164 



THE NERVOUS SYSTEM. 



rami spinales (branches of the Arteria vertebralis}. These branches 

 form a plexus in the pia mater, from which vessels pass at irregular 

 points into the cord ; one set of small vessels, described by Reissner, 

 pass in a straight course from the superior longitudinal sinus 

 towards the substantia reticularis, where they divide. Other branches 

 pass directly from the arteria spinalis anterior, through the ventral 

 longitudinal fissure, and there divide ; the twigs as a rule avoiding 

 the septum medium and passing in greater part towards the ventral 





Fig. no. 



op 



horns. The vessels in the white matter are 

 for the most part radial and straight, w r hile 

 in the more vascular grey matter they are 

 irregular and more sinuous in their course. 



The choroid plexus of the third ventricle 

 (Plexus choroid eus veniriculi tertii, Reissner) 

 lies, as already described, on the roof of the 

 third ventricle; it is somewhat triangular in 

 form (Plexus veuosus triangular is, Schobl), and 

 is evidently a prolongation of the pia mater, 

 with an increased supply of vessels. It re- 

 ceives, at its anterior angle, veins from the 

 cerebral hemispheres and the adjacent parts. 

 The under surface is covered with a layer of 

 ciliated pavement epithelium. At the pos- 

 terior angles of the plexus the veins commu- 

 nicate on either side (Fig. no) with the 

 anterior lateral prolongation of the vena 

 spinalis superior, and with the internal jugular 

 vein. The small body iisually named the 

 pineal body, and placed on the roof of the 

 ventricle, is only a small thickened portion of 

 the plexus, and consists of a group of convo- 

 luted blood-vessels. This plexus sends a con- 

 vessels into the upper part of the third ven- 

 The choroid plexus of the fourth ventricle (Plexus choroiileits 

 ventriculi quarti, Reissner) is a triangular membrane, slightly at- 

 tached to the borders of the fourth ventricle. Its upper surface 

 is flat and bounded at either side by a large vein, the vena 

 spinalis superior ; anteriorly it is bounded by the cerebellum. The 

 under surface is not flat; in the middle line is a slight furrow 

 corresponding to the position of a median vessel, which may, when 

 injected, be seen from the dorsal surface; from it a number of 



spp 



Diagram to show the Vena 

 spinalis posterior, the Ve- 

 nae spinales superiorcs, and 

 the origin of the Vena 

 jugularis intfriui. 



H Hemispheres. 



ji Internal jugular. 



op Optic lobes. 



mo Medulla oblongata. 



sjip Vena spinalis posterior. 



siderable bunch of 

 tricle. 



