482 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



fail in the newly born, and are most commonly found, accord- 

 ing to J. Engel, in the bodies of those who have arrived at 

 the adult period of life. For the same reason the crusta of the 

 cerebral peduncle in the newly born presents a grey instead of 

 a white colour. The circumstance of the white medullary sub- 

 stance developing at different periods in the several parts of 

 the child's brain suggests itself as a subject of sufficient im- 

 portance to demand a searching investigation. 



The nucleus common to the vagus and accessory nerves of 

 Stilling and Clarke appears at the lower angle of the fourth 

 ventricle between the median elevation, chiefly produced by 

 the nucleus of the motor hypoglossal nerve, and the internal 

 auditory nucleus of Clarke, and constitutes, according to 

 Deiters, the nucleus of origin of a median and lateral system 

 of mixed nerve roots. In consequence of its dipping at the 

 upper extremity of the calamus scriptorius between the audi- 

 tory nucleus and Stilling's nucleus of the hypoglossal nerve 

 (seen in fig. 257, X 1 , between the lateral and median emi- 

 nence), it gradually vanishes so as to form, when seen from the 

 fourth ventricle, a triangle with the apex above ; and in the 

 same manner the region of the hypoglossal nucleus forms a 

 triangle with the apex downwards, because as it descends it 

 becomes progressively more and more covered by the nucleus 

 common to the vagus and accessory nerves, which comes 

 down to the middle line behind it. (Compare fig. 257 with 

 fig. 258.) 



These morphological relations coincide with the gradual 

 deepening of the fourth ventricle, so as to form a closed cen- 

 tral canal. The nucleus of the vagus becomes exposed, and 

 appears in the form of a mass of grey matter, Arnold's ala 

 cinerea, covered only by the ependyma. The nucleus of the 

 hypoglossus, however, is not thus exposed, but is invested by 

 medullary fasciculi (fig. 257, -3T 4 ), the white substance of 

 which is sharply differentiated from the grey of the ala cinerea, 

 and which belong to the origin of the vagus and accessory 

 nerves. Their centre of origin at the same time (the eminentia 

 teres of Clarke) is superjacent to the hypoglossal nucleus, so 

 that the elevation named medial by Stilling cannot be con- 

 sidered as the hypoglossal nucleus itself, but only as the region 



