Complex sentence: The great importance of intorsion produced by the two oblique muscles can only be understood when considered with regards to the other muscle actions present: the two obliques prevent the eye from rotating about its long (horizontal) axis when the superior and inferior rectus muscles contract.
Split into simpler sentences: The great importance of the action of intorsion produced by the two oblique muscles can only be understood when it is considered with regards to the other muscle actions present. The two oblique muscles prevent the eye from rotating about its long (horizontal) axis when the superior and inferior rectus muscles contract.