( ( A ( gas bubble ) ) ( ( is ( ( injected ( in ( the eye ) ) ) ( to ( ( ( ( place ( gentle pressure ) ) ( on ( the macula ) ) ) and ) ( help ( the ( hole ( to seal ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN gas) (NN bubble)) (VP (VBZ is) (VP (VBN injected) (PP (IN in) (NP (DT the) (NN eye))) (S (VP (TO to) (VP (VP (VB place) (NP (JJ gentle) (NN pressure)) (PP (IN on) (NP (DT the) (NN macula)))) (CC and) (VP (VB help) (NP (DT the) (NN hole) (S (VP (TO to) (VP (VB seal))))))))))) (. .)))
A gas bubble is injected in the eye to place gentle pressure on the macula and help the hole to seal.
(ROOT (S (NP (NNP Gas) (NN bubble) (NNS holes)) (VP (VBZ helps) (NP (NP (JJ pumice) (NN float)) (PP (IN in) (NP (NN water))))) (. .)))