The brain of Acipenser. 125 
that other cells may receive stimuli from the optic fibres, but the 
cells of the three types mentioned must be regarded as especially 
constituting the end nucleus of the optic nerve. Not all the optic 
fibres which end in the tectum enter it at the cephalic border. A 
small bundle of fibres becomes detached from the caudal border 
of the optic tract at about the level of the nucleus anterior turns 
caudad, courses around the lateral border of the tectum in company 
with the tractus tecto-lobaris and the tractus tecto-cerebellaris I, 
and enters the tectum near its caudal border. These fibres have 
been very difficult to trace owing to their being mingled with the 
fibres of the tractus tecto-lobaris cruciatus via decussatio postoptica. 
They enter the tectum superficially and presumably end in the same 
way as other optic fibres. 
2) Tractus thalamo-tectalis. — I have not demonstrated 
the existence of such a tract to my entire satisfaction. I only know 
that the neurites of a considerable number of cells in the central 
grey of the thalamus seem to run into the tractus tecto-lobaris and 
thus find their way into the tectum. | 
3) Tractus tecto-bulbaris. — This is the largest tract leav- 
ing the tectum. Its constitution can be made out only by studying 
it in connection with other bundles. I have described the fibres 
of the tectum as arranged in two roughly separated zones, one 
superficial and one occupying the middle one-third of the tectum. 
Beside these three there can be recognized a deep zone in the 
vicinity of the dorsal decussation. This deep zone owes its existence 
chiefly to the fibres of the dorsal decussation, which bend down after 
crossing and run for some distance near the cavity. At the lateral 
border of the tectum the deep zone is scarcely recognizable and we 
have only to consider the superficial and middle zones. In hori- 
zontal sections there is conspicuous a small bundle of fibres running 
longitudinally at either side of the mid-dorsal line at about the level 
of the middle zone of fibres. This small bundle bends laterad at 
the caudal border of the tectum to join the tract which I am about 
to describe. This bundle is not shown in Pl. 13. With certain 
exceptions to be mentioned below (tractus strio-tectalis and tractus 
tecto-cerebellaris), the fibres of the middle and superficial zones 
collect toward the lateral border of the tectum and nearer the caudal 
angle. It will be convenient to consider them as forming here an 
enormous bundle which consists of an ectal portion of finer fibres 
and an ental portion of coarser fibres. These two portions are distinct 
