1396 A MANUAL OF ANATOMY 



Hippocampal or Dentate Fissure. — This fissure commences behind 

 the splenium of the corpus callosum, where it is continuous with the 

 callosal sulcus. It is directed forwards, lying between the gyrus 

 dentatus above and the hippocampal gyrus below, and it terminates 

 within the uncus of the hippocampal gyrus. 



The hippocampal fissure is a complete fissure. It appears in the 

 course of the fifth week, and is parallel to the temporal portion of 

 the choroidal fissure, below which it lies. The portion of the 

 vesicular waU between these two fissures represents the gyrus 

 dentatus, and the portion below the hippocampal fissure forms the 

 hippocampal gyrus. 



The hippocampal fissure, being complete, gives rise to an internal 

 elevation, namely, the hippocampus major, on the wall of the de- 

 scending comu of the lateral ventricle. 



Gyrus Dentatus {Fascia Dentata of Tarinus) . — The gyrus dentatus 

 is situated above the hippocampal gyms, and below the fimbria. 

 It is separated from the hippocampal gyrus by the hippocampal 

 fissure, and from the fimbria by a slight groove, called the fimhrio- 

 dentate sulcus. The dentate gyrus is narrow, and its free margin 

 is indented or notched : hence the name dentatus. It commences 

 behind the splenium of the corpus callosum, and is directed forwards 

 above the hippocampal gyrus into the curve of the uncus. Here 

 it describes a bend, after which it emerges from the curve of the 

 uncus, and, crossing the recurved part, is lost on its outer aspect. 

 This portion of the dentate gyrus is called the frenulum Giacomini. 



Posteriorly it is continuous round the splenium with the rudimen- 

 tary gyrus supracallosus, which contains the mesial and lateral longi- 

 tudinal stricB of one half of the upper surface of the corpus callosum. 



Fimbria. — ^The fimbria is the prolongation of the posterior pillar 

 of the fornix. It is situated above the g5mis dentatus, from which 

 it is separated by the fimbrio-dentate sulcus. Posteriorly it turns 

 upwards round tiie posterior extremity of the optic thalamus, and 

 so becomes continuous with the posterior pillar of the fornix. 

 Anteriorly it enters the uncus. 



Development of the Cerebral Hemispheres. — Each hemisphere is developed 

 from the wall of the cerebral vesicle, which is a hollow protrusion from the 

 upper and lateral part of the telencephalon, one of the divisions of the prosen- 

 cephalon. The anterior wall of that portion of the telencephalon which lies 

 between the two cerebral vesicles is called the lamina terminalis. 



The hemispheres grow out of proportion to the other parts of the encephalon 

 in a forward, upward, and backward direction. Their backward growth is 

 so great that they completely cover the other parts of the encephalon by the 

 seventh month of intra-uterine life. 



The sulci and gyri of the hemispheres first appear about the fifth month of 

 intra-uterine life. 



Development of the Insula and Fissure of Sylvius. — The insula, or island of 

 Reil, appears as a depression, called the Sylvian lossa, on the lateral aspect 

 df the cerebral vesicle. The wall of this fossa becomes developed into the 

 opercula insulce, and, as these grow, they cover the insula, and give rise to 

 the limbs of the fissure of Sylvius. 



