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 fibre 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, 



FIG. 151. Parietal, eye of Anguis fragilis, after Nowikoff. ct, connective tissue cells 

 around nerve; gc, ganglion cells; /, lens; n, nerve fibres; pn, parietal nerve; pc, pigment cells; 

 r, retinal cells; vb, vitreous body. 



on either side. These are ganglionic and are closely related to the 

 corpora stria ta. 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 trans versum, 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 



