Sentence:  In February , 1959 , during the second admission to The New York Hospital , a biopsy specimen of the left gastrocnemius showed striking increase in the sarcolemmal <WSD>sheath</WSD> nuclei and shrunken muscle fibers in several sections . 
Meaning of the word marked with <WSD> tags:  an enveloping structure or covering enclosing an animal or plant organ or part