What is "internal gear"

Internal \In*tern"al\, a. [L. internus; akin to interior. See Interior.]

1. Inward; interior; being within any limit or surface; inclosed; -- opposed to external; as, the internal parts of a body, or of the earth.

2. Derived from, or dependent on, the thing itself; inherent; as, the internal evidence of the divine origin of the Scriptures.

3. Pertaining to its own affairs or interests; especially, (said of a country) domestic, as opposed to foreign; as, internal trade; internal troubles or war.

4. Pertaining to the inner being or the heart; spiritual.

   With our Savior, internal purity is everything.
   --Paley.

5. Intrinsic; inherent; real. [R.]

   The internal rectitude of our actions in the sight of God.
   --Rogers.

6. (Anat.) Lying toward the mesial plane; mesial.

   Internal angle (Geom.), an interior angle. See under Interior.

   Internal gear (Mach.), a gear in which the teeth project inward from the rim instead of outward.

   Syn: Inner; interior; inward; inland; inside.