56 



ANATOMY OF THE OENTEAL NBEVOTJS SYSTEM. 



eeplialon. Its roof is almost exclusiTely formed of epithelial plates whose 

 numerous projections will claim our attention later. Even ventrally the 

 wall, which is evaginated to form the sack-like recessus infundibular is, is 

 only a thin one. The hypophysis lies close beside it. The lateral walls con- 

 taining the optic thalami are not figured. The dorsal wall of the Thala- 

 mencephalon is directly continuous with the cerebral mantle, the Pallium, 

 which is the dorsal wall of the Prosencephalon. The floor of the latter is 

 occupied by the corpus striatum and the olfactory apparatus. .Posteriorly 

 the Thalamencephalon merges into the Mesencephalon, whose dorsal seg- 

 ment is called the corpus opticum, because here is the deep origin of the optic 

 nerves. 



The ventral segment of the midbrain, designated as Tegmental promi- 

 nence (Hauhenwulst) and Basis mesencephali, contains almost exclusively 



'CiSiS 



Fig. 25. — Eeptilian brain. (A diagrammatic sagittal section.) 



nerve-tracts, with, also, a few small nuclei. Turning at a sharp angle, the 

 corpus opticum becomes continuous with the cerebellum. In this angle 

 there are, in all vertebrates, two great nerve-decussations, the anterior one of 

 which belongs to the nervus trochlearis. The cerebellum covers a part of 

 the medulla even in the reptile. The greater part, however, is exposed and 

 is covered by a thin choroid plexus: the Plexus ventriculi quarti. Then 

 the cavity of the central system becomes narrower and narrower until it be- 

 comes a narrow canal which traverses the whole length of the spinal cord. 

 Something has been said of the development of the peripheral nerves 

 in the previous chapters. If the central nervous system has progressed so 

 far in its development that the principal parts are clearly distinguished one 

 from another, then from the Thalamencephalon to the end of the spinal 

 cord the central cavity (ventricle, or central canal) is inclosed with gray 

 tissue: masses rich in ganglion-cells. Posterior to the midbrain the periph- 



