TtfB NERVOUS SYSTEM. 131 



—if its bulk be (compared with that of the nerves arising 

 from it; but by no means, if its weight be compare^ 

 with that of the whole body. 



200. Besides the bony cranium, a threefold covering 

 is afforded to the brain,* viz. the dura and pia mater, 

 and, between these two, the tunica arachnoidea. 



SOI. The dura mater, f which lines the inside of the 

 cranium, like a periosteum, forms various processes. 

 By the falx it divides the hemispheres of the cerebrum 

 and cerebellum; by the tentorium J it supports the 

 posterior lobes of the cerebrum and prevents their 

 pressure upon the subjacent cerebellum. 



In its various duplicatures it contains and supports 

 tiie venous sinuses § and prevents their pressure. These 

 receive the blood returning from the brain to the heart, 

 the proportion of which to the rest, Zinn long ago very 

 truly remarked, has been overrated try physiologists. 



* Eustachius, tab. xvii. xviii. 



Haller, Icones anat. fasc. vi. tab. i. ii. iii. 



Santorini, tab. posth. ii. iii. 



+ J. Ladmiral, Tconcs Aura matrix in enncava et convexa superficie vistr. 

 Amst. 173e. fasc. i. ii. 4to. 



X In the skulls of some mammalia, a remarkable lamina of bone .penetrates 

 a duplicative of the tentorium and supports it Cheseldcn [Anat. of the Bones . 

 chapt 8.) supposes this bony tentorium to exist in /era? only ; but it is found in 

 the equine genus, the cercopithecus paniscus, the delphinus phocaana, &c. Its 

 use is uncertain i that which is generally ascribed to it (for instance, by Laur. 

 Nihell, de cerebro. Edin. 1780. p. 4.), — of protecting the cerebellum in those 

 mammalia which move very swiftly, is improbable, because we find it in tbe 

 bear and other animals of still slower motion, and not in the ibex, which moves 

 with the greatest rapidity. 



§ Vicussens, Neurograph. universal, tab. xvii. fig. 1. 



Duverney, CEuvres anatom. vol. i. tab. iv. 



Haller, Icones anat. fasc. i. tab. vi. 



Walter, De morbis peritonei et apoplexia. Bcrol. 1785. 4to. tab. iii. iv. 



Vicq- d'Azyr, Planches Anatomiqves. xxxii. et xxxv, 



k2 



