Surface anatomy: finding the organs from the outside

Most of the body's important structures are hidden, but they sit in predictable places relative to bones you can feel through the skin. Surface anatomy is the skill of mapping those hidden structures onto the outside of the body. Clinicians use it every day: to listen to the heart valves, to examine the abdomen, to find a pulse or to choose where to put a needle.

Remember that these are averages. Landmarks shift with age, body shape, posture and breathing, and every person is slightly different.

The neck

• The thyroid cartilage, the "Adam's apple", sits at about the level of the fourth and fifth cervical vertebrae (C4 to C5).
• Just below it, the cricoid cartilage marks the level of C6, where the larynx becomes the trachea.
• The thyroid gland wraps around the front of the trachea below the cricoid cartilage. Its central bridge, the isthmus, crosses roughly the second to fourth tracheal rings, and the gland moves up when you swallow.
• The carotid pulse can be felt at the front edge of the sternocleidomastoid muscle, at about the level of the thyroid cartilage.

The chest

The most useful landmark in the chest is the sternal angle (the angle of Louis), the ridge where the upper part of the breastbone meets its body. It lines up with:

• the second rib, which makes it the starting point for counting ribs and the spaces between them,
• the disc between the fourth and fifth thoracic vertebrae (T4 to T5),
• the point where the trachea divides into the two main bronchi,
• the start and end of the arch of the aorta.

Other chest landmarks:

• The apex beat, the point where the heartbeat is felt most clearly, is normally in the fifth intercostal space in the midclavicular line. A displaced apex beat can suggest an enlarged heart.
• The classic places to listen to the heart valves are the aortic area (second intercostal space, right of the sternum), the pulmonary area (second intercostal space, left of the sternum), the tricuspid area (lower left sternal edge) and the mitral area (at the apex).
• The lungs rise about 2 to 3 cm above the inner third of the collarbone. Their lower edges sit at roughly the 6th rib in the midclavicular line, the 8th rib at the side of the chest and the 10th rib at the back. The pleura, the lining around the lungs, reaches about two ribs lower at each point.
• In the right lung, the horizontal fissure runs along the fourth rib at the front. The oblique fissures run from about the level of the third thoracic spine at the back, around the side, to the sixth costal cartilage at the front.
• The joint between the body of the sternum and the xiphoid process lies at about T9.

The transpyloric plane

An imaginary horizontal line halfway between the notch at the top of the breastbone and the top of the pubic bone, at about the level of the first lumbar vertebra (L1), is called the transpyloric plane. A surprising number of structures sit close to it:

• the outlet of the stomach (pylorus), although its position varies,
• the fundus of the gallbladder, where the tip of the ninth costal cartilage meets the edge of the rectus abdominis muscle,
• the neck of the pancreas,
• the entry points of the kidneys, with the left usually a little higher than the right,
• the lower end of the spinal cord in adults, around L1 to L2.

The abdomen

• The liver lies mostly under the right ribs. Its lower edge follows the right costal margin, and its upper border reaches about the fifth intercostal space. The span of the liver measured by percussion in the midclavicular line is usually around 6 to 12 cm.
• The gallbladder fundus sits where the right costal margin meets the edge of the rectus muscle. Pressing there while the patient breathes in tests for Murphy's sign.
• The spleen lies high at the back on the left, under the 9th to 11th ribs, with its long axis along the 10th rib. A normal spleen cannot usually be felt; one that can be felt is typically enlarged.
• The kidneys lie against the back wall of the abdomen, from about T12 to L3. The right kidney usually sits a little lower than the left because the liver is above it. Tenderness in the angle between the lowest rib and the spine can indicate kidney inflammation.
• The umbilicus usually lies at about the level of the L3 to L4 disc, and its skin is supplied by the tenth thoracic nerve (the T10 dermatome).
• A line joining the highest points of the iliac crests crosses the spine at about L4. Clinicians use it to find a safe space for a lumbar puncture, below the end of the spinal cord.
• McBurney's point, one third of the way from the bony point at the front of the hip (the anterior superior iliac spine) to the umbilicus, marks the usual position of the base of the appendix.
• The bladder sits behind the pubic bone and can only be felt or tapped out above it when it is full.

The groin and limbs

• The inguinal ligament runs from the anterior superior iliac spine to the pubic tubercle.
• The femoral pulse is felt just below the ligament, at the mid-inguinal point, halfway between the anterior superior iliac spine and the pubic symphysis. Within the femoral triangle, the structures lie from lateral to medial in the order nerve, artery, vein, then lymphatics, often remembered as NAVEL (nerve, artery, vein, empty space, lymphatics).
• The radial pulse is at the wrist on the thumb side, just lateral to the tendon of flexor carpi radialis.
• The dorsalis pedis pulse is on the top of the foot, just lateral to the tendon that lifts the big toe (extensor hallucis longus). The posterior tibial pulse is behind the bony bump on the inside of the ankle (the medial malleolus). Weak or absent foot pulses are a classic sign of peripheral artery disease.

Why this matters clinically

Surface anatomy turns symptoms into possibilities. Pain at McBurney's point suggests the appendix; pain under the right ribs that stops a deep breath suggests the gallbladder; tenderness at the angle between the lowest rib and the spine suggests the kidney. It also keeps procedures safe, from placing a chest drain to taking blood from an artery.

How Tangent uses this

Stat!'s Tangent game hides one structure, an organ, bone, muscle, vessel or nerve, and scores every guess by how close it lies to the target on a map of the body. Knowing which structures share a region, like everything near the transpyloric plane, lets you jump straight to the right neighborhood instead of guessing blindly. The Tangent guide explains how the scoring and the body map work.