# What is the deltoid?

The shoulder cap muscle is called the deltoid. Gray's Anatomy (1918) describes it as a thick, triangular muscle that caps the shoulder.

## What it means

- A thick, triangular muscle that caps the shoulder.
- Named for its resemblance to the Greek letter delta.
- Its main action is to raise the arm to the side (abduction).

## Action steps

1. Over the shoulder joint.
2. Arises from the clavicle, acromion, and spine of the scapula.
3. Inserts halfway down the outer side of the humerus.

## Sources

- [Gray's Anatomy of the Human Body (1918)](https://www.bartleby.com/107/)

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