The brain of Acipenser. 13 
The neurites are coarse and somewhat rough and irregular, be- 
coming thicker and rougher toward their terminations. Fine fibres 
which end among the cells of the lower olive come down around 
the lateral surface of the medulla from the spinal V tract. They 
have about the same calibre as the fibres of the cerebellar crest. 
They mostly or wholly cross the ventral raphe to end among the 
olive cells of the opposite side. Some of these may be collaterals 
of spinal V fibres, but it is probable that at least a part of them 
are neurites of cells in the acusticum or the cerebellum (compare 
page 191). 
Fibre tracts in base of medulla. — The fasciculus lon- 
gitudinalis posterior (Phots. 8—20) is made up of coarse medullated 
fibres, the smaller number of which come from the central diffuse 
nucleus in the thalamus (page 114), and the greater number of which 
come from the motor cells along the course of the fasciculus in the 
medulla. It gives immediate origin to the VI, to the ventral roots ofthe 
IX and XII nerves. The III and IV ‘nerves arise from special nuclei. 
The ventral V and VII are intermediate in character. The motor 
nucleus of the V is elongated so that many fibres run parallel with 
the fasciculus, but separate from it, to reach their point of exit. 
The motor nucleus of the VII is chiefly caudal to the root and the 
fibres form a distinct group in the lateral part of the fasciculus as 
they run forward to their point of exit. It is probable that the 
fibres of the other nerves do not run any great distance in the 
fasciculus before passing out in the nerve roots. 
The ventral and lateral columns are made up of short and long 
tracts. The short tracts consist of the neurites of commissural and 
tract cells. The long tracts include the tractus bulbo-tectalis, tractus 
tecto-bulbaris, tractus lobo-bulbaris, and others to be described 
below (page 124 ff). 
Ventral roots of cranial nerves. — The XII or hypo- 
glossus leaves the ventral surface of the medulla as four small 
rootlets. The fibres come from the fasciculus longitudinalis posterior, 
and are thick and heavily medullated. 
The X or vagus has numerous ventral rootlets (in one case 
nineteen), which arise from the fasciculus, curve close around the 
groove of the ventral horn, and pass in a dorso-lateral direction to 
their points of exit immediately dorsal to the descending tract of 
the trigeminus (spinal V). The ventral vagus roots are composed 
of coarse medullated fibres which are never impregnated in my pre- 
