314 THE RHOMBENCEPHALON. 



The lateral wall (superior triangle) is formed by the brachium 

 conjunctivum of the cerebellum; and (inferior triangle) by the 

 taenia of the fourth ventricle winding across the restiform body, 

 funiculus cuneatus and funiculus gracilis to the obex. The roof 

 is formed by the superior medullary velum (valve of Vieussens) 

 superiorly; and by the inferior medullary velum and roof epithe- 

 lium, inferiorly (Fig. 91). The superior and inferior halves of 

 the roof meet at an acute angle, the fastigium, and form the tent 

 of the fourth ventricle (Fig. 79). On either side, the gable is 

 pushed out over the restiform body and thus is formed the lateral 

 recess. The lateral recess is a tunnel-like extension of the ventric- 

 ular cavity, reaching almost to the posterior lateral sulcus. The 

 recess is bounded, superiorly and ventrally, by the restiform body; 

 dorsally, by the inferior medullary velum; and inferiorly, by the 

 roof epithelium. The chorioid plexuses of the fourth ventricle 

 invaginate the roof epithelium and hang from the roof into the 

 lateral recesses and the inferior part of the cavity (Fig. 91). 



Floor of the Fourth Ventricle. (Fossa rhomboidea). Be- 

 cause it contains the nuclei of one or more roots of the posterior 

 eight (fifth to twelfth) cerebral nerves, the floor of the fourth ven- 

 tricle is a very important area (Figs. 86 and 96). A median groove 

 bounded by the eminentiae mediales forms the long axis of the 

 diamond-shaped floor and divides it into two lateral halves; the 

 lateral halves are bisected transversely by a number of lines, the 

 medullary striae (striae medullares). The striae are produced by 

 bundles of fibers which rise from the cochlear nucleus of the 

 auditory nerve. Diverging somewhat and plunging into the 

 medulla at the median groove, the fibers of the striae enter the 

 opposite trapezoid body and lateral fillet. The medullary striae 

 divide each lateral half of the floor into a superior and an inferior 

 triangle. 



Description to Fig. 96. 



a. Nucleus of olfactory nerves, b. Nucleus of oculomotor nerve, c. Nucleus of trochlear 

 nerve, d. Nucleus of descending root of trigeminus. e. Chief motor nucleus of trigeminus. 

 f. Nucleus of facial, g. Nucleus of abducens. h. Nucleus ambiguus (vagus and glosso- 

 pharyngeus). i. Nucleus of (hypoglossus. j. Nucleus of accessory nerve. Nuclei of optic 

 nerve: k. Pulvinar of thalamus, 1. Lateral geniculate body, m. Nucleus of superior collicu- 

 lus. n. Sensory nucleus of trigeminus. o. Nucleus of vestibular nerve, p. Ventral nucleus 

 of cochlear nerve, q. Lateral nucleus of cochlear nerve, r. Nucleus alaecinereae (vagus and 

 glossopharyngeus) . s. Solitary tract (vagus and glossopharyngeus) . t. Nucleus of spinal 

 tract of trigeminus. 



