- Kingsbury, Brain of Nectiinis. 175 



borhood of the seventh nerve, and cephalad tract a soon re- 

 gains its dorsal position. SHghtly cephalad of the fifth nerve it 

 disappears, the fibers which compose it turning mesad into the 

 neighboring cinerea. This tract does not seem to be increased 

 to any extent from arcuate fibers. 



Tract b likewise first appears in the region of the tenth 

 nerve reaching its strongest development between the seventh 

 and ninth. It lies latero-dorsad of the fasciculus communis. Be- 

 tween the ninth and tenth the tract is largely increased by arcuate 

 fibers which turn cephalad in it. The relation of arcuate 

 fibers to tract b continues throughout its length, but is most 

 marked between the ninth and X\ Most of the bundles which 

 enter it turn cephalad ; others, however, in the region of the 

 ninth nerve, turn caudad ; this is also the case in the region of 

 the fifth. 



Between tracts a and b, the alba is almost entirely amyel- 

 inic. Small fascicles of myelinic fibers occur, however, in it ; 

 these seem to be to a large extent arcuate fibers which pene- 

 trate this region. Fibers also pass into this intermediate re- 

 gion from tracts a and b. Into it enter the dorsal division of 

 the ninth and the ventral portion of the "dorsal seventh" 

 (IX' and IX^ and Vllb"). 



Arcuate fibers. — Arcuate bundles are well developed in 

 Necturus. In the myel fine myelinic fibers pass down from 

 the dorsal cinerea to decussate in the raphe, assuming a longi- 

 tudinal direction in the ventral columns. In the re- 

 gion of the 2nd spinal nerve and cephalad these fibers are 

 greatly increased, in number. Up to the exit of X' the num- 

 ber of fibers is diminished ; cephalad of that point, however, 

 they begin to pass to the dorsal tracts, traversing the alba at all 

 levels, not confined to the border of the cinerea. Between the 

 ninth and X' they largely take part in the formation of tract b, 

 in which they turn both cephalad and caudad. The intermedi- 

 ate region between tracts a and b also receives many of them. 

 In the region of the seventh large bundles of arcuate fibers pass 

 dorsad to terminate in the lateral border of the cinerea adjacent 

 to the dorsal tracts, and the dorsal island of alba into which the 



