DIENCEPHALON.
To the diencephalon, sometimes called the on account of its position between the end- and the mid-brain, belong : The thalamencephalon and the pars mamillaris hypothalami.
The diencephalon surrounds the third ventricle. The immediate roof of the latter is formed by the lamina chorioidea e,t5ithelialis and the Ida chorioidea ventriculi feria, which lies above and fused with the epithelial sheet. As secondary coverings, over the tela, follow the fornix and the corpus callosum.
The dissection of the brain proceeds in the following manner, the display of the fornix being next undertaken. To this end, the callosum is cut through transversely, from i to 2 cm. in advance of the posterior border of the splenium. This is best accomplished by passing the knife, from the side and horizontally, above the crus fornicis and then cutting through the corpus callosum from below upward and somewhat obliquely backward. The callosum is now pulled up, its attachment to the psalterium severed, and separated from the body of the fornix and, farther forward, from the upper border of the septum pellucidum.
After removal of the corpus callosum it is to be noted, how on each side the fim bria ascends from the inferior horn and passes into the crus fornicis, how the crura fornicis approach each other and meet to form the corpus fornicis, and how the columnae fornicis bend downward in front of the foramen interventriculare (Fig. 54). Further to be observed are the partition separating the lateral ventricles, the septum pellucidum, with the cavum septi between its laminae, and the course of the plexus chorioideus ventriculi lateralis from the inferior horn through the pars centralis as far as the foramen interventriculare. By means of a probe or bristle may be readily
demonstrated the manner in which the two lateral ventricles are connected by the foramen of Monro. The latter marks the ¶osition at which the plexus chorioideus ventriculi lateralis is continuous with the choroid plexus of the third ventricle. It must not be forgotten, however, that the plexus really lies extraventricular. On removing the choroid plexus, the taenia chorioidea and the taenia fornicis are recognizable, and, likewise, the anterior part of the thalamus.
Beneath the fornix lies the tela chorioidea ventriculi tertii (Fig. 55). In order to exhibit the latter, we proceed in the following manner : one peduncle of the fornix is lifted and sectioned with a sharp knife medialward and, at the same time, obliquely backward, the section being continued through the hind end of the corpus callosum, thus cutting across the pars occipito-temporalis of the radiatio corporis callosi. A similar section is executed on the other side. The posterior end of the callosum is now raised and turned forward, with the fornix. The latter is cut off at the posterior margin of the septum pellucidum, where the corpus fornicis passes into columnae fornicis. After removal of the fornix, the tela chorioidea lies free, beneath which the lamina chorioidea epithelialis alone remains as the roof of the third ventricle. The removal of the tela chorioidea is carried out from in front ; it is raised behind the columnae fornicis and carefully reflected backward. Compare Figs. 62 and 63 for orientation. We now pass to the consideration of the thalamencephalon.