FARTHER DEVELOPMENT OF THE EXCEPHALON. 



iod 



their interior behind, below, nnd at the sides, the thalami become solid, and at 

 the same time a cleft or fis&ure appears between them above, and jjenetrates 

 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 tJiird rentriclc. 

 Behind, the two thalami continue united by the jyoxfrrinr C(i/ii»iix.\ure, -which, is 

 distinguishable about the end of the third month, and also by the ped7i?icle.i of 

 the 2>i»'''ii glnnrl. The Koft commissure probably exists from an early period, 

 although it could not be detected by Tiedemann until the ninth month. 



At an early period the ojriie tracts may be recognised as hollow i^rolongations 

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

 the fourth month these tracts are distinctly fonned. They subsequently are pro- 

 longed backwards into connection with the coi-pora quadiigemina. 



Prosencephalon. — Each /iemisj>he/T-rcsicIc 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 Eeil, or central lobe ; the other fonns the 

 expanded or covering portion of the hemisphere, and is designated by Reichert 

 the mantle. The aperture existing at the constricted neck of the licmisphere- 

 vesicle, Schmidt and Reichert have recognised as the foramen of Monro. 



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

 thalami ; for, while the latter are formed by thickening of the cu-cumferential 

 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 thicken- 

 ings of the floor of the hemisphere-vesicles, which are lateral ofl:'-shoots 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 taenia semicircularis, and the hemisphere-vesicles, which include the 

 corpora striata and hemispheres. 



Fig. 561. — View of the Inner Fig. 56! 



Surface of the Right Half 



OF THE FffiTAL BrAIN OF ABOUT 



SIX MONTHS (from Reichert). 



F, frontal lohe ; P, parietal ; 

 0, occipital ; T, temporal ; I, 

 olfactory bulb ; II, right optic 

 nerve ; f p, fronto-parietal fis- 

 sure ; p, vertical fissure ; ;/, 

 internal vertical fissui'e ; /(, hip- 

 pocampal fissure ; ff, gyrus formi- 

 catus ; c, c, corpxis callosum ; 

 s, septum lucidum ; /, placed 

 between tlie middle commissure 

 and the foramen of Monro ; 

 V, in the upper part of the 

 third ventricle immediately be- 

 low the velum interpositum ami 

 fornix ; v', in the back part of 



the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of 

 Sylvius ; v" , in the lower part of the third ventricle above tlie infundibuhim ; )•, recessus 

 pinealis passing backwards from tlie tela choroidea ; p r,pons Varolii ; C c, cerebellum. 



The cerebral hemispheres enlarging, and having their walls increased in 

 thickness, fonn, dming the fourth month (Tiedemann), two smooth shell-like 

 lamellfe. which include the cavities afterwards named the lateral ventricles, and 

 the i^arts contained within them. FoUowdng out the subsequent changes affect- 

 ing the exterior of the cerebral hemispheres, it is found that about the foiuth. 

 month the first traces of some of the conrolvtions appear, the intermediate svlci 

 commencing- only as very slight depressions on the hitherto smooth sm-face. 

 Though the hemispheres continue to grow quickly upwards and backwards, the 

 convolutions at first become distinct by comparatively slow degrees ; but towards 



