2IO Journal of Comparative Neurology, 



caudal face of the lobus vagi medullated fibres gather and 

 constitute a spinal portion of the fasciculus communis; 

 some of these cross at once to the opposite side dorsally 

 of the fourth ventricle in small irregular bundles. Others 

 continue caudad, where they are joined by additional 

 fibres arising farther laterally, probably from the dorsal 

 cornu, and just caudad of the lobus vagi they participate 

 in the formation of a compact commissure or decussation. 

 All of the transverse fibres above described belong to the 

 commissura infima Halleri, which in Menidia is but feebly 

 developed. 



From this place, on each side, a close round bundle of 

 medullated fibres extends caudad to the level of the first 

 spinal root, where it breaks up and disappears in an area 

 of loose reticular tissue which contains few medullated 

 fibres and many rather small pale cells with scattered 

 larger deeply stained multipolar cells and which lies ven- 

 trally of the dorsal horn and dorsally and laterally of the 

 canalis centralis. This region is both in Weigert and 

 Nissl preparations very clearly distinguished structurally 

 from the dorsal horns, having a more open reticular struc- 

 ture and larger rather numerous cells. This structure is 

 characteristic of the corresponding region, /. e. , the area 

 adjacent to the floor of the fourth ventricle, dorsally of the 

 fasciculus longitudinalis dorsalis, far cephalad under the 

 lobus vagi, and here it contains, besides the medium sized 

 cells mentioned, the very large cells of the nucleus am- 

 biguus. These latter cells extend nearly to the caudal 

 end of the commissura infima (683), where they terminate 

 abruptly. 



For this nucleus of the spinal cord laterally of the 

 canalis centralis I shall adopt the name ' ' paracentral 

 nucleus" from Onuf and Collins ('98). The nucleus am- 

 biguus seems to be a specialized cranial portion of the 

 paracentral nucleus. 



At the level of the first spinal nerve this reticular area 

 of the paracentral nucleus has spread out laterally and 

 now occupies the region dorsally and laterally of the can- 

 alis centralis, the "intermediate zone" of Onuf and Col- 



