THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 295 



of this line the median or third ventricle extends forward through 

 the whole length of the telencephalon (cf. Fig. 8). It is bounded 

 laterally by thick walls but dorsally and ventrally the walls are 

 much thinner. Just in front of the optic chiasma the floor of the 

 ventricle is suddenly depressed to form the preoptic recess. In 

 front of this recess is the point corresponding to the lower edge 

 of the neuropore in the embryo. From this point forward and 

 upward the brain wall is formed by the closing of the neuropore 

 in the median line. As this is regarded as the end of a closed 

 tube the front wall of the median ventricle is called the lamina 

 terminalis. The lower portion of the lamina terminalis is thick- 

 ened by the anterior commissure, and the upper portion of it is 

 also thickened by a transverse band of fibers known as the olfac- 

 tory decussation. Immediately above the olfactory decussation 

 is seen a short triangular projection of the ventricle which marks 

 the point at which the neural tube remained longest in connection 

 with the ectoderm. This was probably at the upper border of the 

 neuropore and the little sac is called the recessus neuroporicus. 

 From the recessus neuroporicus back to the nuclei habenulae the 

 roof of the median ventricle is membranous. This is a part of 

 the true brain roof and does not belong to the lamina terminalis. 

 There is no such thing as a pars supraneuroporica of the lamina 

 terminalis. In the front part of the forebrain the median ventricle 

 expands laterally into the cavities of the lateral lobes. These 

 cavities are the lateral ventricles and each has an anterior and a 

 posterior branch corresponding to the two parts of the lateral 

 lobes. These must not be confused with the anterior and posterior 

 horns of the lateral ventricles in man, since both together corre- 

 spond more nearly to the anterior horn in man. The connection 

 of the median with the lateral ventricle of each side is knowTi as 

 the foramen of Monro. 



Into the anterior half of each lateral lobe the olfactory nerve 

 enters and its fibers are distributed to all parts of the anterior half 

 of each lobe. This anterior part of the lateral lobe is therefore 

 the olfactory bulb. It consists of a great number of slightly 

 differentiated cells (Fig. 94) from which arise the fibers of the 

 olfactory tract. These fibers pass back into the posterior portion 



