Q5 LONG ANSWER 10M ★ High-yield

Describe the structure of skin and fascia. Add a note on the lymphatic system and the thoracic duct.

Anatomy · 1st Professional — Pre-Clinical

Skin is the largest organ of the body, about 1.8 m2 and 16% of body weight.

Layer	Composition
Epidermis	Stratified squamous keratinised epithelium; avascular. Layers from deep to superficial — basale, spinosum, granulosum, lucidum, corneum
Dermis	Dense irregular connective tissue; papillary and reticular layers; carries vessels, nerves, glands and hair follicles

Mnemonic for epidermal layers — "Come, Let's Get Sun Burnt" from surface inward
Stratum lucidum is present only in thick (glabrous) skin — palms and soles
Melanocytes lie in the stratum basale; racial differences depend on melanin distribution and size of granules, not on the number of melanocytes
Functions — protection, thermoregulation, sensation, excretion, vitamin D synthesis, immunological (Langerhans cells)

Feature	Superficial fascia	Deep fascia
Nature	Loose areolar tissue with fat	Dense fibrous tissue, NO fat
Contents	Cutaneous nerves and vessels, superficial veins and lymphatics	Bounded by it; deep structures lie beneath
Present over	Almost everywhere	Absent over the face, external ear, penis and scrotum
Functions	Insulation, energy store, shape, mobility of skin	Confines muscles into compartments, forms retinacula and intermuscular septa, aids venous return

Superficial fascia has two layers in the lower abdomen — Camper (fatty) and Scarpa (membranous); Scarpa continues into the perineum as Colles fascia, which determines where extravasated urine can spread
Deep fascia forms retinacula — the flexor retinaculum at the wrist and the extensor retinaculum at the ankle, holding tendons in place

Consists of lymph capillaries, vessels, nodes and lymphoid organs
Lymph capillaries are blind-ended, more permeable than blood capillaries, and lack a basement membrane — which is why protein and cells enter them
Absent from — central nervous system, cornea, cartilage, bone marrow, epidermis, nails, hair
Functions — returns tissue fluid and protein to blood; absorbs fat from the gut (chyle); immune defence; a route of tumour spread
Capsule with trabeculae; hilum where the efferent vessel leaves
Cortex — lymphoid follicles with germinal centres; B lymphocytes
Paracortex — T lymphocytes; expands in viral infection
Medulla — medullary cords and sinuses; plasma cells
Afferent vessels are many and enter the convex surface; the efferent is single and leaves at the hilum

Duct	Drains	Ends in
Thoracic duct	The whole body except the right upper quadrant — that is, both lower limbs, abdomen, left half of thorax, left upper limb, left head and neck	Junction of the left subclavian and internal jugular veins
Right lymphatic duct	Right upper quadrant — right head and neck, right upper limb, right thorax	Corresponding junction on the right

The thoracic duct begins as the cisterna chyli at the level of L1–L2, and is about 45 cm long
It is the largest lymphatic vessel in the body

Region	Chief nodes
Breast	Axillary (75%), internal mammary, and across to the opposite breast
Testis	Para-aortic nodes (it develops in the abdomen) — not inguinal
Scrotal skin	Superficial inguinal nodes
Lower limb	Superficial and deep inguinal → external iliac
Anal canal above the pectinate line	Internal iliac
Anal canal below the pectinate line	Superficial inguinal

The testis drains to para-aortic nodes while the scrotum drains to inguinal nodes — a favourite examination point, explained by the descent of the testis from the posterior abdominal wall

Structure	Type	Note
Sweat glands (eccrine)	Simple coiled tubular	Thermoregulation; supplied by sympathetic cholinergic fibres — an exception to the usual rule
Apocrine glands	Larger, open into hair follicles	Axilla, areola, perineum; active after puberty; the source of body odour
Sebaceous glands	Holocrine	Open into hair follicles; absent on palms and soles; blocked in acne
Hair	Keratinised	Arrector pili is smooth muscle with sympathetic supply
Nail	Keratinised plate	Grows from the nail matrix; about 0.1 mm/day

Lymphoedema follows obstruction — after axillary clearance, radiotherapy, or filariasis, which causes elephantiasis and is endemic in parts of India
Lymphatic spread of carcinoma determines staging and surgical clearance; the sentinel node is the first node draining a tumour
Chylothorax follows injury to the thoracic duct, as in oesophageal surgery or trauma
Cellulitis and lymphangitis show as red streaks tracking toward the draining nodes
Knowledge of drainage directs examination — a lesion on the lateral side of the foot drains to the popliteal nodes, not the inguinal, and would otherwise be missed

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