920 



THE NERVOUS SYSTEM 



formed by the denticulate ligaments, which extend laterally from either side of the spinal cord, 

 connecting the pia and dura mater and involving the arachnoid in passing through it. Within 

 the subarachnoid cavity these form more or less complete septa, though outside the arachnoid 

 they are attached to the dura only at the intervals of their pointed dentations. They belong 

 to the pia mater and wiU be described with it. The arachnoid is further continuous with 

 the pia by way of the connective-tissue sheaths of the roots of the spinal nerves and the blood- 

 vessels passing through the subarachnoid cavity. 



Vessels and nerves.- — The arachnoid has no special blood supply and probably no special 

 nerves other than those supplying the walls of the blood-vessels passing through it. 



The cerebro-spinal fluid. — The subarachnoid cavity is the great lymph-space of the central 

 nervous system. That of the spinal region is directly continuous into that of the cranium, and 

 the fluid contained communicates freely with that in the ventricles of the brain and the central 

 canal of the meduUa and spinal cord by way of the foramen of Magendie or medial aperture 

 into the fourth ventricle. In addition, there are the lateral apertures into the fourth ventricle 

 and there is possible an interchange of fluid between the lateral ventricle and the subarachnoid 

 cavity of the base of the brain by diffusion through the thin floor of the chorioid fissure. The 

 arachnoid throughout is not a membrane sufficiently compact to seriously oppose diffusion 

 between the fluid contained in its cavity and that contained in the subdural cavity, and the 

 endotheHum covering it probably even facilitates such activities. The cerebro-spinal fluid 

 occupying the cavities is a transparent fluid of a sfight yeUow tinge, characteristic of the 



Fig. 725. — Diagram of Transverse Section of Upper Thoracic Region of the Spinal 

 Cord showing the Relations of the Spinal Meninges and their Cavities. 

 Dura mater 

 I Arachnoidea 



' { Pia mater 



Septum posticum 



Subdural trabeculae 



Subdural space 



Fila of dorsal root 

 ^., Subarachnoid cavity 



Denticulate ligament 



§^. ■"■ Fila of ventral root 



* Linea splendens with anterior 



^ \ spinal artery 



Epidural trabeculae to periosteum 



lymph in other lymph-spaces of the body. It is not very great in amount, probably never 

 exceeding 200 c.c. in normal conditions. It is greatest in amount in old age, when the cavities 

 are larger, due to atrojjhy and shrinkage of the nervous tissues. It coHects from the lymph 

 spaces in the meninges, and from exudation through the walls of the vascular chorioid plexuses 

 and sinuses of the system it bathes. Its amount may be temporarily increased by a period 

 of increased blood-pressure in the cranial vessels. Pressure due to its abundance may be 

 relieved by diffusion through the membranes containing it, and especially through the villi 

 of the Pacchionian bodies into the venous sinuses and lacunie and thence into the venous system 

 through the internal jugular veins. 



The Pia Mater 



The pia mater, the third of the meninges, is a thin membrane which envelopes 

 and closely adlu^res to the entire central nervous system and sends numerous proc- 

 esses into its substance. It likewise contributes the most i)roxiinal antl compact 

 portion of the sheaths worn by the nerve-roots in their passa{2;e throufijh the menin- 

 geal spaces. It is very vascular in that the superficial i)lexuses of blood-vessels of 



