CHAPTER XXVII 

 THE NERVOUS SYSTEM (Continued) 



E. THE MENLNGES A^D BLOOD SUPPLY 



THE brain and spinal cord are enveloped by the meninges, which 

 include three fairly distinct membranes, the dura mater, arachnoid, 

 and pia mater, and two cavities filled with lymph or a lymph-like 

 fluid ; by this arrangement the cerebro-spinal axis is, as it were, 

 suspended in fluid, and is everywhere surrounded by a watery 

 cushion. 



The dura mater is the outermost of the three coats. Within 

 the cranial cavity it is firmly attached to the bony walls, and serves 

 as a periosteum for the internal surface of the bones which form 

 the cranial cavity. Within the medullary cavity of the spinal cord 

 the dura mater is distinct from the periosteum of the vertebrae, 

 with which it is connected by loose fibrous tissue and masses of fat, 

 which inclose large lymphatic spaces or chambers, lined by endo- 

 thelium and collectively forming the epidural space. 



The dura mater is composed of interlacing bundles of fibrous 

 tissue containing few elastic fibres. The disposition of its fibre 

 bundles varies somewhat in its different portions. In its spinal 

 portion, most of the bundles are longitudinally disposed, compara- 

 tively few passing circularly around the circumference of the spinal 

 canal ; within the cranial vault the bundles cross at acute angles ; 

 in the falces and in the tentorium cerebelli they are radially dis- 

 posed. 



The cranial dura consists of two distinct layers, an outer, which 

 is very vascular and serves as the bony periosteum, and an inner, 

 which is but slightly vascular and may be considered as the dura 

 propria. It is the inner layer only which is prolonged inward to 

 form the falx cerebri and the falx and tentorium cerebelli. The 

 venous sinuses of the cranium are inserted between the two layers 

 of the dura. 



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