Dorsal perspective of the inferior fourth ventricle. For orientation, the bottom of this image is ventral, and the cerebellar tonsils and cerebellum are reflected dorsally. The tela choroidea and the inferior medullary velum comprise the inferior portion of the roof of the fourth ventricle. The inferior medullary velum has been retracted dorsally in this image to reveal the lateral recess of the fourth ventricle on the left side of the image. The prominence of the dorsal cochlear nucleus is evident within the lateral recess. The posterior inferior cerebellar arteries course around the lateral medulla into the tonsillomedullary segments, giving rise to medullary perforating arteries before supplying the posterior inferior cerebellum. (Image courtesy of AL Rhoton, Jr.)