PROTECTIVE MECHANISMS OF THE NERVOUS SYSTEM 717 



of blood-vessels forming the choroid plexus. A similar vascular fringe 

 is suspended from the roof of the fourth ventricle. 



This subarachnoid system is in direct communication with the 

 ventricles of the brain by way of the foramen of Magendie and the 

 foramina of Luschka. It also connects with the lymphatic spaces 

 accompanying the cranial nerves, as well as with the central canal tra- 

 versing the commissure of the gray matter of the spinal cord. In 

 addition, it is placed into relation with the venous sinuses by the 

 Pacchionian bodies. The latter are pouch-like protrusions from the 

 surface of the arachnoid formed by enlargements of the normal villi 

 of this membrane. Most of these bodies are lodged in irregular pits 



FIG. 359. TRANSVERSE SECTION THROUGH THE LONGITUDINAL FISSURE TO SHOW THE 



RELATION OF THE CEREBRUM TO THE MENINGES. 



C.C., Corpus callosum; W, white matter; G, cortical gray matter; P, pia mater 

 closely investing it; A, arachnoid with its membranous prolongations forming the 

 subarachnoid space (S.S.); D, dura mater; B, skull consisting of the external and 

 internal plates separated by a spongy center; F, falx cerebri enveloping LS, the longi- 

 tudinal sinus. Into the latter extend the Pacchionian bodies. 



in the calvaria, but some of them also project into the sinuses. They 

 are particularly numerous along the superior longitudinal sinus. The 

 close contact which is thus established between the liquid filling this 

 entire system and the venous blood, might lead us to suppose that these 

 saccules serve as a means for returning some of the liquor to the blood. 

 They do not, however, constitute perfectly open outlets, and hence the 

 escape of the lymph must be brought about very largely by processes 

 of filtration and osmosis. 



The Growth of the Brain. Broca 1 states that the weight of the pia 

 mater amounts to: 



1 Elements d'Anthropologie gtfne'rale. 1885. 



