132 OF THE FUNCTIONS OF 



202. Next to the dura mater lies the arachnoid, so 

 named from its thinness. Its use is not exactly known; 

 it is destitute of blood-vessels (5), and extended, like 

 the dura mater, merely over the substance of the brain, 

 without following the course of its furrows and promi- 

 nences. 



203. On the contrary, the membrane called pia mater 

 by the ancients, closely follows the cortical substance 

 of the brain,* and possesses innumerable blood-vessels 

 which penetrate into the latter. Hence, if a portion of 

 this membrane is detached, we find the external surface 

 very smooth, while the internal is villous and resem- 

 bles the roots of moss, f (A) 



204. The cerebrum and cerebellum are composed of 

 various parts which differ in texture and figure, but 

 the use of which is unknown. The most remark- 

 able are the four ventricles, in the two anterior and 

 fourth of which are found the choroid plexuses, of 

 whose function also we are ignorant. 



205. The substance of the brain is twofold : the one 

 called cineritious or cortical, though not always situated 

 exteriorly ; the other white or medullary. Between the 

 two, Sb'mmerring || has detected a third substance, 

 most conspicuous in the arbor vitae of the cerebellum 

 and the posterior lobes of the cerebrum. 



* Ruysch, Respow. ad ep. problemat. nonam. Amst. 1670. tab. x. 



f B. S. Albinus, Annot. Acad. L. 1. tab. ii. fig. 1. 5. 



J S. Th. Sommerring, alter das Organ der Seek. Kreningsberg. 1796. 4to. 

 tab. i. ii. 



The importance of these plexuses is shewn in the dissection of maniacs, 

 in whom they alone are very frequently found diseased. 



II De bast fiiccphali. p. 13. 



Compare Genari, Dr peculiari structitra cerebri. Farm*. 1782, 8v<. 

 tab. ii. iii. 





