THE TELENCEPHALON. 1159 



psalterii), sometimes intervenes as the result of imperfect union, between the under 

 surface of the corpus callosum and the middle part of the body of the fornix. It 

 should be understood, however, that this cleft is not a part of the series of true ven- 

 tricular spaces. The under surface of the fornix rests upon the velum interpositum, 

 which thus separates it from the third ventricle and the upper surfaces of the two 

 thalami which it overlies. 



The anterior pillars of the fornix (columnae foraicis) are two slender cylin- 

 drical strands, which, slightly diverging as they leave the anterior angle "of the body, 

 arch downward and forward, then somewhat backward, and descend to the basal 

 surface of the brain, where they end in the mammillary bodies. In their descent 

 they lie in the extreme front part of the lateral walls of the third ventricle, where 

 they show as ridges (Fig. 976), and form on each side, the upper and anterior 

 boundary of the foramen of Monro. A short distance below the latter opening, 

 the pillar disappears from the ventricular wall in consequence of the increasing 

 divergence from the mesial plane. On reaching the mammillary body on the basal 

 surface of the brain, the fibres composing the anterior pillar are interrupted to 

 a large extent in the mammillary nuclei (Fig. 967). The connections of these 

 stations are described elsewhere (page 1129), sufifice it here to recall that while a 

 part of their fibres are continued to lower levels, a very considerable strand, known 

 as the bundle of Vicq d' Azyr, arches upward and completes the connection between 

 the fornix and the thalamus, in the anterior part of which these mammillo-thalamic 

 fibres end. The relations of the anterior pillars to the olfactory paths are noted in 

 connection with the olfactory nerve (page 1222). 



The posterior pillars of the fornix (crura fornicis), the widely diverging 

 backward prolongations from the lateral angles of its body, are at first attached to 

 the under surface of the corpus callosum. They then turn outward, and, sweeping 

 around the posterior ends of the optic thalami, enter the descending horns of the 

 lateral ventricles and arch downward along the dorso-mesial border of the conspicu- 

 ous hippocampi, the elevations which mark the inferior horns of the lateral ventricles. 

 On reaching this situation, however, the posterior pillar no longer retains its previous 

 form, but now appears much reduced in size, as a white flattened band, known as 

 the fimbria, which, broadest in the middle of its course, narrows as it descends, and 

 ends by joining the uncus at the lower extremity of the ventricle. The progressive 

 diminution of the fimbria during its descent is due to the contribution of many of its 

 fibres to the sheet of white matter, the alveiis, which covers the hippocampus. It is 

 evident that the fornix constitutes, by means of its several parts, a continuous tract 

 of longitudinally coursing fibres, which convey impulses from the chief cortical olfac- 

 tory centre, the uncus and the hippocampus, to the mammillary nuclei and thence, in 

 great part, by the bundle of Vicq d'Azyr to the thalamus. 



The fornix may be considered, in a sense, as a tract of white matter representing the lower 

 edge of the hemisphere ; in front and behind these edges remain ununited and more or less 

 widely apart. Beneath the corpus callosum they become attached not only to the under surface 

 of this bridge, but also to each other by the commissural fibres of the psalterium. The peculiar 

 course of the fornix is referable to the backward and downward expansion of the developing 

 hemispheres, as the result of which the posterior end of the fornix follows the hippocampus in 

 its migration into the descending horn of the lateral ventricle as the temporal lobe is de\'el- 

 oped. Further consideration of these changes, however, may be deferred (page 1167) until 

 the associated structures have been described in connection with the lateral ventricle. 



The Septum Lucidum. — The septum lucidum (septum pellitcxdura) is the thin 

 median vertical partition which fills the interval between the corpus callosum above 

 and in front and the fornix behind (Fig. 996), with which structures its margins are 

 firmly attached. It separates the anterior horns and adjoining parts of the lateral 

 ventricles and Is, in a modified form, triangular in shape when viewed laterally. The 

 sides of the triangle are all curved and its anterior angle, received within the bend of 

 the genu, is blunt and rounded. Its posterior angle is narrow and extends for a 

 variable distance between the under surface of the bodv of the corpus callosum 

 and the upper arched surface of the body of the fornix. The lower angle 

 occupies the interval between the thin edge of the rostrum and the anterior pillars 



