894 



THE NERVOUS SYSTEM 



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 medulla and spinal cord by way of the foramen of Magendie or aperture into the 

 fourth ventricle. In addition, there is possible an interchange of fluid between the lateral 

 ventricle and the subaraclmoid 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 endothelium covering it probably even facilitates such activities. 

 The cerebro-spinal fluid occupying the cavities is a transparent fluid of a slight yellow tinge, 

 characteristic of the 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 atrophy and shrinkage of the nervous tissues. It collects 

 t'mm the lymph spaces in the meninges, and from exudation through the walls of the vascular 

 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 



Fio. 665. DIAGRAM OF TRANSVERSE SECTION OF UPPER THORACIC REGION SHOWING THE 

 RELATIONS OF THE SPINAL MENINGES AND THEIR CAVITIES. 



DVRA MATER 



ARACHNOIDEA 

 PIA MA TER 



SEPTl'M 



, Hl'BDL'EAL TRABECUL& 



BJJBDU&AJ, BPAC& 



FILA OFnORSAL 

 HOOT 



SUB A RA CHXO1D 

 CA 11TY 



, Xjigamentum 

 denticulatum 



--- FILA OF VENTRAL 

 ROOT 



- LIXEA 8PLENDBNS with anterior 

 * spinal artery 



"**_% 

 EPWL'RAL TRABECUL& TO PERIOSTEf.M 



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



venous sinuses and iacuna; and 



THE PIA MATER 



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

 and closely adheres to the entire central nervous system and sends numerous pro- 

 cesses into its substance. It likewise contributes the most proximal and compact 

 portion of the sheaths worn by the nerve-roots in their passage through the meningeal 

 paces. It is very vascular in that the superficial plexuses of blood-vessels of both 

 >. brain and spinal cord ramify in it as they give off the central branches into 

 substance The structure and arrangement of the membrane vary 

 somewhat in the cranial and spinal regions. 



The spinal pia mater consists of two layers, an inner and an outer. It is thicker 

 and more compact than that of the encephalon, due to the extra development of its 

 outer layer, which is in the form of a strong, fibrous layer with the fibres arranged 



