The answer is ULNA, four letters. The ulna is the forearm bone on the little finger side, and it is the fill to write whenever the grid gives you four squares. Six squares usually means RADIUS, the other forearm bone, and seven points to HUMERUS, the single long bone of the upper arm.
Two bones below the elbow, one above
Your arm has three long bones, and a bare clue like this one could mean any of them. The humerus runs from the shoulder down to the elbow. Below the elbow the forearm splits into a pair that lie side by side, the ulna on the little finger side and the radius on the thumb side. The ulna is the longer and thinner of the two, and its upper end, a hooked piece called the olecranon, is the pointed part of the elbow you rest on a table.
The two forearm bones do not simply sit parallel. Turn your palm toward the floor and the radius crosses over the ulna, which is how the hand rotates while the elbow stays put. Because the pair work as a unit, setters treat them as interchangeable in wording like “Forearm bone,” which has been answered with both ULNA and RADIUS. The square count does the sorting, and the anatomy only matters once you know which length you are dealing with. Wording that mentions the upper arm is different, because only one bone lives there, and “Upper arm bone” has been answered with HUMERUS.
Five squares means the clue went plural
Add an s to the clue and every answer changes shape. “Arm bones” has been answered with ULNAE, ULNAS and RADII, all five letters, and with HUMERI at six. Dictionaries accept both plurals of ulna, and ULNAE and ULNAS differ only in the last square, so leave that square empty until a crossing entry settles it. RADII is the classical plural of radius, and it also names the lines drawn from the center of a circle, so the same five letters can be reached from geometry as easily as from anatomy. The less common English plural RADIUSES has also turned up, at eight squares.
Every arm bone entry, singular and plural
| Letters | Answer | When it fits |
|---|---|---|
| 4 | ULNA | The default for the singular clue |
| 5 | ULNAE | Plural wording, Latin ending |
| 5 | ULNAS | Plural wording, English ending |
| 5 | RADII | Plural wording, thumb side bones |
| 6 | RADIUS | Six squares or a thumb side hint |
| 6 | HUMERI | Plural wording aimed at the upper arm |
| 7 | HUMERUS | Seven squares or any mention of the upper arm |
| 8 | RADIUSES | Less common English plural of radius |
Latin names with everyday meanings
None of the three names started out as anatomy. Ulna is Latin for the elbow, and the Romans also used it as a measure of length. It is thought to share an ancient root with the English words elbow and ell, an old unit of length, so the bone and the bend of the arm are close relatives in the dictionary. Radius meant a staff or rod, the spoke of a wheel and a ray of light. The Romans already used it for the forearm bone, and English picked up that sense in the early seventeenth century. Humerus comes from the Latin word for the shoulder, and in English it meant the shoulder itself before it settled on the bone of the upper arm in the early eighteenth century.
The funny bone is really a nerve
The jolt you feel after knocking your elbow does not come from a bone. It comes from the ulnar nerve, which runs behind a bony bump at the lower end of the humerus with very little padding over it. The nickname funny bone is on record from the early nineteenth century and refers to that odd tingle, though the pun on humerus is hard to miss. The nerve matters in a grid because it has its own answer. Wording about the funny bone nerve wants ULNAR, the adjective, which shares its first four letters with ULNAE and ULNAS. Once again the last square is the one to confirm before you move on.
