80 J. B. JOHNSTON, 
crosses with a fellow of the opposite side in the commissura ansulata, 
and continues up into the tectum, where the fibres are distributed 
in the middle zone. See description of tractus bulbo-tectalis below, 
page 127. The remainder of the neurites from the acusticum run 
laterad and bend ventrad, coursing around the lateral surface of the 
medulla and crossing the middle line in a superficial position. These 
fibres are medullated and are present through the whole extent of 
the acusticum. They are perhaps to be homologized with the ex- 
ternal arcuate fibres of mammals, while those just described are 
internal arcuate fibres. 
Afferent fibres. — The fibres of the lateral line X 
nerve enter the dorsal part of the acusticum as already stated. 
Few, if any, of these fibres end in immediate proximity to their 
place of entrance, but regularly run forward or backward in the 
acusticum. I have been unable to determine whether these fibres 
branch as they enter the brain, owing to faulty impregnation near 
the point of entrance. A considerable number of these fibres run ~ 
caudad parallel with the spinal V tract and end in part in the nucleus 
funiculi and in part in the nucleus mesial to it (Nuc. acustici spinalis). 
The bundle here described receives additional fibres from the lateral 
line VII and from the VIII. The bundle was described in my 
preliminary paper and homologized with the spinal VIII in man 
(98b, p. 585). In the same paper I stated that a part of the lateral 
line fibres became arcuate fibres to the opposite side of the medulla. . 
I now wish, upon further study, to make the same correction with 
regard to these as I made above concerning the descending fibres 
of the trigeminus (page 77). The most of the lateral line fibres 
which turn forward in the acusticum run to the cerebellum. The 
paucity of cells in the cephalic part of the acusticum is correlated 
with the small number of fibres which end there. 
The ventral root of the lateral line VII nerye enters the dorsal 
part of the acusticum just in front of the VIII. The fibres of this 
root have the same central disposition as do those of the lateral 
line X, except that a considerable number of fibres from this root 
reach the lobus lineae lateralis as will be described below (page 82). 
The single large root of the VIII nerve enters the ventral part 
of the acusticum just caudal to the lateral line VII. Superficially 
its root lies immediately caudal and slightly ventral to the sensory 
root of the VII proper. As the root enters the medulla immediately 
dorsal to the spinal V tract some fibres divide into two unequal 
