878 



NEUROLOGY 





The Arachnoid Villi (granulationes arachnoideales; glandule? Pacchioni; Pacch'w- 

 nian bodies) (Fig. 769) are small, fleshy-looking elevations, usually collected into 

 clusters of variable size, which are present upon the outer surface of the dura 

 mater, in the vicinity of the superior sagittal sinus, and in some other situations. 

 Upon laying open the sagittal sinus and the venous lacunae on either side of it 

 villi will be found protruding into its interior. They are not seen in infancy, 

 and very rarely until the third year. They are usually found after the seventh 

 year; and from this period they increase in number and size as age advances. 

 They are not glandular in structure, but are enlarged normal villi of the arach- 

 noid. As they grow they push the thinned dura mater before them, and cause 

 absorption of the bone from pressure, and so produce the pits or depressions 

 on the inner wall of the calvarium. 



Emissary vein 



Venous lacuna \ Sup. sagittal sinus 



Cerebral vein 



Arachnoid granulation 



Meningeal vein 



Subdural cavity 

 Subarachnoid cavity 



Falx cerebri 

 Pia mater 



Dura mater 

 \ 

 Arachnoid 



Cerebral cortex 



Fio. 769. Diagrammatic representation of a section across the top of the skull, showing the membranes of the 



brain, etc. (Modified from Testut.) 



Structure. An arachnoidal villus represents an invasion of the dura by the arachnoid mem- 

 brane, the latter penetrates the dura in such a manner that the arachnoid mesothelial cells come 

 to lie directly beneath the vascular endothelium of the great dural sinuses. It consists of the 

 following parts: (1) In the interior is a core of subarachnoid tissue, continuous with the mesh- 

 work of the general subarachnoid tissue through a narrow pedicle, by which the villus is attached 

 to the arachnoid. (2) Around this tissue is a layer of arachnoid membrane, limiting and enclosing 

 the subarachnoid tissue. (3) Outside this is the thinned wall of the lacuna, which is separated 

 from the arachnoid by a potential space which corresponds to and is continuous with the subdural 

 cavity. (4) And finally, if the villus projects into the sagittal sinus, it will be covered by the 

 greatly thinned wall of the sinus which may consist merely of endothelium. It will be seen, there- 

 fore, that fluid injected into the subarachnoid cavity will find its way into these villi, and it has 

 been found experimentally that it passes from the villi into the venous sinuses into which they 

 project. 



The Pia Mater. 



The pia mater is a vascular membrane, consisting of a minute plexus of blood- 

 vessels, held together by an extremely fine areolar tissue and covered by a reflexion 

 of the mesothelial cells from the arachnoid trabeculae. It is an incomplete mem- 



