( ( ( ( Spiral arms ) , ) ( ( dust and ) ( the ( interstellar medium ) ) ) ) . )
(ROOT (NP (NP (JJ Spiral) (NNS arms)) (, ,) (NP (NP (NN dust)) (CC and) (NP (DT the) (NN interstellar) (NN medium))) (. .)))
Spiral arms, dust and the interstellar medium.
(ROOT (S (NP (JJ Interstellar) (NN medium)) (VP (VBZ consists) (PP (IN of) (NP (NP (RB thinly) (NN spread) (NN gas)) (CC and) (NP (NN dust))))) (. .)))