1096 



HUMAN ANATOMY. 



THE FOURTH VENTRICLE. 



The fourth ventricle (vcntriculus quartus), the persistent and modified hind-brain 

 segment of the primary neural canal, is an irregular triangular space between the 

 pons and the medulla in front, and the inferior cerebellar worm and the superior and 

 inferior medullary vela behind. The lateral boundaries are contributed by the supe- 

 rior and inferior cerebellar peduncles. Its long axis is approximately vertical and 

 about 3 cm. in length, measured from the lower extremity, where the ventricle is 

 directly continuous with the central canal enclosed within the medulla and spinal 

 cord, to the upper end, where it passes into the aqueduct of Sylvius. Its width is 

 greatest (about 2.75 cm.) somewhat below the middle, where this dimension is 

 increased by two lateral recesses, one on each side, that continue the cavity of the 

 ventricle over the restiform body. 



The Floor of the Fourth Ventricle. The floor of the ventricle, really its 

 anterior wall, when viewed from behind after removal of the cerebellum and the 

 medullary vela, appears as a lozenge-shaped area (fossa rhomboidea). The upper half 

 of the floor is formed by the dorsal or ventricular surface of the pons and is bounded 



FIG. 947. 



Sylvian aqueduct 



Superior 



posterior recess 



IV ventricle 



Lateral recess 



Posterior commissure 



Sylvian aqueduct 

 Isthmus 



Superior median sulcus 



Superior lateral sulcus 



Foramen 

 of Luschka 



Superior posterior recesses 



Lower end of ventricle containing foramen of Magendie 



Cast of cavity of fourth ventricle; A, from the side ; , from above. X J. (Retzius.) 



laterally by the upwardly converging superior cerebellar peduncles. The lower half 

 is formed by the ventricular surface of the open part of the medulla and is bounded 

 by the downwardly converging inferior cerebellar peduncles and the clavae. The 

 narrow lower angle of the rhombic area, long known as the calamus scriptorius, 

 corresponds to the interval between the clavae, where the central canal of the cord 

 communicates with the fourth ventricle. The upper angle, situated beneath the 

 superior medullary velum and, therefore, described by some anatomists as belonging 

 to the isthmus of the hind-brain (rhombencephalon), marks the lower end of the 

 Sylvian aqueduct. The length of the rhombic fossa is about 3 cm., and its breadth, 

 greatest at the level of the auditory nerve, is about 2 cm. 



In consequence of the elevation of its lateral boundaries, the floor appears sunken 

 and corresponds approximately with the frontal plane, being almost vertical. It is 

 divided into symmetrical lateral portions by a median groove (sulcus medianus longi- 

 tudinnlis sinus rhomboidnlis), and into an upper and a lower half by transverse mark- 

 ings, the acoustic striae (striae acusticae), which on each side arise from the nuclei of 

 the cochlear nerve, wind over the restiform body and cross the floor of the ventricle 

 to disappear within the median furrow. At its lower end, where it sinks into the 

 central canal of the cord, the median groove becomes somewhat wider, the resulting 

 depression being sometimes designated the vcntriculus Arantii. Roofing in the 

 ventricle at this point and brid^in^ the cleft separating the posterior columns, lies a 

 thin triangular sheet of loose vascular tissue, the obex, which laterally is continuous 



