THE NERVOUS SYSTEM. 131 



if its bulk be compared with that of the nerves arising 

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

 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. 



201. 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 ; it supports the 

 posterior lobes of the cerebrum and prevents their 

 pressure upon the subjacent cerebellum. 



In its various duplicatures it contains and supports 

 the 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 by physiologists. 



* Eustachius, tab. xvii. xviii. 



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



Santorini, tab. posth. ii. iii. 



f J. Ladmiral, Icones dura matrix in concava et convexa superf.de vixte. 

 Amst. 1738. fasc. i. ii. 4to. 



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

 a duplicature of the tentorium and supports it. Cheselden (/Inat. of the Bones. 

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

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

 use is uncertain : 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 the 

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

 with the" greatest rapidity. 



Vieussens, Neurograph. universal, tab. xvii. fig. 1. 



Duvemey, (Euvres anatom. vol. i. tab. iv. 



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



Walter, DC morbis peritonaii ct apnplexia. Bcrol. 1785. 4to. tab. in. iv. 



Vicq. d'Azyr, Plaitr/ta Anatomiqut*. xxxii. et xxxv. 



K '2 



