Human Anatomy & Histology

Syllabus Unit: Pre & Para Clinical Subjects

Why it matters

Chapter overview

Gross anatomy, neuroanatomy, histology and embryology form the pre-clinical foundation AIIMS PG tests through image-based and viva-style recall — the mediastinum's compartments, the spinal cord's exact termination level, an epithelium's exact classification, and a congenital anomaly's exact embryological failure point are all asked as precise, single-fact questions rather than broad conceptual ones.

Postgraduate Medical Exam Syllabus Pre & Para Clinical Subjects 50% weightage

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Superior MediastinumGreat vessels, trachea, oesophagus, thymus remnant

Anterior MediastinumThymus remnant, lymph nodes, fat

Middle MediastinumHeart, pericardium, phrenic nerves

Posterior MediastinumDescending aorta, oesophagus, thoracic duct, azygos vein

1Gross Anatomy: Thorax & Abdomen

The mediastinum's compartments and the abdomen's peritoneal reflections are the two anatomic frameworks AIIMS PG uses to test surgical and radiological localisation.

Learning Objectives

Divide the mediastinum into superior, anterior, middle and posterior compartments, naming a key structure in each.
Trace the course of the recurrent laryngeal nerve and explain its surgical significance in thyroidectomy.
Describe the peritoneal relations of the stomach, liver and spleen versus retroperitoneal organs.
List the boundaries and contents of the inguinal canal and their relevance to hernias.

Key Terms

MediastinumThe central thoracic compartment between the two pleural cavities, subdivided into the superior and inferior (anterior, middle, posterior) parts, each housing distinct structures.

Recurrent Laryngeal NerveA branch of the vagus that loops under the aortic arch (left) or subclavian artery (right) to ascend in the tracheoesophageal groove and supply all intrinsic laryngeal muscles except cricothyroid; at risk during thyroidectomy.

Retroperitoneal Organs (SAD PUCKER)Suprarenal glands, Aorta/IVC, Duodenum (2nd-4th part), Pancreas (except tail), Ureters, Colon (ascending/descending), Kidneys, Esophagus, Rectum — lie behind the peritoneum, distinct from intraperitoneal organs like the stomach and spleen.

Inguinal CanalAn oblique passage through the anterior abdominal wall (deep ring to superficial ring) transmitting the spermatic cord (male) or round ligament (female); the site of indirect versus direct inguinal hernias.

Thoracic and abdominal gross anatomy is tested through localisation: which mediastinal compartment houses a mass, whether an organ is intra- or retroperitoneal, and which landmark separates one hernia type from another.

The mediastinum's four compartments

Superior — great vessels, trachea, oesophagus, thymus remnant.
Anterior — remnant of the thymus, lymph nodes, fat.
Middle — heart, pericardium, phrenic nerves, roots of the great vessels.
Posterior — descending thoracic aorta, oesophagus, thoracic duct, azygos vein, sympathetic chain.

The recurrent laryngeal nerve

Common MistakeUnilateral RLN injury during thyroidectomy causes hoarseness; bilateral injury can cause airway obstruction from unopposed vocal cord adduction — always identify and preserve it during dissection.

Peritoneal relations

Intraperitoneal organs (stomach, spleen, liver, first part of duodenum) are almost fully covered by visceral peritoneum and mobile; retroperitoneal organs (remembered by SAD PUCKER) lie behind the peritoneum and are relatively fixed.

The inguinal canal

Running from the deep (internal) ring to the superficial (external) ring, the canal transmits the spermatic cord in males; indirect hernias pass through the deep ring lateral to the inferior epigastric vessels, while direct hernias push through Hesselbach's triangle medial to them.