NERVOUS CENTRES. (HUMAN ANATOMY. THE MENINCES.) 



635 



The surface of each choroid plexus presents 

 many slight projections or folds resembling 

 villi, in which are contained loops and plexi- 

 form anastomoses of minute vessels, very si- 

 milar to the arrangement of the vessels of the 

 villous processes of the chorion of the ovum, 

 or those of the tufts of the placenta. These 

 vessels are surrounded by an epithelium which 

 has much the appearance of that of serous 

 membranes. From the great number of these 

 vessels and from the delicate nature of the 

 epithelial covering which surrounds them, it is 

 plain that the choroid plexuses are well suited 

 either for the purpose of pouring out fluid or 

 of absorbing it. 



Fig. 364. 



Side view of villi of the choroid plexus of the lateral 

 ventricle in the brain of a Goose, to show the 

 disposition of the bloodvessels. Not to obscure the 

 mew of t/te bloodvessels, the edge of the epithelium 

 only has been shown. 



a, epithelium ; b, bloodvessels. 

 (After Valentin.) 



The epithelium may be best seen by examin- 

 ing the edge of a fold. It becomes very distinct 

 when acted upon by acetic acid. As its particles 

 are very delicate and consist only of a single 

 layer, they are easily detached. The cells of 

 epithelium are most of them six-sided, and 

 contain a clear nucleus, or several minute gra- 

 nules. Valentin states that cilia may be seen 

 playing upon this surface, especially in the 

 embryo. I have observed the peculiar punc- 

 tiform or spiniform formations to which he 

 alludes, which look like the remains of former 

 vibratile cilia. 



Velum interposition. (Toile Choroidienne, 

 Vicq d'Azyr.) The choroid plexuses are con- 

 nected to each other by the velum interpositum, 

 which is a triangular fold of pia mater that 

 passes in at the transverse fissure between the 

 upper surface of the tubercula quadrigemina 

 and the posterior reflected portion of the corpus 

 callosum. This process is continuous with the 

 pia mater of the inferior surface of the posterior 

 lobes of the brain, and with that of the superior 

 surface of the cerebellum, and it therefore con- 

 sists of two laminae ; as it passes forwards, it 

 sends downwards a little process which em- 

 braces the pineal body ; it forms the roof of the 

 third ventricle, being interposed between that 

 cavity and the fornix, (hence its name,) and 

 at its sides as well as its apex its continuity 



with the choroid plexuses may be readily de- 

 monstrated. At its anterior extremity it corre- 

 sponds to the foramen commune anterius. The 

 velum interpositum is best exposed in the dis- 

 section from above downwards by removing 

 carefully in succession the corpus callosum and 

 the fornix. In raising the velum itself, in order 

 to disclose the cavity of the third ventricle, it 

 is necessary to be very careful, as from the in- 

 timate connexion which the pineal body has 

 with it towards its base, that body may be 

 readily disturbed from its position. 



Choroid plexuses of the fourth ventricle- 

 The choroid plexuses of the fourth ventricle 

 are two small processes of pia mater united 

 along the median line, presenting the same 

 villous character as those of the lateral ventri- 

 cles. These folds seem as if they had been 

 pushed up into the fourth ventricle by the 

 lower lamina of the inferior vermiform process. 

 Their position may be best seen by opening the 

 fourth ventricle from above, where they will be 

 found lying on each side of that portion of the 

 median lobe of the cerebellum which stops up 

 the inferior extremity of the fourth ventricle. 

 These plexuses are in every respect similar, as 

 far as regards structure, to the larger ones 

 which are found in the lateral ventricles, and, 

 like them, exhibit a delicate epithelium upon 

 their surface. Upon the centre of each epithe- 

 lium cell Valentin states that a pigment cor- 

 puscule is deposited. (Fig. 365.) 



Fig. 365. 



A highly magnified villus of the choroid plexus of 



human cerebellum. (After Valentin.) 

 a, the villus ; b, the epithelium cells ; c, the 

 nuclei. 



These internal processes of the pia mater 

 contain minute crystalline formations, a kind 

 of very fine sand, which, however, is not con- 

 stantly present in all brains. 



The grains are deposited in the meshes of the 

 vascular plexuses. Sometimes they accumulate 

 in masses so as to be visible to the naked eye 

 or easily recognized by the touch. In general, 

 however, they are microscopic, in form glo- 

 bular, and connect themselves with the minute 

 vascular ramifications like little bunches of 

 grapes. They are found principally in the 

 choroid plexuses of the lateral ventricles, and 

 in that portion of the velum interpositum 

 which embraces the pineal body. In the for- 

 mer they are most numerous at that part which 

 was called by the Wenzels glomus, where the 

 choroid plexus turns up from the inferior cornu 

 into the horizontal portion of the lateral ven- 

 tricle.* As regards chemical composition this 



* See Van Ghert de plexubus choroideis, 

 Utrecht. 1837 j Valentin, in Soemmering Anat., and 



