Anatomy of the Brain of Bdellostoma Dombeyi 3 



area of fine fibers, a third nucleus, the lohus linece lateralis. In Bdel- 

 lostoma, while the lohus linear lateralis is fairly well marked out, it is 

 not entirely distinct, as in Petromyzon, and there is no differentiation 

 of the other two parts. 



The Acusticus Nucleics. — The acusticus nucleus (Figs. 1, 2, 3, 4, 26 

 and 30) is a long and narrow club-shaped structure, much thicker in 

 its cephalic than in its caudal half. It is about 2.15 mm. long, .675 mm. 

 broad just before it tapers to its cephalic end, about .6 mm. deep at its 

 deepest part about midway of its length, and from .3 mm. to .4 mm. 

 deep through its caudal half. This difference in depth is caused by 

 the arrangement within the nucleus of the entering fibers. It lies in 

 the dorso-mesial part of the dorsal column of the medulla, separated 

 from the dorsal surface of this body by the fasciculus communis. It 

 appears somewhat wedge-shaped as it lies in the medulla, its meso- 

 ventral angle being its most ventral part, the dorso-mesial and ventro- 

 lateral surfaces both sloping dorsad. The ventro-lateral surface is 

 comparatively fiat, but the dorso-mesial surface, although quite flat in 

 the caudal third of the nucleus, is very convex in the cephalic part. 

 The long axis of the nucleus runs fore and aft in its hind part, but 

 about three-quarters of the length of the nucleus from its cephalic end 

 it bends laterad, at an angle of about 120°, following the curvature of 

 the dorsal column of the medulla. 



There are five sets of fibers that enter the acusticus nucleus, those of 

 r. acusticus utricularis, of r. acusticus saccularis, of N. lateralis pos- 

 terior, and two sets for N. lateralis anterior. Of these entering fibers 

 those of r. acusticus saccularis are the most caudal set (Figs. 26 and 

 30). They enter along the lateral face of the nucleus from about 1.1 

 mm. to 1.3 mm. from the caudal end. Immediately cephalad of the 

 fibers of r. saccularis, and with only a break of about .03 mm. between 

 the two groups, the fibers of r. acusticus utricularis enter. 



The Auditory Fibers. — The auditory fibers of the r. utricularis and 

 those of the r. saccularis will bp considered together because their 

 internal distribution is the same. The fibers are straight and of varying 

 diameters when they enter the brain, some measuring 2.2 microns, 

 while some are no more than .9 microns in thickness. They run 

 cephalo-mesad, at an angle of about thirty degrees with the long axis 

 of the brain until they reach the lateral border of the acusticus nucleus. 

 After entering the nucleus the fibers divide into two branches, one 

 running cephalad and one caudad within the nucleus (Figs. 5, 6, 26 



