Lesson 1.2 · Unit 1 · HS-LS1-2
Anatomical position, the directional terms and the planes
"Above the elbow" is ambiguous, because it depends entirely on where the arm is. Anatomy solves this the way navigation solves it, by fixing a reference position and describing everything relative to it. Every directional term in this course assumes that position, whatever the body is actually doing.
- Anatomical position is defined precisely: standing erect, feet slightly apart and flat on the floor, head and eyes facing forward, arms at the sides with the palms facing forward and the thumbs pointing away from the body. That last detail is the one students forget, and it is why the radius is described as lateral.
- Left and right always mean the body's own left and right. Not yours as you look at the diagram. A drawing's left side is the subject's right.
- The paired terms: superior and inferior (toward the head, toward the feet), anterior and posterior (front, back), medial and lateral (toward the midline, away from it), superficial and deep (toward the surface, away from it).
- Proximal and distal describe limbs only. They mean closer to and farther from the point where the limb attaches to the trunk. The wrist is distal to the elbow. Applying them to the trunk itself is an error.
- Regional terms name specific areas: brachial (arm), antebrachial (forearm), carpal (wrist), femoral (thigh), crural (leg proper), tarsal (ankle), sural (calf), popliteal (back of knee), cubital (front of elbow), axillary (armpit), inguinal (groin), cervical (neck), occipital (back of head).
- Three planes cut the body: sagittal divides left from right (midsagittal exactly down the middle), frontal or coronal divides anterior from posterior, and transverse or horizontal divides superior from inferior. A transverse section is what a CT scan produces.
- Prone means face down and supine means face up. A useful mnemonic: you lie on your spine when supine.
Where students lose marks: describing a hand raised above the head as superior to the shoulder. The terms assume anatomical position, not the position the body happens to be in, so the hand is distal to the shoulder no matter where the arm is waving.
The problem. A patient has a laceration described in a chart as "on the anteromedial surface of the left crural region, approximately 8 cm distal to the patella." Locate it exactly, and explain how each word narrows the position.
Step one: settle the side. "Left" means the patient's left, so if you are facing them it is on your right. Establish this before anything else, because getting it backwards makes every other term point to the wrong place.
Step two: find the region. Crural means the leg proper, which in anatomy is the part between the knee and the ankle, not the whole lower limb. Everyday speech uses "leg" for the entire limb; anatomy does not, and this is a real source of error.
Step three: apply the first directional term. Anterior means the front surface. So you are on the front of the shin, not the calf, which would be the posterior surface of the same region.
Step four: apply the second. Medial means toward the midline of the body. On the front of the left shin, the medial side is the inner side, the one nearer the other leg. Anteromedial therefore means the front-inner surface, which is the flat area over the tibia that has almost no muscle covering it.
Step five: apply the measurement with its reference. Distal to the patella means farther from the trunk attachment than the kneecap is, so 8 cm down the shin from the kneecap. If the term had been proximal, the same 8 cm would have put the wound on the thigh.
Step six: state the location in plain words. On the patient's left shin, on the inner half of the front surface, about 8 cm below the kneecap.
Step seven: check why the precision matters. The anteromedial shin is where the tibia lies directly under skin with little soft tissue between. A laceration there is more likely to reach bone and slower to heal than the same cut on the calf, which is thickly muscled. The directional terms are not bureaucracy; they carried clinically relevant information in seven words.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- Describe anatomical position completely.
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Standing erect, feet slightly apart and flat, head and eyes facing forward, arms at the sides with palms facing forward
- Give the term meaning toward the midline of the body.
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Medial
- Name the plane that divides the body into anterior and posterior portions.
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The frontal, or coronal, plane
- Define prone and supine.
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Prone is lying face down; supine is lying face up
- Which region does "popliteal" name?
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The back of the knee
- State the relationship of the elbow to the wrist, and of the elbow to the shoulder,
using the correct terms.
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Both comparisons are within a limb, so proximal and distal apply. The elbow is closer to the shoulder attachment than the wrist is, and farther from it than the shoulder is. The elbow is proximal to the wrist and distal to the shoulder
- A student writes that the stomach is "distal to the heart." Correct the error and
give the proper term.
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Proximal and distal apply only to the limbs, where there is a defined attachment point to measure from. The trunk has no such point, so the comparison must use the head-to-foot axis instead. The stomach is inferior to the heart
- Explain why the radius is described as the lateral bone of the forearm, given that in
everyday standing posture it often lies on the inner side.
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All terms assume anatomical position, in which the palms face forward. With the palm forward the radius lies on the thumb side, which is the side away from the midline. When you rotate the forearm so the palm faces backward the radius crosses over the ulna and physically moves, but the name does not change, because the name was assigned in the reference position. In anatomical position the palm faces forward, placing the radius on the thumb side, which is lateral
- A CT scanner produces images showing a slice through both lungs, the vertebral column
and the sternum at the same level. Name the plane and justify it.
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Structures at the front of the body (sternum) and the back (vertebra) appear in the same image, so the cut cannot be frontal, which would separate them. Both left and right lungs appear, so it cannot be sagittal either. The only plane that shows anterior, posterior, left and right together is one cut across the body's long axis. A transverse (horizontal) plane
- A surgeon needs to reach a structure described as deep to the sternum and slightly to
the left of the midsagittal plane. Name the structure and state what "deep" rules out.
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Deep means farther from the body surface than the sternum, so the structure sits behind the breastbone in the thoracic cavity, ruling out anything superficial such as skin, subcutaneous fat or the pectoral muscles. Slightly left of the exact midline, behind the sternum, is where the heart sits. The heart; "deep" rules out the skin, fat and chest wall muscles that lie superficial to the sternum