186 ANATOHY OF THE CENTRAL NERVOUS SYSTEM. 



over each lateral ventricle, it is cut off anteriorly and posteriorly. It is then 

 seen that it is united by its under surface to thin, white bundles of fibers, 

 which, arching over the cavity of the ventricle, descend anteriorly and pos- 

 teriorly into the depth of the same. They belong to the fornix. 



The fornix is a combination of bundles of fibers which pass con- 

 tinually along the edge of the hemispheres. They arise, on each side, from 

 the mesial border of the inferior horn as the crura fornicis (Fig. 135, pos- 

 terior); then converge over the posterior portion of the thalamus and unite 

 with one another above the ventricle to form a broad tract: the corpus 

 fornicis. In the angle where they meet a number of fibers pass across them 

 transversely, thus forming a triangle. This triangle is known as the lyra 

 Davidis, or psalterium. It lies under the posterior end of the corpus cal- 

 losum and, for the most part, is united with this. At this point, conse- 

 quently, the corpus callosum lies close to the edge of the hemisphere. It 

 occasionally happens, however, that the fornix is situated at some distance 

 from it; a small cavity, the ventriculus Verga, is then observed between the 

 fornix and corpus callosum. In the anterior part of the brain the corpus 

 callosum recedes constantly from the edge of the hemispheres, and a portion 

 of the inner sagittal wall of the hemisphere remains between it and the 

 ventricle. This portion of the median wall, situated below (posterior, in the 

 horizontal section) the corpus callosum, is the septum pellucidum. That part 

 of the great fissure found between the two septi is called the ventriculus 

 septi pellucidi. Imagine the corpus callosum removed in Fig. 125; the con- 

 tinuation of the wall of the hemisphere into the septum and the significance 

 of the ventricle are then evident at once. This is not a true ventricle, but, 

 as already stated, only a portion of the fissure between the hemispheres 

 covered over by the corpus callosum. 



The fornix naturally borders this portion of the wall of the hemisphere 

 also. It again divides at the anterior end of the corpus callosum into two 

 bundles, the columnce fornicis, which descend in front of the thalamus as a 

 posterior thickening of each membrane of the septum pellucidum, and ter- 

 minate provisionally at the base of the brain at the boundary-line between 

 the forebrain and interbrain. 



The corpus fornicis has been removed, along with the corpus callosum, 

 in Fig. 125 and only the anterior and posterior portions remain visible. 

 In the right, where the section passes somewhat deeper through the white 

 substance, the fornix is divided in that part known as the fimbria. It there 

 lies near to its point of origin, the cornu Ammonis. On the left I have 

 divided it just where it arches over the surface of the thalamus. 



Unite the points F and F^ by a gentle curve passing over the thalamus, 

 and the course of the fornix is reproduced. It will also be clear to you from 

 the accompanying median, longitudinal section through an embryonic brain. 



