midsagittal plane

The midsagittal plane is the vertical plane that runs down the body's midline and divides it into equal left and right halves. It is a plane through the body, not a structure in it: the plane is imaginary, and what makes it the midsagittal one is that the two halves come out equal. A sagittal plane that sits off the midline divides the body into unequal left and right parts, and it is simply sagittal. The other two reference planes answer the other questions: a frontal plane divides front from back, and a transverse plane divides top from bottom. The planes matter because the directional words depend on them — medial means toward the midline the sagittal plane runs through, lateral means away from it, and bilateral means on both sides. Two structures on the same side are ipsilateral.