FASCIA DENTATA FORNIX. 371 



longed from the central cavity of the ventricle, and is attached along 

 the inner border of the hippocampus major to its termination. 



Fascia dentata: if the corpus fimbriatum be carefully raised a 

 narrow serrated band of gray substance will be seen beneath it ; 

 this is the fascia dentata. 



Beneath the corpus fimbriatum will be likewise seen the transverse 

 fissure of the brain, which has been before described as extending 

 from the fissure of Sylvius on one side, across to the same fissure 

 on the opposite side of the brain. It is through this fissure that the 

 pia mater communicates with the choroid plexus, and the latter 

 obtains its supply of blood. The fissure is bounded on one side by 

 the corpus fimbriatum, and on" the other by the under surface of the 

 thalamus opticus. 



The internal boundary of the lateral ventricle is the septum luci- 

 dum. This septum is thin and semi-transparent, and consists of 

 two laminee of cerebral substance attached above to the under sur- 

 face of the corpus callosum at its anterior part, and below to the 

 fornix. Between the two layers is a narrow space, the fifth ven- 

 tricle, which is lined by a proper membrane. The fifth ventricle 

 may be shown, by snipping through the septum lucidum transversely 

 with the scissors. 



The corpus callosum should now be cut across towards its ante- 

 rior extremity, and the two ends carefully dissected away. The 

 anterior portion will be retained only by the septum lucidum, but 

 the posterior will be found incorporated with the white layer beneath, 

 which is the fornix. 



The fornix (arch) is a triangular lamina of white substance, 

 broad behind, and extending into each lateral ventricle: narrow in 

 front, where it terminates in two crura, which arch downwards to 

 the base of the brain. The two crura descend through the foramen 

 commune anterius of the third ventricle, and terminate in the cor- 

 pora albicantia. Opening transversely beneath these two crura, 

 just as they are about to arch downwards, is the foramen of Munro, 

 through which the two lateral ventricles communicate, and the cho- 

 roid plexuses are connected anteriorly. 



The lateral thin edges of the fornix are continuous posteriorly 

 with the concave border of the hippocampus major at each side, 

 and form the narrow white band called corpus fimbriatum. In the 

 middle line the fornix is continuous with the corpus callosum, and 

 at each side with the hippocampus major and minor. Upon the 

 under surface of the fornix towards its posterior part, some trans- 

 verse lines are seen passing between the diverging lateral fasciculi : 

 this appearance is termed the tyra, from a fancied resemblance to 

 the strings of a harp. 



The fornix may now be removed by dividing it across anteriorly, 

 and turning it backwards, at the same time separating its lateral 

 connexions with the hippocampi. If the student examine its under 

 surface, he will perceive the lyra above described. 



Beneath the fornix is the velum interpositum, a reflection of pia 

 mater introduced into the interior of the brain, through the trans- 



