CRANIAL CAVITY. 



Floor. This is a lozenge-shaped area, covered by central 

 gray matter and an ependymal layer, the upper half formed 

 by the pons and the lower by the medulla : its lower end tapers 

 (calamus scriptorius) between the clavse. A median groove 

 bisects it; each lateral half is divided into upper and lower 

 triangular portions by transverse fibres (striae acusticce) run- 

 ning from the mesial groove across the upper part of the resti- 

 form body. Lower portion: Each is divided into three areas 

 by grooves prolonging the sides of a triangular depression (in- 

 ferior fovea) lying, apex upward, just below the striae. These 

 areas are mesial (trigonum hypoglossi), inferior (trigonum 

 vagi), and external (trigonum acustici); the latter has an 

 eminence (acustic area) at its base. Upper portion: Along- 

 side the median groove is a ridge (eminentia teres) continuous 

 with the trigonum hypoglossi; outside this, just above the 

 striae, is a slight depression (fovea superior) whence a shallow 

 grayish depression (locus coeruleus) extends toward the aque- 

 duct; outside of and below the fovea is an elevated extension 

 of the area acustica. [487] 



Roof. The upper part is formed by the superior medullary 

 velum and, slightly, superior peduncles. The lower part is 

 formed above by the inferior medullary velum and below by 

 ependyma supported on pia. A thin white lamina passes a 

 short distance over the ependyma at (obex) the calamus 

 scriptorius, and also (ligula) along each lateral boundary; it 

 runs upward from the clava over the cuneate tubercle and 

 then outward over the restiform body, bounding the lateral 

 recess below and sometimes becoming continuous with the in- 

 ferior medullary velum. An orifice (foramen of Magendie) 

 pierces the epithelial and pial roofs mesially, just above the 

 calamus scriptorius ; another is found at the tip of each lateral 

 recess. [511] 



The choroid plexuses are vascular infoldings of pia which 

 invaginate the epithelial roof, one on each side, along the mid- 

 line, and project into the cavity ; an offshoot is prolonged into 



[29] 



