THE CEREBRAL HEMISPHERES 



941 



It is along this arcuate and fissure-like gap (Fig. 693) that the richly vascular (pial) 

 choroid plexus invaginates the atrophied parietes of the secondary fore-brain to 

 form the choroid plexus which is everywhere covered by ependyraa. The choroid 

 fissure extends from the foramen of Monro to near the tip of the middle cornu 

 in an arcuate course, and ependymal reflections everywhere close in this gap 

 except at the foramen of Monro. The manner in which this is accomplished may 

 best be understood by a study of a trans-section showing the ependymal reflections 

 from the ventricular wall onto the invaginated choroid plexus (Fig. 666). The 

 caudatothalamic fusion and the intrusion of the great fibre masses constituting 

 the cerebral crura play their parts in complicating the relations in brains of 

 higher type. 



FIG. 693. Diagram showing the choroid fissure. 



The Choroid Plexus of the Lateral Ventricle (paraplcxus] and Velum Inter positum. 

 The choroid plexus is a highly vascular, fringe-like structure composed of pia 

 which is invaginated into the lateral ventricle along the choroid fissure, or gap 

 between cerebral hemisphere and diencephalon. The portion of the choroid 

 plexus protruding into the "body" of the lateral ventricle is the fringed vascular 

 border a triangular fold of pia the velum interpositum (tela choroidea superior], 

 which, as its name implies, is interposed between the relatively small primary 

 fore-brain and the enormous overlapping secondary fore-brain, and is produced 

 by the overgrowth of the latter onto the former. Inasmuch as the nerve tissue 

 in the roof of the third ventricle atrophies totally, the ventral fold of the pia comes 

 into contact with the ependyma of that ventricle and here permits a similar 

 vascular invagination in the form of two parallel fringes hanging into the cavity 

 (diaplexus or choroid plexuses of the third ventricle). The dorsal leaf of the pial 

 fold is in contact with the ventral face of the body of the fornix. Frontad, the 

 velum interpositum tapers toward the region of the two foramina of Monro, 

 where the choroid plexuses of the two sides are continuous with each other. 

 The ventricular surface of the choroid plexuses is everywhere covered by ependyma 

 which is reflected from it to the fimbriated edge of the fornix on the one hand and 

 to the line of the taenia semicircularis (over the thalamus by the lamina affixa) 

 on the other. Its vascular components, in addition to undefined lymphatic 

 channels, are the anterior choroid artery, a branch of the internal carotid, entering 

 the plexus of the middle cornu ; and the posterior choroid artery from the posterior 

 cerebral artery reaching the choroid plexus in the neighborhood of the splenium. 

 The venules of the plexus join to form a tortuous middle cornual vein which 

 terminates frontad by joining one of the velar veins. 



