336 ARACHNOID, 



posterior lobes of the cerebrum, and prevents their pressure upon 

 the subjacent cerebellum* 



11 



i. lalx. 2. Tentorium. 3, 3. Superior longitudinal sinus. . Inferior longi- 

 tudinal sinus, and, 5. Venae Galeni ; both opening into, 6. The straight sinus. 

 7. Posterior extremity of the superior longitudinal sinus. 8. A small portion 

 of the left lateral sinus. 9. The confluence of the sinuses, or Torcular Herophili. 

 10. Right lateral sinus. 11. Internal jugular vein. 



** In its various duplicatures it contains and supports the venous 

 sinuses b ," which receive blood from the veins of the brain, and 

 convey it to the internal jugular vein, " and it prevents their 

 pressure." It is prolonged in a tubular form throughout the 

 canal of the spine, and has openings for the various encephalo- 

 spinal nerves, and is prolonged a very short distance around the 

 greater part. 



" Next to the durea mater lies the arachnoid) so named from its 

 thinness/' It is a serous membrane, " destitute of blood-vessels, 

 and extended, like the dura mater, merely over the substance of 



orycteropus capensis, &c. Its use is uncertain : that which is generally ascribed 

 to it (for instance, by Laur. Nihell, De Cerebro. Edin. 1780. p. 4.), of pro- 

 tecting the cerebellum in those mammalia which leap 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." 



b (i Vieussens, Neurograph. universal., tab. xvii. fig. 1. 



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



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



Walter, De morbis peritontei et apoplexia. Berol. 1785. 4to. tab. iii. iv. 



Vicq d'Azyr, Planches Anatomiqites. xxxii. etxxxv." 



