868 



THE NERVE SYSTEM 



gata, the dorsal wall hypertrophies in its cephalic portion to form the cerebellum, 

 while caudad thereof the roof atrophies and expands and becomes so attenuated 

 as to be represented merely by a thin epithelial membrane. The outward folding 

 of the walls of the neural tube in this region creates an expansion of the central 

 cavity in the form of a rhomboidal fossa roofed in by the cerebellum and a thin 

 epithelial layer. A time-honored custom enumerates this as the fourth of a system 

 of ventricles of which the other three lie in the fore-brain. 



A cast of the cavity (Fig. 689) shows it to be irregularly pyramidal, with a 

 lozenge-shaped base and ridge-like apex extending from side to side, corresponding 

 to the acute-angled recessus tecti in the fastigium ("gable roof") formed by the 

 valve of Vieussens and superior medullary velum. Such a cast also indicates 

 the ventral extension of the cavity from the lateral angles of the rhomboidal 

 base in the form of the lateral recesses. 



It is customary to describe for the fourth ventricle a roof and a floor, although an 

 examination of a sagittal section of a brain hardened in situ shows the floor to be 

 in a vertical plane in the erect attitude. Caudad the cavity is continuous with 

 the minute central canal of the spinal cord and postoblongata (in part); cephalad 

 it passes into the aqueduct or mesocele. The dorsal wall or "roof" is formed 



FIG. 633. Varieties of fourth ventricle. 



by the valvula (of Vieussens}, the superior peduncles, tela choroidea ventriculi 

 quarti, and fastigium of the cerebellum. The ventral wall or "floor" is the rhom- 

 boidal fossa occupied by the expanded central gray of the pre- and postoblongatal 

 portions of the hind-brain. 



"Floor" of the Fourth Ventricle (fossa rhomboidea). The "floor" of the fourth 

 ventricle is lozenge-shaped and exhibits regional elevations, depressions, and 

 color differences which are in relation with the deep anatomy of the medulla ob- 

 longata and tegmentum of the pons (preoblongata). It is divided longitudinally 

 into symmetrical halves by a median groove, and each lateral half is subdivided 

 into a larger cephalic and a smaller caudal triangle by white, transverse striae, 

 composed of bundles of myelinic fibres connected with the acoustic tract and ap- 

 pearing to sink beneath the surface near the median groove. The portion occupied 

 by these striae acusticae (striae medullares; striae transver sales) is termed by His 

 the pars intermedia as distinguished from the pars superior and pars inferior, or frontal 

 and caudal triangles respectively. Much variation is met with in regard to the 

 course and degree of prominence of the striae acusticae (Fig. 633). There may 

 be none visible or as many as twelve distinct -bundles; bilateral symmetry is the 

 exception, and not infrequently one or more bundles run obliquely cephalolaterad 



