the suprasternal interaponeurotic space, which contains the jugular venous arch (arcus venosus juguli).
To understand the cause of air embolism during a neck injury, imagine a "tented pavilion": the outer shell (adventitia) of the jugular vein is tightly anchored to the edges of the dense fasciae. If such a vein is lacerated, its walls cannot physically collapse—they are stretched apart by the fasciae, and the vein begins to suck in air during inspiration.
Class and exam questions
What are the components of the medial neurovascular bundle of the neck?
The bundle includes the common carotid artery, the internal jugular vein, and the vagus nerve. They are enclosed in a common carotid sheath located posterior to the sternocleidomastoid muscle.
Which nerves emerge onto the surface of the sternocleidomastoid muscle?
The branches of the cervical plexus—greater auricular nerve, transverse cervical nerve, and lesser occipital nerve—are projected onto the surface of the muscular sheath.
What is the clinical significance of the external jugular vein anatomy?
Its walls are firmly fused with the cervical fasciae (second and third); therefore, upon mechanical injury, the vessel lumen remains patent, creating an immediate threat of air suction and the development of an air embolism.
Where is the cervical plexus formed?
It is formed beneath the upper half of the sternocleidomastoid muscle, situated between the longus capitis and longus colli muscles and the scalene muscles covered by the prevertebral fascia.
What else to study on this topic
- Topography and composition of the medial neurovascular bundle of the neck
- Mechanism of formation and functions of the ansa cervicalis
- Anatomical boundaries and contents of the omotrapezoid triangle
- Structure of the blind pouch (saccus caecus retrosternocleidomastoideus)
- Topography of the scalenovertebral triangle
Master the complete topic in the "Topographic Anatomy" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the "Sternocleidomastoid region" topic and 2,777 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 91 video lessons on the subject
Other topics in "Topographic Anatomy of the Neck"
Free video lessons: topographic anatomyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.