The brain of Acipenser. 135 
cerebellum and forms a medullated commissure in the caudal part 
of the body. This is the largest of the commissures through the 
cerebellum. It is sometimes divided into two parts. Since its fibres 
join the corresponding tract of the opposite side, it is not a true 
commissure but a decussation. If its fibres come from the post- 
optic decussation as do the other fibres of this tract, they must re- 
cross here and end in the medulla on the side of their origin. I 
regard it as more probable that these are fibres which join this 
tract without crossing in the postoptic decussation, and cross here. 
The bulbar portion of this tract can be traced into the medulla 
about as far as the direct tract above described. It lies at first 
dorsal, then lateral and finally ventral to the direct tract. 
In the postoptic decussation and in its dorso-caudal course 
through the thalamus this tract has mingled with it the fine 
fibres of the tractus tecto-lobaris cruciatus as described above 
(page 127). 
In Pl. 13 the direct and crossed tracts just described have 
been shown as united, in order to simplify the figure. It would be 
impossible in this plate to show the bundles to the cerebellum in 
their relative position. I have drawn one orange band for both the 
direct and crossed fibres to the cerebellum and have let one branch 
of this represent the fine fibres, a second branch the coarse fibres, 
and a third branch the fibres of the commissure. 
3) Tractus mammillo-bulbaris. — This is a direct tract 
of fine fibres coming from the lateral and caudal parts of the corpus 
mammillare and bending back into the medulla ventral to the tracts 
from the lobi inferiores. It is a relatively small tract and its fibres 
are lost in the medulla sooner than those of the tracts last described- 
The tract is slightly indicated in Phot. 20 just as it leaves the corpus 
mammillare. In Pl. 13 this tract is shown as joining the tracts 
from the lobus, by which it is meant to indicate merely that they 
probably have the same destination. 
e) Summary of Tracts running from the Side Wall of 
the Mid Brain into the Cerebellum. 
I shall enumerate here in the order of their cephalo-caudal 
succession the bundles which pass through the bridge of tissue con- 
necting the cerebellum with the side walls of the mid brain and 
medulla, adding such further description of the bundles as may be 
necessary. 
