The brain of Acipenser. 65 
larger, so that before it leaves the cranium it becomes nearly twice 
as thick as the medulla at its caudal end. On the left side only 
the root of the nerve is in position and two of the most caudal 
rootlets have been pulled out, leaving two distinct holes ventro- 
cephalad from the calamus. In front of X, and slightly more ventral 
in position, is IX, which stands next to X in point of size. Farther 
cephalad and dorsad is the N. lineae lateralis X which in the 
photograph appears in its natural position along the dorsal surface 
of IX and X, except that it has been loosened and raised slightly 
for the sake of making it more distinct. Farther forward, and more 
ventral than the nerves already described, is the VIII nerve. In 
Phot. 4 it appears to be divided into cephalic and caudal halves. 
In front of VIII are four nerve roots in a dorso-ventral row, the 
most dorsal being somewhat more caudal. The dorsal two are the 
two roots of the N. lineae lateralis VII, the dorsal one of which 
enters the lobus lineae lateralis, while the ventral one enters the 
acusticum proper. The ventral two are the sensory and motor roots 
of VII, the motor root being small and indistinctly visible ventro- 
caudal to the sensory root. It is more clearly seen in Phot. 4. 
The VI. nerve arises from the ventral surface of the medulla op- 
posite VII and VIII by four or five extremely slender rootlets in 
cephalo-caudal succession. It was removed in making this dissection. 
Cephalad from VII is V, whose large sensory and small motor 
roots are distinctly seen in Phots. 1 and 4. 
The internal surface of the medulla is shown in several photo- 
graphs of the choroid roof in Phot. 1, and of the nervous wall in 
Phots. 3, 5, 6 and 7. Upon the floor at either side of the median 
groove is seen a large ridge caused chiefly by the fasciculus longi- 
tudinalis posterior, which continues on forward to the ansulate 
commissure. These ridges are bounded laterally by the grooves of 
the ventral horns. The ridges are large in the middle part of the 
medulla. Opposite the middle of the lobus lineae lateralis they are 
greatly diminished by the loss of the fibres of the ventral VII, 
which go off to either side as large bundles which completely ob- 
literate the grooves and form transverse ridges projecting into the 
ventricle. These are described by GORONOWITSCH, but seem to be 
not so large in A. ruthenus as in A. rubicundus. Farther cephalad 
the grooves grow rapidly smaller to near the level of the cephalic 
end of the lobus lineae lateralis, and again grow slowly larger until 
the commissura ansulata is reached. This enlargement and the 
Zool. Jahrb. XV. _ Abth. f. Morph. 5 
