132 OP 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, Sbmmerring || has detected a third substance, 

 most conspicuous in the arbor vitae of the cerebellum 

 and the posterior lobes of the cerebrum. 



• Ruysch, Retpam. ad ep. probletnat. nonam. Amst 1670. tab. x. 



t B. S. Albimis, .-In not. Acad. L. 1. tab. ii. fifr. 1. 5. 



J S. Th. Sommerr'mir, iibrr das Organ der Seele. Koeningsberg. 1796. 4t<i. 

 tab. i. ii. 



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

 in whom they alone are very frequently found diseased. 



|| De host enrephali. p. 13. 



Compare dYmui, I)r pro/linn ttructnra cerebri. Farm*. 1782. 8vo. 

 tab. ii. iii. 



