THE NERVOUS SYSTEM. 555 



old fissure, and known as the posterior arcuate fissure. This fissure does not 

 extend at first as far forward as the chorioid, but extends farther caudally, 

 arching downward in the temporal lobe around the caudal end of the chorioid 

 fissure (Fig. 481). The posterior arcuate fissure is a total fissure, involving the 

 whole wall and producing a fold on the inner surface of the medial hemisphere 

 wall (plica arcuata). The temporal or caudal part of this whole formation 

 persists in the adult without much further change. The fissure here becomes 

 the hippocampal fissure separating the fascia dentata from the gyrus hippocam- 

 pus; the part rolled in by the hippocampal fissure produces the eminence in 

 the lateral ventricle known as the cornu ammonis or hippocampus major; 



Frhl'' 



vRh Vmr Fstr hRh 



FIG. 480. Diagram of a graphic reconstruction of the mesial hemisphere wall of a 16 mm. human 

 embryo (about six weeks). His, Ziehen. Cavities are dotted, cut surfaces are lined. 



Apt, Angulus praethalamicus; Atr, preterminal area; Fpr, anterior arcuate fissure (fissura prima); 

 Frhl, mesial termination of lateral rhinal fissure; hRh, posterior olfactory lobe (tuberculum 

 olfactorium + substantia perforata anterior) ; Lt, lamina terminalis (lined) ; Vmr, depression 

 between the two olfactory lobes; vRh, anterior olfactory lobe (bulbus olfactorius + tractus 

 olfactorius + trigonum olfactorium). 



the edge of the limbus corticalis forms the fascia dentata; the limbus medullaris 

 or exposed fibrous part is thefimbria which is continued by its thinning edge 

 or taniafimbria into the ependymal or epithelial portion (lamina chorioidea) 

 of the chorioid plexus of the lateral ventricle. The chorioid plexus is attached 

 by the taenia chorioidea and lamina infrachorioidea (here the lamina affixa) to 

 the brain wall, usually near the junction of corpus striatum and thalamus, 

 thereby forming a part of the wall of the inferior horn of the lateral ventricle. 

 At this line of junction of thalamus and hemisphere wall is formed the stria 

 terminalis. The fimbria is continuous anteriorly with the posterior pillar of 

 the fornix. (Fig. 482.) 



The anterior part of the hippocampal formation above described. undergoes 



