The phrase "to cross the midline" means to move or perform activities that involve crossing an imaginary line in the middle of our body. Full definition
And I am especially in favor of crossing the midline of the body in my 1st grade classroom to spell words and so on! (onnit.com)
We will have fun exploring the ways letters are formed and shaped with full body games and practice fine and gross motor skills like jumping out sight words and practice crossing the midline in ways to make letter sounds STICK! (bubblesacademy.com)
Interestingly, deafferented AN2 dendrites appeared to cross the midline in sema - injected deafferents as they do in non-injected controls (not shown). (bowdoin.edu)