The brain of Acipenser. 71 
l. Base of Medulla. 
This includes the follwing structures which are continuous with 
structures of the same name in the cord: posterior longitudinal 
fasciculus, ventral horn or motor cells, ventral and lateral tracts 
with their tract and commissural cells. In this part there are situated 
also: the olive, the secondary vagus tract and nucleus, arcuate fibres, 
ascending and descending tracts connecting the medulla with higher 
parts of the brain. 
Cells of the base of the medulla. — The motor cells sur- 
round the groove of the ventral horn but are not sharply separated 
from the tract and commissural cells. They measure 10—26 by 
28—128 u. The cell body is irregular, usually long and curved, 
sometimes pyramidal or cubical, The dendrites are large and 
numerous, and are directed toward the periphery, ending in numerous 
branches among the cells and fibres of the ventral and lateral tracts. 
The neurites arise either from the cell body or from the base of 
one of the dendrites, and usually or always enter the fasciculus 
longitudinalis posterior, to become the root-fibres of motor nerves at 
some other level. Only the cells of the nuclei of the III, IV, and V 
motor nerves send their neurites directly into the nerve roots. These 
will be referred to again in describing the origin of the several nerves. 
Among the cells of the ventral horn are two large cells which 
give rise to MAUTHNER’s fibres. As noted above, the root fibres of 
the ventral VII form a strong and compact bundle which crosses the 
groove of the ventral horn. MAUTHNER’s cells lie immediately be- 
hind this bundle and partly enveloped by it. They measure about 
60 by 375 u, and have a form similar to that of the motor cells. 
Owing to their size and position these cells materially aid the 
ventral VII fibres in obliterating the groove. The neurite arises 
from the mesial end of the cell body and makes a convex bend 
cephalad as it passes toward the median raphe. The fibres pass 
caudad gradually approaching one another and decussate beneath 
the median groove (in one case about 260 u caudad from the cells). 
The neurites are about 11 or 12 a in thickness. 
The tract cells are distributed throughout the entire area of the 
base of the medulla, except the external one-third or one-half of the 
thickness of the wall. They measure 10 to 16 by 16 to 60 w. 
They are provided with from two to several dendrites, and vary 
greatly in the shape ot the cell body and in the manner of branching 
