CHAPTER XII. 

 The Cekebeum, ok Peosenoephalon (Continued). 



III. THE CEEEBEAL MANTLE. 



With the olfactory apparatus and the corpus striatum, we have de- 

 scribed everything that is common to the Prosencephala of all vertebrates. 

 "We come now to the consideration of the variable portion of the forebrain, 

 namely, the Mantle. 



As mantle, or pallium, we have designated all of those portions of the 

 wall of the cerebral vesicle not included in the olfactory apparatus and the 

 striatum; that is, the dorsal and lateral walls of the cerebrum. It has 

 already been mentioned that in several lower vertebrates the largest part of 

 it is formed of a simple epithelial plate. Of the epithelial mantle of the 

 teleost Pigs. 86 and 107 furnish a sufficient picture. In cyclostomes portions 

 of the wall on either side of the basal ganglion extend upward, ending in a 

 folded epithelial membrane. Studniczka has recently designated these 

 structures as lateral areas of the mantle. Their minute structure is, how- 

 ever, too little known to justify a decision. It is possible that we have to deal 

 here with simply a dorsally directed extension of the striatum. In rays and 

 sharks, representatives of the selachians, the mantle is developed; indeed 

 the most anterior portion is so enormously thickened and the lateral por- 

 tions project so far inward that in the greater part of the forebrain of 

 selachians the ventricle is obliterated, and in rays it is to be demonstrated in 

 only the most posterior portions. In most sharks it is present, and even its 

 projections into the olfactory lobes are to be recognized. But even here, 

 since the anterior wall of the brain is disproportionately thickened, it pro- 

 jects mostly far over the region of origin of the olfactory lobes, so that they 

 do not lie anteriorly, as in the other vertebrates, hut laterally and remote 

 from the frontal portion of the cerebrum. In this way the brain of the 

 selachian diverges much in form from the brains of other vertebrates, as is 

 shown in Pig. 106. It thus comes that through the thickening of the walls 

 the division into hemispheres is often so masked (Fig. 106, B) that it is only 

 recognizable microscopically through the finer fibers and through the nar- 

 row vascular cleft between the right and left hemispheres. 



In the mantle of all other vertebrates a deep groove separates the right 

 from the left ventricle. It reaches posteriorly to the Lamina terminalis, 

 near which the cerebral vesicles have evaginated. All of the commissures 



(159) 



