146 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
tract leading to the eye of the opposite side. In the lower groups the 
lobes contain an epiccele (p. 143), but in the higher they are solid, the 
cavity being reduced to the aqueduct. The floor of the mid-brain is 
‘formed of large fibte tracts (see below), the floor plate having been 
invaded by their fibres. 
In the thalamencephalon (’twixt-brain) the lateral walls are thick- 
ened, the dorsal zones developing a nerve centre, the optic thalamus, 
IN (ely Te 
g 
ESSE 
l 
0 
AiG 
Fic. 151.—Parietal, eye of Anguis fragilis, after Nowikoff. ct, connective tissue cells 
around nerve; gc, ganglion cells; /, lens; , nerve fibres; pm, parietal nerve; fc, pigment cells; 
r, retinal cells; vb, vitreous body. 
on either side. These are ganglionic and are closely related to the 
corpora striata. Frequently the thalami of the two sides touch or 
even unite above, forming’ the so-called soft commissure (commis- 
sura mollis, fig. 152)—really not commissural in character. Still 
more dorsal is a small habenular ganglion on either side, in front 
of the pinealis to be described in a moment. 
Under the head of epiphysial structures are several parts devel- 
oped in the roof plate of the primitive fore-brain. At the junction of 
cerebral hemispheres and twixt-brain (fig. 150) there is an internal epi- 
thelial fold, the velum transversum, depending from the cerebral 
roof. In front of this an outgrowth, the paraphysis, arises on the top 
of the brain in nearly all vertebrates. It is non-nervous and apparently 
-is an extra-ventricular chorioid plexus with secretory functions. The 
other epiphysial structures belong to the ’twixt-brain and consist of a 
parietal organ and a pinealis. Both arise from the roof between the 
