Sentence:  One simple method of measuring the expansion of the heart is to tie a thin <WSD>rubber</WSD> tube , filled with mercury , around the heart and record the change in resistance as the tube is stretched . 
Meaning of the word marked with <WSD> tags:  an elastic material obtained from the latex sap of trees (especially trees of the genera Hevea and Ficus) that can be vulcanized and finished into a variety of products