ORIGIN OF THE CEREBRAL NERVES. 



483 



in its vicinity. The nuclei common to the vagus and acces- 

 sory nerves are connected below in the form of a horse-shoe 

 curving round the bolt commissure (Biegels-Obex) (Clarke, 

 Deiters) (fig. 258, Ob), the fasciculi of which cause the external 

 border of the lower half of this nucleus to project, and appear 

 in the fourth ventricle, though more constantly in Mammals 

 than in Man.* 



The obex is concentrically surrounded by the adhering border of 

 the embryonic roof of the fourth ventricle, the taaniola (Kiemchen), 

 which proceeds from the substance of the slender column. The roof 

 of the fourth ventricle continuous with it, investing the auditory nerve, 

 cuts off two lateral diverticula of the fourth ventricle, which, according 

 to Reichert, are the analogues of the lateral ventricles. 



The fourth ventricle of Mammals (excluding Monkeys), on account 

 of the absence of the striae and the slighter projection inwards of the 

 auditory nucleus, renders the region of origin of the facial and hypo- 

 glossal nerves far more evidently a continuation of the median motor 

 column (Lenhossek) than in Man. Moreover their shallow slightly 

 excavated fourth ventricle wants the apex of the calamus scriptorius, 

 since the lower extremity of the hypoglossal nucleus, in consequence 

 of the insignificant alaa cinereae, remains broader, and has a curved 

 boundary in the transverse projection of the obex. 



The posterior division of the pons contains below its greatest 

 convexity, (at those planes, namely, which are defined poste- 

 riorly by the superior peduncle no longer covered by the loop, 

 and which lie anteriorly below the emergence of the fifth be- 

 tween the transverse fasciculi of the middle peduncle), one after 

 the other, the origin of the fifth, sixth, and seventh, and lastly 

 of the eighth pair of cerebral nerves. 



The small root of the fifth arises from the uppermost of 

 these origins, the superior trigeminal nucleus of Stilling, which 

 forms, in the lateral regions of the motor area in front of the 

 descending roots of the sensory portion of the fifth, internally 

 to its point of emergence, and behind the fillet, an oblong 



* For a fuller account and representation of the obex (Clarke's slender 

 column connected with the vagus and spinal accessory), see Clarke's 

 Researches on the Brain, in Phil. Trans., 1868, Part i., pp. 274, 275, and 

 276. Plate x., figs. 25, 27, 30. 



i i 2 



