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transverse section thorax T4 level anatomy labelled diagram structures

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https://mapkmc.manipal.edu/specimen/anat231

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A clean, detailed, labelled anatomical diagram of a transverse section (cross-section) of the thorax at the level of T4 vertebra (sternal angle / angle of Louis). Show the following structures clearly labelled: Sternum (anteriorly), Body of T4 vertebra (posteriorly), Spinal cord within vertebral canal, Right and left lungs (in pleural cavities), Superior mediastinum contents: Arch of Aorta (left side), Superior Vena Cava (right side), Trachea bifurcating into Right and Left principal bronchi (carina at T4), Oesophagus (posterior to trachea), Azygos vein (right side, posteriorly), Left phrenic nerve, Right phrenic nerve, Left vagus nerve, Right vagus nerve, Thoracic duct, Parietal pleura lining the cavity. Draw on a white background with clear black labels and arrows, anatomical orientation (anterior at top, posterior at bottom, right on left side of diagram, left on right side). Medical textbook style diagram.

Key structures at the level of T4 (sternal angle / angle of Louis):

This is a clinically important level because it marks:

The bifurcation of the trachea (carina) into right and left principal bronchi
The beginning and end of the arch of aorta
The junction between the superior and inferior mediastinum
The level of the 2nd costal cartilage anteriorly (sternal angle)

Structure	Position
Sternum	Anterior
T4 vertebral body + spinal cord	Posterior
Arch of Aorta	Left side of superior mediastinum
Superior Vena Cava (SVC)	Right side, anterior mediastinum
Tracheal bifurcation / Carina	Central, posterior
Right & Left principal bronchi	Diverging laterally from carina
Oesophagus	Posterior to trachea, anterior to vertebra
Azygos vein	Right side, posteriorly
Thoracic duct	Left side, posterior
Right & Left phrenic nerves	Lateral to pericardium
Right & Left vagus nerves	Posterior, on either side of trachea
Right & Left lungs	Lateral (in pleural cavities)
Parietal & visceral pleura	Lining pleural cavities

(Manipal Museum of Anatomy; aneskey.com thorax anatomy)

Memory tip for T4 structures - "4 things happen at T4":

Trachea bifurcates
Arch of aorta begins/ends
Superior/inferior mediastinum junction
Sternal angle (angle of Louis) anteriorly

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The full answer for Q1 - Submandibular Salivary Gland was already provided above in detail. Here it is compiled cleanly:

Q1. Submandibular Salivary Gland

a) External Features

Shape: Irregular, roughly the size of a walnut
Has a larger superficial part and a smaller deep part, both continuous around the posterior border of mylohyoid muscle
Partially enclosed between two layers of deep cervical fascia (from the greater cornu of hyoid bone)
It is a mixed but predominantly serous gland (~90% serous, 10% mucous)
The superficial part lies in the digastric triangle
Wharton's duct (~5 cm long) opens at the sublingual papilla beside the frenulum of the tongue
The lingual nerve crosses the duct laterally on the hyoglossus muscle (crosses it twice - hooks around it)

b) Relations

Superficial Part:

Surface	Relations
Inferior	Skin, platysma, deep fascia; crossed by facial vein + cervical branch of facial nerve
Lateral	Submandibular fossa of mandible + medial pterygoid (mandibular attachment)
Medial - anterior part	Mylohyoid muscle
Medial - posterior part	Styloglossus muscle
Medial - intermediate part	Hyoglossus, separated from it by: lingual nerve, submandibular ganglion, hypoglossal nerve, deep lingual vein

Deep Part:

Lies between mylohyoid (inferolaterally) and hyoglossus/styloglossus (medially)
Extends forwards to the posterior end of the sublingual gland

c) Nerve Supply

Parasympathetic (secretomotor) - the main supply:

Pre-ganglionic fibres: CN VII (facial nerve) → Chorda tympani → joins Lingual nerve → Submandibular ganglion (synapse here)
The ganglion is a small fusiform body suspended from the lingual nerve, lying on hyoglossus muscle
Post-ganglionic fibres from the ganglion supply the submandibular and sublingual glands

Sympathetic:

Post-ganglionic fibres from superior cervical ganglion via plexus on the facial artery
Effect: vasoconstriction, reduces secretion

Sensory:

Lingual nerve (branch of V3 - mandibular division of trigeminal)

d) Micro-anatomy (Histology)

Acinar cells - 3 types:

Cell Type	Granules	Product
Serous	Small, dense	Proteins, enzymes (amylase)
Mucous	Large, pale	Mucoproteins
Seromucinous	Intermediate	Mixed

Submandibular gland specific: Predominantly serous acini + some mucous acini capped by serous demilunes

Duct system (acinus → mouth):

Intercalated ducts - low cuboidal epithelium; surrounded by myoepithelial cells
Striated ducts - columnar cells with basal folding (striations); active water and electrolyte transport
Excretory ducts - tall columnar epithelium
Wharton's duct - main duct opening at sublingual papilla

Acini are organized into lobules (each with its own duct), linked by fibrous septa to form lobes.

Salivary gland microanatomy - serous acinus, mucous acinus, serous demilune, intercalated duct, striated duct and myoepithelial cells

e) Clinical Anatomy

Sialolithiasis (Calculi) - ~80% of all salivary calculi occur in Wharton's duct. Reasons:
- Duct is long (5 cm) and runs upward against gravity
- Saliva is viscous and alkaline
- Causes: painful swelling at mealtimes ("dinner-time syndrome")
Submandibular sialadenitis - Chronic inflammation, often secondary to stones; gland becomes densely fibrotic and tethered
Nerves at risk in submandibulectomy:
- Marginal mandibular nerve (runs ~2 cm below mandible over facial vessels) → damage = drooping corner of mouth
- Lingual nerve → damage = loss of taste + sensation to anterior 2/3 of tongue
- Hypoglossal nerve → damage = ipsilateral tongue paralysis and wasting
Tumours - ~50% of submandibular gland tumours are malignant (much higher malignancy rate than parotid). Total excision is always indicated.
Ludwig's angina - Severe bilateral cellulitis of submandibular and sublingual spaces, usually from lower molar infections; can cause fatal airway obstruction.
Frey's syndrome - Rare after submandibulectomy; aberrant regeneration of parasympathetic fibres causing gustatory sweating.

Key memory aid for nerve supply:

Facial → Chorda tympani → Lingual → Submandibular ganglion → Gland "Fools Can Love Salivary Glands"

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