120 THE POCKET ANATOMY 



tract which join the restiform body, whilst the rest of the 

 column passes upwards as far as the lower end of the olive 

 which conceals it. 



The olivary body is an oval prominence lying in the upper 

 part of the medulla, between the pyramid and the restiform 

 body. It is separated from the pons by a groove, which con- 

 tains some of the external arcuate fibres. 



On the inner side lie the nerve-roots of the hypoglossal ; 

 and on the outer side, but separated from it by a groove, the 

 roots of the accessory, vagus, and glosso-pharyngeal nerves 

 issue. Longitudinal fibres, prolonged from the lateral 

 column, lie in the groove on the outer side of the olive, 

 and occasionally a few bundles of longitudinal fibres are seen 

 on the inner side. These, together with the arched fibres 

 above and below, give it the appearance of being enclosed in 

 a capsule. 



The fillet of the olivary body is a small bundle of fibres derived 

 from the capsule, and from the longitudinal fibres from the 

 lateral tract. The fibres ascend in the crus cerebri. 



Anterior area of the medulla. Most of the fibres of the 

 anterior columns of the cord are prolonged upwards deeply 

 beneath the pyramids, but a bundle varying in size passes 

 upwards on the outer side of the crossed portion of the lateral 

 column (crossed pyramidal tract). These fibres form the 

 direct Pyramidal tract. 



The pyramids are two oval prominences, broader above 

 than below. They are placed one on each side of the anterior 

 median fissure. They consist of two sets of fibres, the outer 

 derived from the anterior column of the cord, the inner from 

 the lateral column of the opposite side. The crossing of the 

 two sets of these latter fibres constitutes the decussation of the 

 Pyramids. 



The fourth ventricle. The central canal of the cord ex- 

 pands in the upper and posterior part of the medulla until 

 opposite the middle peduncles of the cerebellum, where it 

 again gradually narrows, and becomes continuous with the 

 Sylvian aqueduct above. The floor or anterior (ventral) wall 

 of the ventricle is thus diamond-shaped. The lower end has 

 been compared in shape to a pen, and is hence termed the 

 calamus scriptorius. 



The floor in the lower half is formed by the upper part of 

 the posterior surface of the medulla, and in the upper half by 

 the back of pons Varolii. 



