746 



NEUROLOGY 





the horns of the future lateral ventricle; the hinder end of the vesicle is carried down- 

 ward and forward and forms the inferior horn; the posterior horn is produced 

 somewhat later, in association with the backward growth of the occipital lobe of 

 the hemisphere. The roof-plate of the primitive fore-brain remains thin and of an 

 epithelial character; it is invaginated into the lateral ventricle along the medial 

 wall of the hemisphere. This invagination constitutes the choroidal fissure, and 

 extends from the interventricular foramen to the posterior end of the vesicle. Meso- 

 dermal tissue, continuous with that of the primitive falx cerebri, and carrying 

 bloodvessels with it, spreads between the two layers of the invaginated fold and 

 forms the rudiment of the tela choroidea; the margins of the tela become highly 

 vascular and form the choroid plexuses which for some months almost completely 

 fill the ventricular cavities; the tela at the same time invaginates the epithelial 

 roof of the diencephalon to form the choroid plexuses of the third ventricle. By 

 the downward and forward growth of the posterior end of the vesicle to form the 

 temporal lobe the choroidal fissure finally reaches from the interventricular fora- 

 men to the extremity of the inferior horn of the ventricle. 



Gyrus dentatus 

 Tania thalami 



Choroidal fissure 



Thalamus 

 Post, commissure 



Corpora quadrigemina 

 Cerebral aqueduct 

 Cerebral peduncle 



Cerebellum 

 IV. ventricle 



Corpus callosum 

 Septum pcllucidum 

 Anterior commissure 



Lamina terminalis 



- Rhinencephalon 

 Optic chiasma 

 Hypophysis 



III. ventricle 

 Pans 



Medulla oblongata 



FIG. 657. Median sagittal section of brain of human embryo of four months. (Marchand.) 



Parallel with but above and in front of the choroidal fissure the medial wall of 

 the cerebral vesicle becomes folded outward and gives rise to the hippocampal 

 fissure on the medial surface and to a corresponding elevation, the hippocampus, 

 within the ventricular cavity. The gray or ganglionic covering of the wall of the 

 vesicle ends at the inferior margin of the fissure is a thickened edge; beneath this 

 the marginal or reticular layer (future white substance) is exposed and its lower 

 thinned edge is continuous with the epithelial invagination covering the choroid 

 plexus (Fig. 656). As a result of the later downward and forward growth of the 

 temporal lobe the hippocampal fissure and the parts associated with it extend from 

 the interventricular foramen to the end of the inferior horn of the ventricle. 

 The thickened edge of gray substance becomes the gyms dentatus, the fasciola 

 cinerea and the supra- and subcallosal gyri, while the free edge of the white sub- 

 stance forms the fimbria hippocampi and the body and cms of the fornix. The 

 corpus callosum is developed within the arch of the hippocampal fissure, and the 

 upper part of the fissure forms, in the adult brain, the callosal fissure on the medial 

 surface of the hemisphere. 



The .Commissures (Fig. 657). The development of the posterior commissure 

 has already been referred to (page 743). The great commisssures of the hemi- 



