Sentence:  If a "hyperdeviation" is present in an eye treated with such a <WSD>myotomy</WSD> of a horizontal rectus, the proximal <WSD>myotomy</WSD> should begin above to shift the insertion downward and make the horizontal rectus more effective as a depressor. 
Meaning of the word marked with <WSD> tags:  The surgical procedure of cutting or making an incision in a muscle or muscular organ.