( ( ( ( ( ( These junctions ) ( are ( impervious ( to tracer ) ) ) ) , ) so ) ( ( many substances ) ( can ( affect ( ( the metabolism ) ( of ( the eyeball ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT These) (NNS junctions)) (VP (VBP are) (ADJP (JJ impervious) (S (VP (TO to) (VP (VB tracer))))))) (, ,) (IN so) (S (NP (JJ many) (NNS substances)) (VP (MD can) (VP (VB affect) (NP (NP (DT the) (NN metabolism)) (PP (IN of) (NP (DT the) (NN eyeball))))))) (. .)))
These junctions are impervious to tracer, so many substances can affect the metabolism of the eyeball.
(ROOT (S (NP (DT A) (NN tracer)) (VP (VBZ is) (NP (NP (DT the) (NN name)) (PP (IN for) (NP (NP (DT a) (NN substance)) (SBAR (WHNP (WDT that)) (S (VP (MD can) (VP (VB be) (VP (VBN used) (S (VP (TO to) (VP (VB follow) (NP (NP (DT the) (NN pathway)) (PP (IN of) (NP (DT that) (NN substance)))) (PP (IN through) (NP (DT some) (NN structure))))))))))))))) (. .)))