THE INTERBHAIN OR THALAMENCEPHALON. 



143 



pare Figs. 81 and 82). The tract thus becomes little poorer in fibers. 

 Splitting np into numerous branches it now passes to the roof of the mid- 

 brain, where it is already familiar to you. These tracts ascending to the 

 corpora quadrigemina might be called the peduncles of the quadrigemina, 

 as they have been in mammals, but it is preferable to retain the term 

 "roots" because these tracts contain in mammals also fibers from the cere- 

 brum. 



In Fig. 66 one sees a part of the ends of the optic fibers. One may 

 note here that they stand in close relation to those tracts which arise in 

 the deep medullated stratum of the midbrain. Not only do the dendrites 

 of these cells from which arise the bundles to the sensory nuclei of the 

 medulla and spinal cord dip into the midst of the optic system, but hundles 



- Comm. haloDul. p. front. 

 " Fas. Tr. habenulo-peduno. 



- Opticus 

 Tr. strio-thalam. 



j " Decuss. transversa 



Deo. Tr. pallii 



Hypophysis 



Mg. 93. — Cross-section through anterior portion of thalamus of Scyllium canicula. 



of axis-cylinders pass into the optic layer from the sensory fiber-system above 

 mentioned. 



In mammals a majority of the fibers of the optic end in the Geniculatum 

 laterale and the remainder in the Tectum mesencephali. 



We will not leave the consideration of the Thalameneephalon without 

 impressing the fact that this segment of the brain, in lower vertebrates at 

 any rate, is joined to the cerebrum by relatively small tracts. I, therefore, 

 present at the close of this chapter a section from the most anterior portion 

 of the interbrain of the Scyllium canicula just behind the G-gl. habenulse. 



Note that nearly the whole section is filled with the opticus and with 

 the commissures associated with the optic system; also a few which cor- 

 respond to the system of the ganglion habenulse. Note that to the cerebrum 



