the.com/muscular system

means The organ system made up of all the body's muscles — skeletal, smooth, and cardiac — that contracts to produce movement, maintain posture, and move substances through the body.

from Muscle traces back to Latin musculus, literally "little mouse" — a diminutive of mus, mouse. The popular story is that a flexing bicep looked to Roman eyes like a little mouse rippling under the skin, and that image is the standard explanation historians give for the leap, though the exact imaginative moment is lost to time. System comes from Greek systema, "organized whole," from synistanai, "to combine, to stand together" — syn- (together) plus histanai (to stand). So the phrase quietly describes itself: little mice, standing together.