( ( 1The ( first operation ) ) ( ( involved ( ( ( putting ( a ( gas bubble ) ) ) ( in his1eye ) ) ( to ( help ( ( close ( the holes ) ) ( in ( his retina ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ 1The) (JJ first) (NN operation)) (VP (VBD involved) (S (VP (VBG putting) (NP (DT a) (NN gas) (NN bubble)) (PP (IN in) (NP (CD his1eye))) (S (VP (TO to) (VP (VB help) (VP (VB close) (NP (DT the) (NNS holes)) (PP (IN in) (NP (PRP$ his) (NN retina)))))))))) (. .)))
1The first operation involved putting a gas bubble in his1eye to help close the holes in his retina.
(ROOT (S (NP (NNP Gas) (NN bubble) (NNS holes)) (VP (VBZ helps) (NP (NP (JJ pumice) (NN float)) (PP (IN in) (NP (NN water))))) (. .)))