📝 STANDARD DESCRIPTION:
1. Cranial Vena Cava:
- Formed by union of right & left subclavian + jugular veins (or via brachiocephalic trunks).
- Returns blood from head, neck, forelimbs, cranial chest wall.
- Opens into right atrium.
2. Caudal Vena Cava:
- Brings blood from abdomen, pelvis, hindlimbs.
- Passes through CAVAL FORAMEN of diaphragm.
- Opens into right atrium.
3. Azygos Vein:
- RIGHT azygos in OX & HORSE (drains right intercostal spaces).
- LEFT azygos in DOG, PIG, CAT.
- Empties into the cranial vena cava (or coronary sinus in dog/pig).
4. Pulmonary Veins: 4-6 veins — return OXYGENATED blood from lungs to LEFT atrium.
5. Coronary Sinus: Receives veins of heart wall; opens into right atrium.
📚 DETAILED DESCRIPTION:
THE BIG VEINS OF THE CHEST
Veins of the chest collect blood from the head, limbs, body wall, and the lungs, and bring it back to the heart.
CRANIAL VENA CAVA (Anterior Vena Cava):
Large short vein lying in the cranial mediastinum. Formed by the union of the right and left brachiocephalic veins (each formed in turn by jugular + subclavian veins). Drains:
• Head and neck (via jugulars).
• Both forelimbs (via subclavians).
• Cranial chest wall (via internal thoracic & cranial intercostals).
It empties into the cranial part of the right atrium.
CAUDAL VENA CAVA (Posterior Vena Cava):
Largest vein of the body. Brings deoxygenated blood from the abdomen, pelvis, and hindlimbs. Pierces the diaphragm through the caval foramen (held open in the tendinous centre — never collapses) and then runs in a special pleural fold (plica venae cavae) to enter the right atrium from behind.
AZYGOS VEIN — important species difference:
This vein drains the dorsal intercostal spaces (chest wall behind the heart).
• In OX, HORSE, SHEEP → RIGHT azygos vein is present. It runs along the right side of the spine and opens into the cranial vena cava just before the heart.
• In DOG, PIG, CAT → LEFT azygos (hemiazygos) is present. It runs on the left side and opens into the coronary sinus (and thus into the right atrium).
Memory aid: 'Right is RuminantHorse, Left is for Carnivore-Pig'.
PULMONARY VEINS:
4 to 6 veins (varies). They are unusual: they carry oxygenated blood (red), opposite to all other veins. Bring blood from the lungs and empty into the left atrium.
CORONARY SINUS:
The 'big vein' of the heart wall. Lies in the coronary groove on the back of the heart and empties into the right atrium just below the cranial vena cava opening.
// COMPARATIVE ANALYSIS
| OX |
RIGHT azygos only. |
| HORSE |
RIGHT azygos only. |
| DOG |
LEFT azygos (hemiazygos) only. |
| PIG |
LEFT azygos only. |
| FOWL |
Two cranial venae cavae (right & left) — both open separately into right atrium. |
CLINICAL RELEVANCE
Jugular vein thrombosis: Can extend down into the cranial vena cava, blocking venous return → swelling of head and forelimbs.
Cranial vena cava obstruction (e.g. by mediastinal tumour like lymphoma in cattle): Causes brisket oedema (bottle jaw / brisket swelling).
