BRAIN. 145 
fibre tracts soon to be mentioned, some with nerves arising from this 
region. 
Most noticeable of these ganglia are the olivary bodies (oliva) near the roots of 
the hypoglossal or first spinal nerves; the nuclei of the cuneate and slender funiculi 
connected with the posterior columns of the cord; the eminentia medialis in the 
floor of the fourth ventricle, connected with the anterior and lateral columns; and 
the tuber acusticum, an enlargement connected with the eighth nerve; its anterior 
end in the ichthyopsida is specialized as the lobe of the lateral line system. 
The cerebellum is developed from the dorsal zones and the roof 
plate, the latter invaded by nerve cells from the sides. In front it dips 
deeply into the fourth ventricle, its anterior portion being vertical and 
together with part of the roof of the isthmus, forming the valve of 
Fic. 150.—Diagrammatic longitudinal section of brain. ac, anterior commissure in 
lamina terminalis; ag, aqueduct; c, cerebrum; cb, cerebellum; cp, chorioid plexus; cs, corpus 
striatum; cv, cerebellar ventricle; kh, hypophysis; hc, habenular commissure; 7p, inferior 
chorioid plexus; m, mesencephalon; ml, myelencephalon; , pinealis; pa, paraphysis; pc, 
posterior commissure; pe, parietal eye; v, valve of Vieussens; vt, velum transversum with 
aberrant commissure. 
Vieussens (velum medullare anterius,fig.150). In the ichthyopsida 
and lower reptiles there is no special differentiation of parts in the cere- 
bellum, but in the higher reptiles and in the birds a central portion, the 
vermis, and a pair of lateral lobes, the flocculi (fig. 161) occur. In the 
mammals the cerebellum is still farther enlarged, chiefly by the develop- 
ment of large cerebellar hemispheres between vermis and flocculi, the 
latter being forced by them to the lower side of the cerebellum. In the 
walls of each hemisphere, besides others, there is a large nerve centre, 
the nucleus dentatus, connected with the posterior peduncle of the 
cerebellum to be mentioned shortly, and with the fibres which go 
farther forward in the brain. 
The mesencephalon is relatively largest in the lower vertebrates, 
less conspicuous and tending to be covered by cerebrum and cerebellum 
in the higher groups. On its dorsal surface are the two optic lobes 
(transversely divided in the mammals) each connected with an optic 
Io 
