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CEREBELLUM. 93 



which they form the transverse fibres, uniting with the 

 fibres of the cms of the opposite side. The inferior pedun- 

 cle, or processus ad medullam, passes downward to the 

 medulla oblongata and forms part of the restiform body ; 

 it is also the inferior portion of the lateral boundary of the 

 fourth ventricle. 



The FOURTH VENTRICLE is a space bet ween the cerebellum 

 nd the posterior aspect of the medulla oblongata and pons 



roliij it has the shape of a lozenge, the points of which 

 directed upward and downward, the upper extending 

 as high as the superior border of the pons Yarolii, and the 

 wer to a level with the inferior part of the olivary body. 



e lower half of this space has been called the calamus 

 riptorius, from a fancied resemblance to the tip of a writ- 

 ing pen ; its lateral boundaries are formed by the superior 

 peduncles of the cerebellum above, and by the restiform 

 bodies below ; its roof is formed by the valve of Yieussens 

 and the sicles of the peduncles with which it is connected, 

 and by the under part of the superior vermiform process ; 

 its floor corresponds to the posterior surfaces of the medulla 

 and pons Yarolii. Along the centre of its floor is a median 

 groove, ending at the point of the calamus, and on each side 

 of the groove a fusiform elevation called the eminentia teres. 

 The eminentia teres is crossed by white streaks, varying 

 in their arrangement and not always recognizable, called 

 linese transversse. The fourth ventricle communicates with 

 the third ventricle through the Her a tertio ad quartum 

 rrnfriculum, and by this canal the lining membrane of the 

 other ventricles is prolonged into the fourth. 



The arachnoid and pia mater stretch across between the 

 medulla oblongata and cerebellum, at the lower part of the 

 ventricle, under the name of the valve of the arachnoid. 

 On the inner surface of these membranes is a vascular fold, 

 called the choroid plexus of the fourth ventricle. The ven- 

 tricles communicate with the sub-arachnoid space of the 

 encephalon and spinal cord, through an aperture in this 

 membrane just above the calamus scriptorius. 



A vertical section of the cerebellum shows that the 

 laminae of its hemispheres are composed of an external 

 layer of gray matter and an internal layer of white medul- 

 lary substance, the white part being derived from a large 

 medullary centre in the interior of each hemisphere, which 

 in one direction gives rise to the peduncles of the cerebel- 

 lum, and in the other, diverging like the branches of a tree, 



