( ( Light ( ( passes ( into ( the eye ) ) ) ( through ( the pupil ) ) ) ) ( , ( ( the ( choroid coat ) ) ( ( ensures ( that ( ( little ( of ( ( the light ) ( entering ( the eye ) ) ) ) ) ( is ( ( reflected back ) ( so ( ( the pupil ) ( normally ( looks black ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (S (NP (NNP Light)) (VP (VBZ passes) (PP (IN into) (NP (DT the) (NN eye))) (PP (IN through) (NP (DT the) (NN pupil))))) (, ,) (NP (DT the) (JJ choroid) (NN coat)) (VP (VBZ ensures) (SBAR (IN that) (S (NP (NP (RB little)) (PP (IN of) (NP (NP (DT the) (NN light)) (VP (VBG entering) (NP (DT the) (NN eye)))))) (VP (VBZ is) (VP (VBN reflected) (ADVP (RB back)) (SBAR (IN so) (S (NP (DT the) (NN pupil)) (ADVP (RB normally)) (VP (VBZ looks) (ADJP (JJ black)))))))))) (. .)))
Light passes into the eye through the pupil, the choroid coat ensures that little of the light entering the eye is reflected back so the  pupil normally looks black.
(ROOT (S (NP (DT The) (NN pupil)) (VP (VBZ is) (NP (NP (DT the) (NN name)) (PP (IN of) (NP (NP (DT the) (JJ black) (NN opening)) (PP (IN in) (NP (NP (DT the) (NN eye)) (SBAR (WHNP (WDT that)) (S (VP (VBZ lets) (SBAR (S (NP (NN light)) (VP (VBP enter))))))))))))) (. .)))