462 THE TISSUES. 



ventricle as the posterior pyramids, and being thence continued on 

 the floor of the ventricle, side by side, into the tegmentum of the 

 crura cerebri. The posterior columns (Fig. 307) in part constitute 

 the corpora restiformia, and finally enter the crura cerebelli; while 

 the remainder, situated externally to the posterior pyramids, also 

 enters the tegmentum of the crura cerebri. There is also a system of 

 horizontal nerve-fibres which are independent of the cord, and which 

 are probably commissural. 



2. The gray matter of the medulla oblongata is collected into larger 

 masses principally in three situations ; viz., in the olivary and the 

 restiform bodies, and in the floor of the fourth ventricle. 1. The 

 gray matter of the olivary body constitutes a capsule, closed on all 

 sides except the inner; and is entirely isolated from all other gray 

 substance. It is traversed by very numerous nerve-fibres of the 

 horizontal system. 2. The gray matter of the restiform bodies may be 

 regarded as a continuation of the posterior horns of the spinal cord, 

 and even presents some resemblance to their substantia gelatinosa. 

 (Stilling.) 3. The gray substance of the floor of the fourth ventricle 

 is a continuation of the central gray matter of the cord, and forms 

 a tolerably thick layer from the calamus scriptorius to the aqve- 

 ductus Sylvii. The portion in the anterior half of this ventricle 

 belongs properly to the pons Yarolii. Besides these three masses of 

 gray matter, there are other very small ones in the medulla oblon- 

 gata, not requiring a description here. In no case are the nerve- 

 fibres (the horizontal, or those from the cord) known to be conti- 

 nuous with the processes of the nerve-cells. 



Do the ten pairs of encephalic nerves, mentioned on page 461, 

 rise from the gray matter of the medulla oblongata to which they 

 have been mostly traced by Stilling and others? We deem it most 

 probable that they rise in the corpora striata and optic thalami, for 

 reasons assigned by Kolliker. 1 That the gray matter, however, 

 influences the nerve-fibres which traverse it, is at the same time 

 most probable. 



2. The Cerebellum. 



The gray matter of the cerebellum occurs only on the surface of 

 the convolutions, in the nucleus dentatus, and the roof of the fourth 

 ventricle; all the rest being white substance. The latter consists 

 of parallel nerve-fibres, presenting all the characters of central 



1 Pp. 377-8. 



