Henry Gray (18251861). Anatomy of the Human Body. 1918.
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.
The rhomboid fossa is divided into symmetrical halves by a median sulcus which reaches from the upper to the lower angles of the fossa and is deeper below than above. On either side of this sulcus is an elevation, the medial eminence, bounded laterally by a sulcus, the sulcus limitans. In the superior part of the fossa the medial eminence has a width equal to that of the corresponding half of the fossa, but opposite the superior fovea it forms an elongated swelling, the colliculus facialis, which overlies the nucleus of the abducent nerve, and is, in part at least, produced by the ascending portion of the root of the facial nerve. In the inferior part of the fossa the medial eminence assumes the form of a triangular area, the trigonum hypoglossi. When examined under water with a lens this trigone is seen to consist of a medial and a lateral area separated by a series of oblique furrows; the medial area corresponds with the upper part of the nucleus of the hypoglossal nerve, the lateral with a small nucleus, the nucleus intercalatus.
The sulcus limitans forms the lateral boundary of the medial eminence. In the superior part of the rhomboid fossa it corresponds with the lateral limit of the