THE NERVOUS SYSTEM 155 



fissure is opened out, so that the central canal comes to the 

 surface, and expands into a lozenge-shaped area the floor 

 of the fourth ventricle. The lateral columns of the cord 

 pass outwards to the cerebellum to form part of its inferior 

 peduncles the restiform bodies. Between the lateral and 

 the anterior columns an almond-shaped swelling, the olive, 

 appears (Fig. 84, 0.). Above this the medulla is encircled 

 by a mass of transverse fibres the middle peduncles of 

 the cerebellum, or the pons Varolii (Fig. 86, P.). The 

 floor of the fourth ventricle is constricted above by the 

 approximation of the superior peduncles of the cerebellum 

 to again become a canal. 



The grey matter of the cord gets broken up into separate 

 masses, of which the most important are : 



1. The nuclei of the posterior columns the nucleus gracilis 

 and nucleus cuneatus (Fig. 83, N.C. and N.G.) masses of 

 cells and synapses in which the fibres of the posterior 

 columns end, and from which the upgoing fibres of the fillet 

 start. 



2. The inferior olivary nucleus (Fig. 84, 0.), which lies in 

 the olive, and which is connected by bands of fibres with the 

 dentate nucleus of the cerebellum (Fig. 86, Deit.). 



3. The nucleus of Deiters (Fig. 86, Deit.), lying higher 

 up in the pons Varolii, and connected with fibres from 

 the cerebellum and from the semicircular canals (see 

 Fig. 73). 



4. The masses of cells from which the cranial nerves take 

 origin (Fig. 85). 



2. Conducting Paths. 



A. Ingoing. 1. The posterior columns of the spinal cord 

 terminate in two masses of grey matter on each side, the 

 nucleus gracilis and nucleus cuneatus. From these, fibres 

 pass downwards and across the middle line forming the 

 decussation of the fillet (Fig. 83, F.). The crossed fibres 

 (Fig. 84, F.) then pass up in a vertical series on each side 

 of the middle line until the pons Varolii is reached, when 

 they spread out horizontally like a fan (Fig. 86, F.) above 

 the deep transverse fibres. Above the pons they divide 

 into two sets (Fig. 90, F.) a lateral fillet, which ends in 



