The brain of Acipenser. 215 
sending their neurites probably to the tectum, indicating the presence 
in the diffuse central grey of the nucleus of the fasciculus and the 
nucleus rotundus. I can not agree with EDINGER’s statement in the 
Vorlesungen (5th ed., p. 130) that the nucleus of the fasciculus longi- 
tudinalis posterior certainly belongs to the hypothalamus in fishes, 
since it lies wholly within the thalamus in Acipenser. 
In addition to these I have described a relatively distinct nucleus 
closely applied to the ventricular surface at the angle between the 
thalamus and hypothalamus, which receives the fibres of the afferent 
saccus bundle. As I shall show below (page 222 ff.), this nucleus 
is part of a vaso-regulator mechanism governing the pressure in 
the cerebro-spinal fluid. It is probable that the fibres from this 
nucleus, which I have traced caudad into the base of the mid brain, 
go to a vaso-motor center in the medulla. 
3. Nucleus ruber tegmenti. 
This nucleus corresponds in position to the nucleus of the same 
name in higher forms (birds and Mammals) and in the entrance into 
it of fibres from the central grey of the thalamus. I have discussed 
the tractus tegmento-cerebellaris above (page 115, 136, 195). 
c) Hypothalamus. 
The histology of the nuclei in the hypothalamus has not been 
studied with any degree of fulness by modern methods. The authors 
who have given most attention to this region in recent years, EDIN- 
GER, S. RAMON Y CAJAL and KÖLLIKER, have confined their de- 
scriptions chiefly to the general relations of the nuclei and the 
disposition of the fibre tracts. Davip (92a) has described several 
collections of cells and figured the cells in different parts of the lobi 
inferiores of Teleosts and Ganoids. His descriptions are so brief 
and fragmentary as to be of little value. 
1. Fibre Tracts. 
In the hypothalamus in fishes there are longitudinal and decus- 
sating tracts arising from and ending in two distinct nuclei, the lobus 
inferior and the corpus mammillare with the ectomammillare. The 
longitudinal tracts set up connections with the fore brain, tectum, 
cerebellum, and medulla. The decussations are chiefly or wholly 
decussations of longitudinal tracts. 
