164 



THE NEEYOUS SYSTEM. 



raj/i'i sp'inalex (branches of the Arteria rertebraliii). 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 straig-ht 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. 



horns. The vessels in the white matter are 

 for the most ])art radial and straight, while 

 in the more vascular grey matter they are 

 irregular and more sinuous in their course. 



The choroid plexus of the third ventricle 

 [Plexus c/ioroiileus ventriculi fertii, Reissner) 

 lies, as already described, on the roof of the 

 third ventricle; it is somewhat triangular in 

 form [Plexus venosus triangularis, Sehobl), 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 prolong'ation of the vena 

 spinalis superior, and with the internal jugvilar 

 vein. The small body usually named the 

 pineal body, and ]3laced 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- 

 siderable bunch of vessels into the tapper part of the third ven- 

 tricle. The choroid plexus of the fourth ventricle [Plexus choroideus 

 rentricuU quarti, Reissner) is a triangidar 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 vtna 

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

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

 corresponding to the j)osition 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 

 sjnnalis posterior, the Ve- 

 »/«(• spinales superiorcs, and 

 the origin of tlie Vena 

 jnijularis interna. 



H Hemispheres. 



ji Internal jugular. 



op Ojitic lobes. 



iiio Medulla oblongata. 



spj) Vena spinalis posterior. 



