The anterior part of the fourth ventricle is named, from its shape, the rhomboid fossa, and its anterior wall, formed by the back of the pons and medulla oblongata, constitutes the floor of the fourth ventricle. It is covered by a thin layer of gray substance continuous with that of the medulla spinalis; superficial to this is a thin lamina of neuroglia which constitutes the ependyma of the ventricle and supports a layer of ciliated epithelium. The fossa consists of three parts, superior, intermediate, and inferior. The superior part is triangular in shape and limited laterally by the superior cerebellar peduncle; its apex, directed upward, is continuous with the cerebral aqueduct; its base it represented by an imaginary line at the level of the upper ends of the superior foveæ. The intermediate part extends from this level to that of the horizontal portions of the tæniæ of the ventricle; it is narrow above where it is limited laterally by the middle peduncle, but widens below and is prolonged into the lateral recesses of the ventricle. The inferior part is triangular, and its downwardly directed apex, named the calamus scriptorius, is continuous with the central canal of the closed part of the medulla oblongata.(

This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from


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