THE NERVOUS SYSTEM. 



511 



earliest appearance of the fossa Sylvii. The external rhinal fissure, as it 

 becomes more pronounced, may be regarded as an extension forward of the 

 fossa (anterior crus of the corpus striatum) . On the mesial surface the incisura 

 prima marks this constriction. With the further curvature of the hemispheres, 

 the anterior lobe becomes bent back under the posterior (third month), but 

 later is again directed forward. It contains a diverticulum of the fore-brain 

 "cavity. The cavity of the posterior lobe is not so well marked off and is 

 bounded by the corpus striatum and the inward projection of the incisura 

 prima. (Figs. 424, 425, 427, 428 and 442.) 



The olfactory nerve at the end of five weeks has reached the anterior lobe 

 on its ventral and posterior side. The lobe develops into the receptive centei 5 for 

 the nerve the olfactory bulb; into the stalk in which the secondary olfactory 



Gyrus olfact. medialis 



Gyrus olfact. medius 



Gyrus diagonalis 



Cerebellum 



Insula 



Gyrus olfact. lat. 



Gyrus ambiens 

 Gyrus semilunaris 



Olive 



FIG. 435. Ventral view of the brain of human foetus at the beginning of the 4th month. Kollmann. 



tract proceeds; and also into a triangular area where the tract divides the 

 trigonum. The posterior olfactory lobe develops into the anterior perforated 

 space and an eminence known as the lobus pyriformis which becomes reduced 

 later (comp. Fig. 3 70, G and H). From it is developed the gyms olfactorius later- 

 alis, connected with the lateral division of the olfactory tract and thegyri ambiens 

 and semilunaris (Fig. 435). On the mesial wall, the posterior lobe is especially 

 connected with the region between the anterior arcuate fissure and the lamina 

 terminalis (trapezoid area of His, parolfactory or preterminal area of G. Elliot 

 Smith) (Fig. 442). Part of this mesial region represents the anterior portion 

 of the archipallium (comp. Fig. 370, G and H and p. 482). 



Corpora Striata and Pallium. The leading feature of the development 

 of this part of the brain is the great expansion of the pallial hemispheres. That 

 part of the brain wall marked externally by the fossa Sylvii and internally by the 

 body of the corpus striatum, and especially that part where the corpus striatum 



