580 DEVELOPMENT OF THE CEREBRO-SPINAL AXIS. 



behind, below, and at the sides, the thalami become solid, and at the same time a 

 cleft or fissure appears between them above, and penetrates down to the internal 

 cavity, which continues open at the back part opposite the entrance of the 

 Sylvian aqueduct. This cleft or fissure is the third ventricle. Behind, the two 

 thalami continue united by the posterior commissure, which is distinguishable 

 about the end of the third month, and also by the peduncles of the pineal gland. The 

 soft commissure could not be detected by Tiedemann until the ninth month ; but its 

 apparent absence at earlier dates may perhaps be attributed to the effects of laceration. 



At an early period the optic tracts may be recognised as hollow prolongations from 

 the outer part of the wall of the thalami while they are still vesicular. At the fourth 

 month these tracts are distinctly formed. 



The hemisphere-vesicle becomes divisible into two parts : one of these is the part 

 which from the interior appears as the corpus striatum, and from the exterior as the 

 island of Reil, or central lobe; the other forms the expanded or covering portion 

 of the hemisphere, and is designated by Reichert the mantle. The aperture existing 

 at the constricted neck of the hemisphere vesicle, Schmidt and Reichert have recognised 

 as the foramen of Monro. 



The corpora striata, it will be observed, have a very different origin from the optic 

 thalami ; for, while the optic thalami are formed by thickening of the circumferential 

 wall of a part of the first cerebral vesicle, and thus correspond in their origin 

 with all the parts of the encephalon behind them, which are likewise derived from 

 portions of the cerebro-spinal tube, the corpora striata appear as thickenings of the 

 floor of the hemisphere-vesicles, which are lateral offshoots from the original cerebro- 

 spinal tube. On this account, Reichert considers the brain primarily divisible into the 

 stem, which comprises the whole encephalon forwards to the tsenia semicircularis, and 

 the hemisphere-vesicles, which include the corpora striata and hemispheres. 



The cerebral hemispheres enlarging, and having their walls increased in thickness 

 form, during the fourth month (Tiedemann), two smooth shell-like lamellee, which 

 include the cavities afterwards named the lateral ventricles, and the parts contained 

 within them. Following out the subsequent changes affecting the exterior of the 

 cerebral hemispheres, it is found that about the fourth month the first traces of some 

 of the convolutions appear, the intermediate sulci commencing only as very slight 



Fig. 395. THE SURFACE OF THE F<ETAL BRAIN AT Six MONTHS (from R. Wagner). 



This figure is intended to show the commencement of the formation of the principal 

 fissures and convolutions. A, from above ; B, from the left side. F, frontal lobe ; P, 

 parietal ; 0, occipital ; T, temporal ; a, a, a, slight appearance of the several frontal 

 convolutions ; s, the Sylvian fissure ; s', its anterior division ; within it, C, the central 

 lobe or convolutions of the island ; r } fissure of Rolando ; p, the vertical fissure (external 

 part) ; t } the parallel fissure. 



depressions on the hitherto smooth surface. Though the hemispheres continue to 

 grow quickly upwards and backwards, the convolutions at first become distinct by 



