THE CEEEBEAL COMMISSUEES AND SEPTUM PELLUCIDUM. 629 



upper end of the hippocampal formation so that it becomes removed far from the 

 lamina terminalis. The, fibres of the fimbria which are prolonged forwards under 

 the corpus callosum and septum pellucidum to bridge this great gap form the cms 

 fornicis on each side. As a rule in the human "adult brain the crura fornicis of the 

 two hemispheres become crowded together at the median plane so as to obscure 

 the connecting lamella which serves as a matrix for the commissura hippocampi 

 (Fig. 557, C) ; but the true arrangement can be seen in the brains of foetuses of 

 the sixth, seventh, and eighth months, and is at once revealed in the adult if 

 the corpus callosum is raised up by an accumulation of fluid in the lateral 

 ventricles (hydrocephalus), so as to put a strain upon the septum pellucidum. 

 The mass formed by the crura fornicis and their commissure is called the corpus 

 fornicis. 



The fascia dentata appears as a notched band behind and below the fimbria ; 

 its upper end passes on to the under surface of the splenium of the corpus 

 callosum, where it tapers and ends (fasciola cinerea) ; but as it dwindles the upper 

 end of the hippocampus emerges upon the surface below and behind it and passes 

 into a thin film of gray matter indusium griseum which is prolonged on to the 

 upper surface of the corpus callosum. It proceeds forwards, becoming as a rule still 



Nfestiges of the supracallosal 

 hippocampus 



Riratermmal \, 



B 



Vestiges of the supracallosal 

 x x hippocampus 



\ Septum pellucidum 



/ Olfactory bulb 

 Vestiges of the precallosal 

 hippocampus 



Epithelial 

 roof of third 

 ventricle 



Septu 

 pellucidum 



Paraterminal body 



Roof 



terminalis 



Commissura 

 hippocampi 



I 

 Paraterminaf body ( . 



Commissura 

 hippocampi 



FIG. 557. THREE STAGES IN THE DEVELOPMENT OP THE CORPUS CALLOSUM. 



more attenuated, and after surrounding the anterior end (genu) of the corpus 

 callosum it passes downwards towards the trigonum olfactorium along the line that 

 separates the corpus paraterminale from the neopalliuin. The indusium represents 

 the atrophied remains of the anterior part of the hippocampal arc of the foetal brain 

 Tig. 555), from which the fascia dentata has entirely disappeared. It is accom- 

 panied by longitudinal fibres homologous to the fornix system : in other words, the 

 fornix fibres of the atrophied supracallosal hippocampus; they form the striae 

 longitudinales of the corpus callosum (Fig. 558; Fig. 564, p. 635; Fig. 559, 

 p. 631). 



The inferior (or anterior) extremity of the fascia dentata dips into a deep furrow, 



around which the area piriformis is bent in a hook-like manner (uncus) ; in this 



becomes considerably reduced in diameter and then emerges (at right angles 



its previous direction) to form Giacomini's " banderella," which we may call 



the cauda fasciae dentatae. Behind this the inferior end of the hippocampus 



'mes to the surface, but is turned inside out, hippocampus inversus. Just in 



ont of the upper ending of the cauda fasciae dentatae a little knob of solid gray 



matter appears upon the surface, surrounded by area piriformis. It is the nucleus 



amygdalae (Fig. 558). 



Corpus Callosum. The corpus callosum is the great transverse commissure 



.ch passes between the two cerebral hemispheres. It is placed nearer the 



r than the posterior aspect of the brain, and it unites the medial sur- 



>s of the hemispheres throughout very nearly a half of their antero-posterior 



