CHAPTER IV. 



THE RHOMBENCEPHALON. 

 SECTION I. THE CEREBELLUM. 



The rhombencephalon is composed of the cerebellum, the pons 

 and the medulla oblongata (Figs. 20, 21 and 27). It is the lozenge- 

 shaped brain. The cerebellum is the dorsal and largest portion 

 of it, and is developed in the roof-plate and dorsal zones of the 

 metencephalon. Together with the pons it forms the hind-brain. 

 Its weight is about five ounces, slightly more than one-tenth of 

 the whole brain. It is situated in the posterior fossa of the skull, 

 under the tentorium cerebelli and dorsal to the pons and medulla 

 oblongata. Between it and the last two structures is enclosed 

 the fourth ventricle. The cerebellum is distinguished from the 

 cerebrum by its stratification. Its surface is composed of gray 

 substance, the cortex (substantia corticalis); its interior is white 

 and is called the medullary body (corpus medullare, Figs. 79 and 81). 



Function. The cerebellum is an important relay in the in- 

 direct motor and indirect sensory paths. In response to impulses 

 received from skin, muscles, tendons, joints and viscera, it is also 

 believed to originate impulses which co-ordinate muscles and 

 maintain equilibrium. Moreover, according to Russel, each 

 cerebellar hemisphere exercises an important inhibitory function, 

 through the brachia conjunctiva, upon the opposite side of the 

 cerebrum. 



Divisions. The cerebellum is made up of two lateral parts, 

 the hemispheres, and a central part, uniting the hemispheres to- 

 gether, called the vermis cerebelli, or worm (Figs. 76, 77, and 80). 

 In the early embryo the cerebellum is a transverse ridge in the 

 roof of the fourth ventricle, partially divided for a time by a median 

 groove on its ventricular surface; and it remains undifferentiated 

 into medial and lateral parts in many lower animals (Edinger). 



The cerebellar hemispheres (hemispheria cerebelli) measure 



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