242 J. B. JOHNSTON, 
the central cavity in the base of the mid brain and cephalic part 
of the medulla. In the history of Vertebrates the habenular apparatus 
seems to have changed its functions. In primitive Vertebrates it 
served the parietal eye and the olfactory apparatus. In higher Verte- 
brates the parietal eye is no longer functional and in some Mammals. 
the olfactory apparatus is reduced. In the higher forms the ganglion 
habenulae serves as part of certain indirect paths and possibly as 
an optic center (KOLLIKER). 
18) The epiphysis sends fibres to the ganglia habenulae in Aci- 
penser, about an equal number going to each ganglion. Over the 
enlarged base of the epiphysis there is a commissure formed in part 
of fibres arising from the epithelial cells of this enlarged base of 
the epiphysis. These fibres join a longitudinal tract through the 
dorsal part of the thalamus and probably go to the nucleus anterior. 
Other fibres in the commissure end freely among the epithelial cells. 
The source of these fibres and the function of the epithelial cells 
are unknown. 
19) The larger size of the right ganglion habenulae is due toa 
larger number of fibres of the tractus olfacto-habenularis ending in 
that ganglion. 
20) The tractus thalamo-mammillaris arises in the nucleus anterior 
and ends in the corpus mammillare. The bundle of Vicq D’AzyYR 
which comes as collateral fibres from the Haubenbiindel and ends 
in the nucleus anterior (CAJAL, KÖLLIKER) in Mammals must be 
regarded as a recent development in higher Vertebrates. 
21) The optic nerve crosses completely in the optic chiasma and 
ends in the tectum and in the nucleus anterior. No other thalamus 
nucleus receives optic fibres in Acipenser. 
22) The nucleus ruber tegmenti sends its neurites in a direct 
tract to the cerebellum. 
23) The tractus lobo-cerebellaris et bulbaris forms a large de- 
cussation behind the chiasma, not before described. The remainder 
of the decussation in this position is formed.by the tractus tecto- 
lobaris (and tractus strio-tectalis?). The nomenclature of the de- 
cussation requires revision. 
24) The saccus vasculosus probably forms part of a mechanism 
for secreting, or otherwise controlling the pressure of, the cerebro- 
spinal fluid. It may affect the heart beat and blood pressure by 
way of the vagus. 
