480 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



only so far as a crossed conduction can be definitively admitted 

 for the tract of the crusta, and] as it can be shown that this 

 decussation is effected by the straight fibres of the raphe, can 

 we be certain that the columnar portions in question constitute 

 the innermost fasciculi of the anterior longitudinal fasciculi of the 

 pons and of the pyramids before their entrance into the raphe, 

 .and were perhaps the same ever since their descent in the cere- 

 bral peduncle (p. 419, fillet or loop of the cerebral peduncle). 



But inasmuch as for reflex actions a crossed arrangement 

 cannot be completely taken for granted, this argument is wanting 

 to determine the localization of the columns of the cerebral 

 nerves within the area of the posterior division of the pons and 

 medulla oblongata, and the view of Schroder v. d. Kolk, that the 

 actions in question are effected by the sectional area situated 

 between the raphe' and the hypoglossus, obtains no support 

 from facts, except in the undeniable diminution of this area 

 downwards. We know, however, the place better than the 

 mode of the termination of these portions of the crus cerebri, 

 the former being indicated by the origins of the cerebral 

 nerves from the same spots. 



A great part of these so-called nerve nuclei, the relations of 

 which to the roots Stilling did good service in discovering, 

 belong to the grey floor of the fourth ventricle, which, in con- 

 sequence of their projections and intervening furrows, acquires 

 a well-marked division into areas. This rhomboidal space 

 (fourth ventricle) formed by the convergence of the superior 

 peduncles above, and the fasciculi graciles below, is divided by 

 the median furrow, (the calamus scriptorius,) into two symme- 

 trical lateral halves, and by the strice medullares (or if these 

 fail, by an imaginary line connecting the two auditory nerves) 

 into an upper and a lower half. 



The superior angle of the fourth ventricle, as it gradually 

 expands, exhibits, in addition to the eminentise teretes belonging 

 to the aquseductus, a furrow extending laterally as a sharp angle 

 between the floor and the lateral wall, through the ependyma 

 of which the dark cells of the substantia ferruginea (figs. 250 

 and 252, F, 5) glimmer of a bluish colour, in accordance with 

 the laws of refraction of light through cloudy media. This is 

 the fossa ccerulea. As the pigmented cell clusters belong to the 



