Anatomy of the Brain of Bdellostoma Dombeyi 9 



nucleus. We will now describe the outgoing fiber tracts. First are the 

 fibers that cross in the ventral raphe. These fibers come from all parts 

 of the nucleus, run ventro-mesad to its ventro-mesial angle, and cross 

 to the other side of the brain in the raphe without forming a distinct 

 bundle (Figs. 26 and 30). All of the fibers of the raphe distribute 

 themselves in the ventral part of the brain of the opposite side, and it 

 is impossible to differentiate those derived from the acusticum from 

 the fibers of a different origin. All that can be said is that by this 

 means a motor connection is established with the opposite side of the 

 brain. A motor connection is also established between the acusticus 

 nucleus and its own side of the brain by means of a large tract that 

 forms about the level of the entering auditory fibers, and crosses the 

 raphe to run directly to the ventral motor tract of its own side of the 

 brain (Figs. 1, 2 and 26). Arrived there, many of its fibers turn 

 caudad, running with the other fibers of this tract. How far they 

 continue cannot at present be stated. Individual cells, both large and 

 small, taking part in the formation of this tract have already been 

 mentioned, and are shown in Figs. 33, 35, 36 and 39. 



Another important outgoing tract may be called the tr. acustico- 

 funiculi. This tract is forked in the acusticum. The dorsal fork arises 

 at the level of the entering auditory fibers and lies on the lateral border 

 of the acusticum. It receives many fibers from the interior of the 

 nucleus, fibers that run laterad or caudo-laterad until they reach this 

 tract, but which, upon reaching it, turn caudad and form part of it. 

 Toward the hind part of the acusticum this division joins the ventral 

 fork of the tr. acustico- funiculi. This ventral division arises in the 

 extreme cephalic part of the nucleus and runs caudad along its lateral 

 border imtil it nears its hind end. Here, turning dorso-meso-caudad, 

 it rises to the dorsal part of the nucleus, receiving on its way the dorsal 

 division of the tract (Fig. 26). The combined tract leaves the nucleus 

 at its dorso-caudal angle and runs caudad, immediately ventrad of the 

 nucleus fasciculus communis, some of whose cells send processes into it, 

 into the nucleus funiculi (Figs. 1, 2, 3, 4, 26 and 30). There is also a 

 small tract us acustico-cerehellaris. This tract arises near the cephalic 

 end of the acusticum along the mesial border, near the mid dorso- 

 ventral plane. It runs caudo-mesad close to the mesial surface for 

 about 1.2 mm.; then it turns mesad into the cerebellum, runs slightly 

 dorso-mesad, and crosses to the other side. After crossing its fibers 

 separate and turn cephalad. They run dorso-latero-cephalad in small 



