762 



THE NERVOUS SYSTEM. 



optic thalamus, and then along the floor of the descending horn of the lateral 

 ventricle, where some of its fibres blend with the white matter of the hippocampus 

 major., while the remainder are prolonged along its inner border as the corpus 

 fimbriatum (Figs. 445, 454), which extends into the white matter of the uncus 

 of the hippocampal gyrus. Upon examining the under surface of the fornix, 

 between its diverging posterior pillars a triangular portion of the under surface 

 of the corpus callosum may be seen, the base of which is the splenium. On it 

 are a number of lines, some transverse, others longitudinal or oblique. This 



FIG. 462. The fornix, velum interpositum, and middle or descending cornu of the lateral ventricle. 



portion has been termed the lyra, from the fancied resemblance it bears to the 

 strings of a harp (Fig. 452). The corpus fimbriatum is often called the fimbria. 



The anterior commissure is a round bundle of white fibres placed in front of 

 the anterior pillars of the fornix, and appears to connect together the corpora 

 striata. It passes outward through the corpus striatum on each side, and then 

 curves backward into the substance of the temporal lobe. 



The septum lucidum (or pellucidum) (Figs. 446, 449) forms the internal bound- 

 ary of the body and anterior cornu of the lateral ventricle. It is a thin septum 

 attached, above, to the under surface of the corpus callosum ; belong to the ante- 

 rior part of the fornix, and in front of this to the reflected portion of the corpus 

 callosum and anterior commissure ; behind, to the anterior pillars of the fornix ; 

 in front, to the posterior surface of genu of the corpus callosum. It is broad in 

 front, and narrow behind, its external surfaces looking toward the cavities of the 

 ventricles. The septum consists of two laminae, separated by a narrow interval, 

 the fifth ventricle. 



