ぼそぼそ is a mimetic for dry, crumbly, powdery texture — here applied to bread that's gone stale. The construction ~ながらの converts a simultaneous action into an attributive modifier: 'a journey of chewing bread' rather than 'chewing bread while on a journey.' 大蛇 (literally 'large snake') could be read おろち in mythology or だいじゃ in everyday use. Subject omitted throughout.