972 



THE NERVE SYSTEM 



which contains the anterior cerebral artery, and a cisterna between the corpora 

 quadrigemina which contains the vena magna Galeni. 



The cerebrospinal fluid (coeliolympha; liquor cerebrospinalis) fills the subarach- 

 noid space. It is a clear, limpid fluid, having a saltish taste and a slightly 

 alkaline reaction. According to Lassaigne, it consists of 98.5 parts of water, 

 the remaining 1.5 per cent, being solid matters, animal and saline. It varies in 

 quantity, being most abundant in old persons, and is quickly reproduced. Its 

 chief use is probably to afford mechanical protection to the nerve centres, and 

 to prevent the effects of concussions communicated from without. 



Structure. The arachnoid consists of bundles of connective tissue, the fine fibres of which 

 form one layer and cross each other in every direction. At the level of the large fissures, and 

 especially around the circle of Willis, it is reenforced by thick fibrous tissue. Both surfaces are 

 covered with endothelium. There are no bloodvessels in the arachnoid; the vessels which 

 appear to be in it are really in the pia. There is no positive proof that nerves are present in 

 the arachnoid. It is true that Bochdalek and Luschka long ago described arachnoid nerves, 

 but these observations have never been corroborated. 



The Arachnoid Villi or Pacchionian Bodies (Granulationes Arachnoideales). 



The arachnoid villi, erroneously called glandulae Pacchioni, are numerous 

 small whitish or purplish projections, usually collected into clusters of variable 

 size, which are found in the following situations: (1) Upon the outer surface of the 

 dura, in the vicinity of the superior longitudinal sinus, being received into small 

 depressions on the inner surface of the calvarium. (2) On the inner surface of the 

 dura. (3) In the superior longitudinal sinus and the other sinuses. 



CAUDATE 

 NUCLEUS 



VENA 



GALE N I 



FIG. 723. Velum interposition. 



(Poirier and Charpy.) 



A hasty examination would lead us to suppose that these bodies spring from 

 the dura, but, as a matter of fact, they originate from the arachnoid. They are 

 not glandular in structure, but are simply enlarged normal villi of the arachnoid. 

 In their growth they appear to perforate the dura, and when a group of villi is 

 of large size it causes absorption of the bone, and comes to be lodged in a pit or 

 depression (foveola granularis [Pacchioni]) on the inner table of the skull. Their 

 manner of growth is as follows: At an early period they project through minute 



