INTERIOR OF THE CEREBRUM. 665 



from each other by a narrow interval. They are composed of white fibres, which 

 descend through the gray matter in the lateral wall of the third ventricle, and 

 are placed immediately behind the anterior commissure. At the base of the brain, 

 each pillar becomes twisted upon itself to form a loop, somewhat resembling the 

 figure of 8. The lowest part of the loop constitutes the white matter of the cor- 

 responding corpus albicans, from which the fibres can apparently be traced upward 

 and backward, as the bundle of Vicq d'Azyr, into the substance of the corre- 

 sponding optic thalamus (Fig. 353). It must be stated, however, that there is 

 probably no direct continuity between this bundle and the anterior pillar of the 

 fornix the latter possibly ending in the gray matter of the corpus albicans. The 

 anterior crura of the fornix are joined in their course by the peduncles of the pineal 

 gland and the superficial fibres of the taenia sernicircularis, and receive fibres from 

 the septum lucidum. Zuckerkandl describes an olfactory fasciculus, which becomes 

 detached from the main portion of the anterior pillar of the fornix, and passes 

 downward, in front of the anterior commissure, to the base of the brain, where it 

 divides into two bundles, one joining the inner root of the olfactory tract; the 

 other, the peduncle of the corpus callosum, and through it reaching the hippo- 

 campal convolution. 



Between the anterior pillars of the fornix and the anterior extremity of the 

 optic thalamus, an oval aperture is seen on each side ; this is the foramen of Monro 

 (Fig. 359). The two openings descend toward the middle line, and lead into the 

 upper part of the third ventricle. Through this foramen the lateral ventricles 

 communicate with the third ventricle, and consequently with each other ; through 

 it also the two choroid plexuses become joined with each other across the middle 

 line. The boundaries of the opening are, above and in front, the anterior pillars 

 of the fornix ; behind, the anterior extremity of the optic thalamus. 



The posterior pillars are the backward prolongations of the two halves of the 

 body of the fornix. They are flattened bands, and, at their commencement, are 

 intimately connected by their upper surfaces with the corpus callosum. Diverging 

 from one another, each curves round the posterior extremity of the optic thalamus, 

 and then passes downward and forward into the descending horn of the lateral 

 ventricle. Here it lies along the concavity of the hippocampus major, on the sur- 

 face of which some of its fibres are spread out, while the remainder are continued, 

 as the corpus fimbriatum or tcenia hippocampi, into the hook or uncus of the hip- 

 pocampal convolution. Upon examining the under surface of the fornix, between 

 its diverging posterior pillars, a triangular portion of the under surface of the cor- 

 pus callosum may be seen. On it are a number of curved or oblique lines passing 

 between the two pillars of the fornix. This portion has been termed the tyra, 

 from the fancied resemblance it bears to a harp. 



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

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

 On transverse section it is seen to be oval in shape, its long diameter being vertical 

 in direction and measuring about one-fifth of an inch. Its fibres can be traced 

 backward and downward through the globus pallidus and below the putamen on 

 each side into the substance of the temporal lobe. It serves in this way to con- 

 nect the two temporal lobes, but it also contains fibres from the olfactory tract of 

 the opposite side, the decussation of which in the anterior commissure may serve 

 to explain the condition of crossed anosmia, e.g., where there is a lesion in one 

 temporal lobe with a loss of smell in the olfactory area of the opposite side of 

 the nose. 



The septum lucidum is a thin, double, vertically placed partition, which forms 

 the internal boundary of the body and anterior horn of the lateral ventricle. It 

 consists of two distinct lamin?e, separated in part of their extent by a narrow chink 

 or interval, called the fifth ventricle. It is a thin, semitransparent septum, at- 

 tached, above, to the under surface of the corpus callosum ; below, to the anterior 

 part of the fornix behind, and the reflected portion of the corpus callosum in front. 

 It is triangular in form, broad in front and narrow behind ; its inferior angle cor- 



