Anatomy — Applied Anatomy, NMC MBBS licence examination syllabus (Nepal Medical Council).
Every "safe site" in medicine is really a statement about what is not there.
Anatomy taught as lists of origins and insertions is almost impossible to retain and rarely tested. Anatomy taught as what lies next to what is retained easily, because each fact does work: it predicts an injury, explains a sign, or tells you where a needle can go.
That is how this chapter is organised, and how the examination approaches it.
The questions worth preparing for take three forms:
Three nerves supply the hand, and each has one place where it is vulnerable — the place where it lies against bone or crosses a joint with little to protect it.
The radial nerve spirals around the shaft of the humerus, held against the bone. So a mid-shaft humeral fracture is the classic cause of radial nerve injury, and prolonged pressure on the upper arm — an arm hanging over a chair back, a badly fitted crutch — does the same thing without a fracture.
It supplies the extensors, so the deficit is wrist drop: the patient cannot extend the wrist and fingers against gravity.
The ulnar nerve passes behind the medial epicondyle at the elbow, where it is close enough to the surface to be felt — the "funny bone". So elbow fractures and prolonged leaning on the elbow injure it. It supplies most of the small muscles of the hand, so the deficit includes clawing, loss of finger abduction and adduction, and sensory loss over the little finger side.
The median nerve passes through the carpal tunnel at the wrist, in a space that cannot expand. Anything that swells the contents compresses it, giving the sensory symptoms of carpal tunnel syndrome and, later, weakness of thumb abduction.
Notice the pattern rather than memorising three separate facts: a nerve is injured where it is held against bone or squeezed in a confined space. Learn the course and the injury follows; learn what it supplies and the deficit follows. That is two facts per nerve instead of a list.The abdominal wall is a continuous sheet except where it must let something through — and the inguinal canal is exactly that. It exists because the testis descends from inside the abdomen into the scrotum during development, taking a passage through the wall with it.
So the commonest site of abdominal wall herniation is a gap the body built on purpose, and inguinal hernia is far commoner in men, whose canal transmits the spermatic cord rather than the much smaller round ligament.
Anything that raises intra-abdominal pressure repeatedly — chronic cough, straining, heavy lifting — pushes contents toward that existing weakness.
Just below the inguinal ligament, three structures run side by side in a fixed order from lateral to medial: nerve, artery, vein — conventionally remembered as NAVY, with the Y standing for the y-fronts, i.e. medially.
Why this is worth knowing rather than just memorising: the artery is the only one you can feel. Once you have the pulse, you know where the vein is (immediately medial) and where the nerve is (immediately lateral) without seeing either. Every procedure in the region is oriented from that pulse — femoral venous access, arterial sampling, and femoral nerve block.
Medial to the vein lies the femoral canal, a small space that allows the vein to expand. It is also the route a femoral hernia takes, and because it is narrow and rigid, it is why femoral hernias strangulate.
Three everyday procedures illustrate the principle that a safe site is an anatomical claim.
Lumbar puncture. The spinal cord ends well above the bottom of the vertebral canal; below that point the canal contains only nerve roots, floating in cerebrospinal fluid. A needle entering below the end of the cord meets roots that move out of its way rather than a cord that cannot. That is the entire reason the procedure is safe where it is done.
No vertebral level is given here on purpose. Levels vary between individuals, the surface landmark used to estimate them is itself unreliable, and a remembered number applied confidently is how a needle ends up too high. Use the landmarks and the level your protocol specifies.
Chest drain insertion. Each rib has a groove on its under surface carrying the intercostal vessels and nerve. So a needle or drain is passed immediately above a rib, hugging its upper border — passing below the rib runs straight into the neurovascular bundle. This is a single anatomical fact that prevents a serious bleed.
Intramuscular injection into the buttock. The sciatic nerve — the largest nerve in the body — runs through the buttock. The upper outer quadrant is used precisely because the nerve is not there. An injection placed elsewhere in the buttock can cause a permanent sciatic palsy, and in children, where the muscle mass is small, this is a real and recognised injury.
The diaphragm and referred shoulder pain. The diaphragm's motor and sensory supply comes from the neck, because it develops in the neck and migrates downward, dragging its nerve with it. So irritation under the diaphragm — blood, pus, air — is felt in the shoulder tip. A patient with shoulder pain after abdominal trauma may be describing diaphragmatic irritation from bleeding, not a shoulder injury.
The recurrent laryngeal nerve and the thyroid. The nerve supplying the vocal cords runs immediately behind the thyroid gland. That is why hoarseness after thyroid surgery is a recognised complication, and why a hoarse patient with a thyroid mass raises concern about invasion — the anatomy makes the two adjacent.
No vertebral levels, measurements or dermatome lists are quoted here: they vary between individuals and between sources, and a remembered level applied to a procedure is how a needle goes to the wrong place. Use your local protocol and the landmarks on the patient in front of you.
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