524 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



of the cerebellar peduncle, progressively break up below the 

 point where the floor of the fourth ventricle is of the greatest 

 width (fig. 257) into the fibrse arcuatse, which, after traversing 

 the olivary bodies (p. 470 et seq.), reappear upon the opposite 

 side, situated internally to the restiform bodies, in the form of 

 the slender and cuneate fasciculi (posterior column), the resti- 

 form body must evidently, in consequence of the passage of 

 its fasciculi into the opposite posterior' column, progressively 

 diminish in size, whilst the medullary masses bounding the 

 posterior column on the inner side augment in the same pro- 

 portion. But if in a given area a certain amount is con- 

 tinuously detached from one side, and equally continuously 

 applied to the opposite, this area must be transposed in the 

 direction of the additions it receives. The transverse section 

 of the cerebellar peduncle diminishing in bulk externally, and 

 augmenting internally, must consequently approximate the 

 median line. As a result of this, it must press upon, and 

 displace inwards, the external column forming the sensory 

 division of the grey floor of the fourth ventricle bounding its 

 internal portion, which appears above as the auditory, but 

 below as the vagal, nucleus. Thus it will progressively deepen 

 the floor of the fourth ventricle ; and at a lower plane these 

 grey masses, instead of being external to the hypoglossal 

 nucleus (fig. 257), become posterior to it (fig. 258); and, being 

 continuously pressed inwards, firstly, the inner surfaces of what 

 are now called the posterior columns of the grey substance, 

 and subsequently the inner surfaces of the posterior columns, 

 continuously increasing upon their inner side, will at length 

 come into contact. This contact of the symmetrically arranged 

 grey matter leads to fusion with inclusion of tlie central 

 canal. 



Between the two posterior columns, however, the posterior 

 fissure of the medulla oblongata is formed. And, in this respect, 

 in this direction it approximates to the form of the spinal cord 

 (fig. 258, 6'). This approximation appears far more remarkable if 

 at the same time it be considered that the form of the grey 

 nucleus of the spinal cord, apart from the bilateral disposition 

 of its masses, already pre-exists in the medulla oblongata. 



The median nu'clei of the hypoglossal nerves, unaffected by 



