THE FOURTH VENTRICLE 311 



the postgracile fissure separates the former two lobes, which 

 are called sometimes the posterior inferior lobules; they com- 

 prise at least two-thirds of the inferior surface of the cerebellum. 

 The bi ventral, tonsilla (amygdala), and flocculus lobes are 

 found on the inferior surface of the cerebellar hemispheres. 



The ventral notch is in relation with the brain stem (teg- 

 mental part of the pons and corpora quadrigemina) ; the 

 dorsal notch is smaller and narrower and lodges the falx cere- 

 belli, which separates the hemispheres as they project beyond 

 the inferior vermis. 



The arbor vita? is the name given to the arrangement of the 

 white substance of the cerebellum, seen on a median section. 

 The cerebellum weighs 5.8 ounces in the male, and 5.4 ounces 

 in the female. The proportion between the cerebellum and 

 cerebrum is 1 to 7.5 in the adult; 1 to 8.5 among eminent men; 

 1 to 20 in the newborn. (Gray.) 



SUMMARY OF THE GRAY MASSES IN THE CEREBELLUM 



Embedded in the white substance are the following four 

 on each side: 



Dentate nucleus or dentatum. 

 Nucleus emboliformis or embolus. 

 Nucleus globosus or globulus. 

 Nucleus fastigi or fastigium. (Gray.) 



The Fourth Ventricle 



The fourth ventricle (or fossa rhomboidea) is an irregularly 

 pyramidal shaped cavity, with a lozenge-shaped base and 

 ridge-like apex; found between the medulla oblongata and 

 the pars dorsalis pontis in front and the cerebellum behind. 

 It is divided into a roof and a floor. The roof is formed by 

 the valvula (of Vieussens), the superior peduncles, tela choroidea, 

 ventricula quarti, and fastigium of the cerebellum. The floor 

 is divided into a larger cephalic (pars superior), and a 

 smaller caudal (pars inferior) triangle by white, transverse 

 striae, the striae acusticae, termed the pars intermedia by His. 

 The pars superior is separated into two halves by a longitudinal 

 groove, the fovea mediana. On each side of the fovea mediana, 



