THE MENINGES AND BLOOD SUPPLY 623 



coming intimately associated with neuroglia. In both the spinal cord 

 and the brain the pial septa serve for the support of numerous- Wood- 

 vessels and perivascular lymphatics which are distributed through this 

 connective tissue to all portions of the brain and spinal cord. 



Within the cranium, reduplications of the pia mater, carrying be- 

 tween their folds a layer of arachnoidal tissue and an extensive plexus 

 of small blood-vessels, push their way into the cerebral ventricles to form 

 the superior and inferior telce choroidece. These choroid plexuses are 

 separated from the ventricular cavities by an investment of cuboidal 

 cells, which in fetal and infantile life are ciliated, and which are derived 

 from and are continuous with the ependyma cells lining the walls of the 

 ventricles. Thus the blood-vessels of the telse choroidese, in the strictest 

 anatomical sense, lie without and not within the cavity of the cerebral 

 ventricles, for they are everywhere separated from those cavities by the 

 ependyma cells, which, ontogenetically at least, form a portion of the 

 wall of these vesicles. The cerebrospinal fluid is supposed to originate 

 largely by process of filtration from the blood-vessels of these choroid 

 plexuses. 



The pia mater contains mostly sympathetic fibers, but probably also a 

 few sensory cerebrospinal fibers. 



The peculiar arrangement of the three constituent membranes of the 

 meninges leaves three distinct spaces or connected groups of spaces 

 which are filled with fluid. These are the epidural, subdural, and sub- 

 arachnoidal spaces. 



The epidural space comprises a connected series of lymph cavi- 

 ties, which is of limited extent within the cranium, but of broad extent 

 within the spinal canal. These spaces are lined by endothelium which 

 is at many points continuous with the perivascular and perineural lym- 

 phatics and through them with the lymphatic vessels of the general 

 systemic circulation. Obviously the epidural spaces serve as large 

 lymphatic vessels and their cavities are consequently filled with 

 lymph. 



The subdural space has a complete lining of rather thick endothe- 

 lial cells. The walls of this cavity are formed by the dura on the outer, 

 and the arachnoid on the inner side. Its cavity is occupied by lymph 

 and is continuous with the lymphatic vessels of the dura, and through 

 them with the epidural spaces and systemic lymphatics. 



This space is penetrated by the outgoing cranial and spinal nerves, 

 which receive an investment from all three of the meningeal coats. The 

 three layers composing this investment soon lose their distinctive charac- 



