ORGANS OF THE BODY. 585 



296. 



Let us now endeavour, as far as the scope of our manual will permit, to 

 clear up some of the difficulties connected with this most complicated 

 arrangement of parts in the medulla oblongata. 



And first, as regards the grey substance. 



In the upper portions of the spinal cord we may remark, in transverse 

 sections, that at the external angle of contact of the two cornua, at one 

 particular spot, the grey matter is prolonged outwards into a pointed 

 process. This has been named by Jacubowitsch the lateral accessory 

 cornti, and by Clarke and Dean the tractus intermedia -later alls. It 

 attains, in its passage into the medulla oblongata, a higher degree of 

 development, and assumes, at the same time, a decidedly retiform appear- 

 ance, bands of fibres from the lateral columns occupying its meshes. We 

 shall see later on what great importance this lateral region of the medulla 

 oblongata possesses, being, as it is, the origin of a special system of nerves 

 commencing with the spinal accessory. 



This is the beginning of the fonaatio reticularis. 



Advancing further through the medulla oblongata towards the brain, 

 we find this reticulated and banded mass encroaching more and more 

 upon the anterior and base of the posterior cornu. This change gradually 

 advances so far that the whole of the upper portion of the medulla may 

 be regarded as a mesh work of grey matter, traversed by bundles of white 

 nerve fibres. The grey substance is, in fact, spread out almost to the 

 periphery, and is connected with the grey nuclei situated there. From 

 the fact, however, that the most internal portions of grey matter, i.e., 

 those originally surrounding the axis canal, remain for the most part 

 unchanged, they may present an appearance liable to deceive, namely, as 

 though they alone were the prolongations of the cornua of the spinal cord. 



That in this very extensive band work of grey matter, as well as in the 

 nuclei, we should encounter ganglion cells of the greatest variety of form, 

 and in certain cases of remarkable size, with primary and secondary axis 

 cylinders, will excite no wonder. And that the tracts of this grey reti- 

 culum should also give origin to part of the cranial nerves might likewise 

 be expected. 



The reader will also understand very easily, from the foregoing descrip- 

 tion, that the posterior cornu suffers the most displacement from the 

 opening out of the central canal, and that it must now lie much more to 

 the side than formerly. 



We have already mentioned, in the preceding section, the separation of 

 the posterior band of Gall, and its transformation into ihefuniculnsgracilis. 



In it likewise, and about it, the grey reticulated mass becomes more and 

 more expanded, pressing down the remainder of the posterior column. Thus 

 the fourth ventricle obtains a lining of grey matter almost over its entire 

 floor. The suste'ntacular substance, also formed more exclusively of connec- 

 tive-tissue, which surrounded the central canal, experiences here a great 

 increase in quantity likewise, and plays, later on, an important part in the 

 fromation of the aquceductus Silvii, the third ventricle, and infundibulum. 



Leaving for the present the grey matter of the medulla oblongata, let 

 us now take a preliminary survey of another very important series of 

 parts, let us inquire into the origin of the ten cranial nerves. 



A most important discovery was made in regard to these by Deiters. 

 Besides the two modes of origin, corresponding to the anterior and pos- 



