(1) Pertaining to the essence or nature of a thing; inherent. (2) anatomy denoting those muscles with an origin and insertion that are both within the structure under consideration, distinguished from the extrinsic muscles that have their origin outside the structure under consideration; applied especially to the limbs but also to the ciliary muscle as distinguished from the recti and other orbital muscles, which are outside the eyeball. SYN: essential (6). [L. intrinsecus, on the inside]