The brain of Acipenser. oO 
common tract does not appear as wide as it actually is at the 
lateral border of the tectum. 
4) Tractus tecto-lobaris. — This tract consists of fine 
fibres derived from the ectal portion of the common bundle de- 
scribed above, some of them at least arising as collaterals from the 
fibres of the common bundle. The tract receives fibres from all 
parts of the tectum, from both superficial and middle fibre zones. 
I have mentioned above a crossed portion which descends to the 
ansulate commissure. The direct tract descends over the lateral 
surface of the thalamus, lying immediately caudal to the optic tract 
and covering superficially a large part of the nucleus ruber tegmenti. 
The crossed portion, which is much the smaller, joins the direct 
and both are distributed to the dorsal and lateral walls of the lobi. 
Another bundle of fine fibres which is probably to be reckoned with 
this tract has the following course: coming from the caudal part of 
the tectum it soon becomes situated a little deeper, mingles with 
the fibres of the tractus lobo-cerebellaris et bulbaris cruciatus, and 
descends in the outer part of this tract to the postoptic decussation, 
where it crosses to end in the cephalic wall of the lobus of the 
opposite side. The actual crossing of the fibres is difficult to de- 
monstrate, but I believe that this description is correct. There is 
thus to be recognized a direct tractus tecto-lobaris and two crossed 
tracts, one by way of the commissura ansulata and one by way of 
the decussatio postoptica. This last is much the smallest part of 
the whole tract and is not shown in PI. 13. 
5) Tractus bulbo-tectalis. — The ental, coarse-fibred 
portion of the common bundle mentioned above consists of ascending 
fibres from the medulla to the tectum. This coarse-fibred tract runs 
mesial to the fine-fibred tractus tecto-bulbaris (3 above), between it 
and the direct tractus lobo-bulbaris (page 134 below). It is to be 
traced in the medulla about as far as the latter tract, namely about 
to the level of the VIII nerve. This coarse tract as it enters 
the tectum runs along the ectal border or base of the torus semi- 
circularis Halleri. Here it receives a bundle from the commissura 
ansulata which runs ental to the crossed tractus tecto-bulbaris (cf. 
Com. ans. below, page 137) and accompanies the portion of that 
tract which comes from the torus. The tract is composed of second- 
ary fibres from the acusticum, most of which have crossed the 
ventral raphe in the medulla as internal arcuate fibres (page 79). 
A part of the fibres do not cross immediately on leaving the acu- 
