FOUETH VENTEICLE. 



513 



lozenge, truncated at its upper part. Below, it is bounded by the 

 diverging posterior pyramids and restiform bodies, surmounted by the 

 ligula. 



The portion below the auditory strife has been already described. 

 Just above the stria3 the eminentia teres forms a convex prominence 

 on each side of the median furrow. To its outer side is a depression, 

 fovea anterior, and above this, near the upper part of the ventricle, is a 

 collection of pigment beneath the surface of a shallow depression, 

 which thus has a greyish tint, and is known as the locus cKrideus. 



The lining membrane of the ventricle is continuous with that of the 

 ventricles in the interior of the cerebrum, through the aqueduct of 

 Sylvius, in which situation it is marked by delicate rugte, oblique or 

 longitudinal in direction. At the sides it is reflected from the medulla 

 to the cerebellum, and extends for a considerable distance outwards, 

 in the form of a pouch, between the flocculus and the seventh and 

 eighth nerves. At the lower end of the ventricle, there is, as was 

 ascertained by Magendie, a narrow orifice in the membrane by which 

 the cavity communicates with the subarachnoid space. 



Projecting into the fourth ventricle at each side, and passing from the 

 point of the inferior vermiform process outwards and upwards to the 

 outer border of the restiform bodies, are two small vascular processes, 

 which have been named the chloroid plexuses of the fourth ventricle. 



The grey matter in the floor of the fourth ventricle has been 

 in part described in connection with the medulla oblongata. 



Opposite the middle of the fourth ventricle, beneath the eminentia 

 teres on each side, lies the common nucleus of the sixth and seventh 

 {facial) nerves (fig. 359, vii'). These nerves to reach it pass upwards as 



Fig. 359. 



Fig. 359. — Section through the Pons Varolii, opposite middle op Eminentia 



Teres. 



VI, sixth nerve ; vii, facial neiTe ; vii', common nucleus of seventh and sixth nerves j 

 vii", descending portion of seventh; y s, sensory nucleus of fifth nerve; \ m, motor 

 nucleus of fifth nerve (Lockhart Clarke); au, auditory nucleus; s o, superior olivary 

 body; t, transverse fibres of "trapezium; ' below these are seen the cut bundles of 

 fibres from the anterior pyramids. C P. m, middle peduncle of cerebellum cut across. 

 M, superficial transverse fibres. 



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