The brain of Acipenser. 129 
rough and irregular like a dendrite. One branch of each fibre 
ends in the valvula, and these have no regular arrangement. The 
other branch takes a straight course with its fellows in a bundle 
along the lateral surface of the body, just ventral to the line of 
connection with the lateral lobes. They break up in end branches 
in the lateral portion (granular layer) of the body, although a few 
fibres reach the keel (molecular layer) at the caudal end of the body. 
The fine-fibred portion (II) of this tract assembles from the 
lateral part of the tectum and becomes at. the latero-caudal angle 
of the tectum a large and conspicuous bundle lying immediately 
ventral to the coarse-fibred portion. The fibres are interlaced with 
those of the tractus tecto-bulbaris and tecto-lobaris, and seem to 
come from the same source. The fibres enter the cerebellum ventral 
to the coarse-fibred portion and are distributed to the valvula, body, 
and the median parts of the lateral lobes. They do not divide at 
the point of entering the cerebellum and are without any regular 
arrangement. Their endings are within the granular layer (cf. page 97 
and Fig. E). 
8) Tractus tecto-thalamicus. — I have described this 
bundle as a part of the common longitudinal bundle in the dorsal 
part of the thalamus (see page 112). It is not shown in Pl. 13. 
b) Tracts of the Thalamus. 
1) Tractus thalamo-mammillaris. — From the nucleus 
anterior to the corpus mammillare. A small but diffuse tract nearly 
parallel with the bundle of MEYNERT (cf. page 113). 
2) Tracts of the central grey matter. The tractus thalamo- 
tectalis has been described above. Other fibres from the central 
grey run cephalad or caudad. 
3) Fasciculus longitudinalis posterior. — In sagittal 
sections this tract is seen to take nearly a straight course through 
the base of the medulla and mid brain. In the ‘tween brain, after 
crossing MEYNERT’s bundle, the fibres spread somewhat in fan-shape 
and turn a little ventrad. This spreading of the fasciculus is due 
to the diffuse nature of the nucleus, as described above (page 114). 
As the fasciculus passes back through the mid brain it is much 
interrupted in the region of the commissura ansulata and broken 
into several bundles by fibres of that commissure running through 
it. At one time I thought that I had found a decussation of the 
fibres of the fasciculus in this region (see figs. 8 and 9 in my paper 
Zool. Jahrb. XV. Abth. f. Morph, 
