THE INTER-BRAIN. 669 



In front, the choroid plexus of the lateral ventricle is small and tapering, and 

 communicates with that of the opposite side through the foramen of Monro. In 

 structure it consists of minute arid highly vascular villous processes, containing 

 an afferent and efferent vessel, and covered by a single layer of flattened epithelium, 

 the cells of which often contain a yellowish fat molecule. The anterior choroidal 

 artery is derived from the internal carotid, and enters the ventricle at the extremity 

 of the descending cornu, and, after ramifying in the plexus, sends branches into 

 the adjacent parts of the brain. The posterior choroidal arteries, one or two in 

 number, are derived from the posterior cerebral artery, and reach the plexus by 

 passing forward under the splenium of the corpus callosum. The veins of the 

 choroid plexus unite to form a prominent vein, which courses from behind forward 

 to the foramen of Monro, and joins with the vein of the corpus striatum to form 

 the corresponding vein of Galen. 



The transverse fissure is not a real fissure or cleft, because it is filled by the 

 invagination of the pia mater, forming the velum interpositum and the choroid 

 plexuses, covered by the lining of the ventricular cavities. If this involution 

 of pia mater is pulled out, the ventricular lining will necessarily be torn away with 

 it, and a cleft-like space will be left on either side, extending from the foramen of 

 Monro to the bottom of the descending horn of the lateral ventricle. The upper 

 part of this cleft, that is to say, the part nearest the foramen of Monro, is between 

 the lateral border of the body of the fornix and the optic thalamus ; below this, 

 at the commencement of the middle horn, it is between the commencing corpus 

 fimbriatum of the fornix and the pulvinar of the optic thalamus ; and lower still, 

 in the descending horn, between the corpus fimbriatum on the floor and the taenia 

 semicircularis in the roof of the cornu. Posteriorly the transverse fissure opens 

 between the splenium of the corpus callosum above, and the corpora quadrigemina 

 and pineal gland below. Through the fissure the venae Galeni emerge to join the 

 straight sinus. 



The velum interpositum or tela choroidea superior (Fig. 358) is a vascular mem- 

 brane, and is a prolongation of the pia mater into the interior of the brain through 

 the middle part of the transverse fissure. It is of a triangular form, and separates 

 the under surface of the body and posterior pillars of the fornix from the cavity of 

 the third ventricle. Laterally it covers the inner part of the upper surface of the 

 optic thalamus. Its posterior border or base lies beneath the splenium of the corpus 

 callosum above, and the optic thalamus, the corpora quadrigemina, and pineal body 

 below. Its anterior extremity, or apex, ends just behind the anterior pillars of the 

 fornix, where it is connected with the anterior extremities of the choroid plexuses, 

 which are here united through the foramen of Monro, and are then prolonged back- 

 ward on the under surface of the velum as the choroid plexuses of the third ventricle ; 

 in front, these plexuses of the third ventricle lie close to the middle line, but diverge 

 from each other behind. The lateral margins of the velum interpositum form the 

 choroid plexuses of the lateral ventricles. It is supplied by the anterior and pos- 

 terior choroidal arteries, already described. The veins of the velum interpositum, 

 the vence Graleni, two in number, run between its layers, each being formed by the 

 union of the vein of the corpus striatum with the choroid vein. The venae Galeni 

 unite posteriorly into a single trunk, the vena magna Galeni, which terminates in 

 the straight sinus (Fig. 326). 



II. The Inter-brain. 



The inter-brain (thalamencephalon) is the region of the third ventricle, and 

 comprises the parts developed from the second cerebral vesicle, together with that 

 portion of the first vesicle which is not concerned in the formation of the cerebral 

 hemispheres. 



The inter-brain is connected above and in front with the cerebral hemispheres ; 

 behind, with the mid-brain or mesencephalon. On its upper surface it is entirely 

 concealed from view, as it is covered by those portions of the internal surfaces of 



