HEMISPHERES. CORPUS CALLOSUM. 



581 



comparatively slow degrees ; but towards the seventh and eighth months they are 

 developed with great rapidity, and, at the beginning of the last month of intra-uterine 

 life, all the principal ones are marked out. 



The S'/lvian fissure, which afterwards separates the anterior from the middle lobe 

 of each hemisphere, begins as a depression or cleft between them about the fourth 

 month, and, after the gre-it longitudinal, is the first of the fissures to make its 

 appearance. It is followed by the fissure of Rolando, and the vertical fissure, and 

 somewhat later by the internal fronto -parietal fissure. After this, the various sub- 

 ordinate fissures dividing the convolutions gradually make their appearance. By the 

 end of the third month the hemispheres have extended so far backwards as to cover 

 the thalami ; at the fourth they reach the corpora quadrigemina ; at the sixth they 

 cover those bodies and great part of the cerebellum, beyond which they project still 

 further backwards by the end of the seventh month. 



During the growth of the hemisphere the aperture of the foramen of Monro is 

 extended backwards ; the arched margin of this opening is curved downwards at 

 its extremities, and forms anteriorly the fornix, and posteriorly the corpus fimbriatum 

 and hippocampus major ; above the margin a part of the wall of each hemisphere 

 comes into contact with its fellow, and in the lower part forms the septum lucidum, 

 while above this the hemispheres are united by the development of the great 

 commissure, the corpus callosum. 



The corpus callosum is described by Tiedemann as being first seen about the end of 

 the third month, as a narrow vertical band, extending across between the forepart of 

 the two hemispheres, and subsequently growing backwards. With this view the 

 observations of Schmidt coincide. Reichert, however, maintains that the commissural 

 structure seen at the forepart of the hemispheres is the anterior white commissure, 

 and that the corpus callosum appears in its whole extent at once. 



The corpora albicantia at first form a single mass : so also do the anterior pillars 

 of the fornix, which are distinguished before the posterior pillars. The posterior 

 pillars are not seen until the fourth or fifth month. At that period the hippocampus 

 minor is also discernible. 



Fig. 396. VIEW OP THE INNER Fig. 396. 



SURFACE OF THE RIGHT HALF 

 OF THE FCETAL BRAIN OF ABOUT 

 six MONTHS (from Reichert). 



F, frontal lobe; P, parietal; 

 0, occipital; T, temporal; I, 

 olfactory bulb ; II, right optic 

 nerve ; fp, fronto-parietal fissure ; 

 p, vertical fissure ; />', inter- 

 nal vertical fissure ; Ti, hippo- 

 campal fissure ; g, gyrus forni- 

 catus ; c, c, corpus callosum ; 

 s, septum lucidum ; f, placed 

 between the middle commissure 

 and the foramen of Monro ; 

 v, in the upper part of the 

 third ventricle immediately below 

 the velum interpositum and for- 

 nix : v', in the back part of the 



third ventricle below the pineal gland, and pointing by a line to the aqueduct of Sylvius ; 

 v", in the lower part of the third ventricle above the infundibulum ; r, recessus pinealis 

 passing backwards from the tela choroidea; p v, pons Varolii; Ce, cerebellum. 



MEMBRANES OF THE ENCEPHALON. 



It is remarked by Bischoff, that the pia mater and arachnoid are formed by the 

 separation of the outer layer of the primitive cephalic mass ; and thus that the pia 

 mater does not send inwards processes into the fissures or sulci, nor into the ventri- 

 cular cavities ; but that every part of this vascular membrane, including the choroid 

 plexuses and velum interpositum, is formed in its proper position in connection 

 Avith the nervous matter. The dura mater, on the other hand, is developed from the 

 inner surface of the dorsal plates. 



