BRAIN AND SPINAL CORD. 



CHAPTER I. 



THE MENINGES OF THE BRAIN. 



(Meninges Encephali.} 



Three membranes invest the brain and spinal cord. They 

 are, from without inward, the dura mater, the arachnoid, and 

 the pia mater. Each membrane forms a protecting sheath for 

 the cerebral or spinal nerves piercing it. 



THE DURA MATER OF THE BRAIN. 



(Dura Mater Encephali.) 



Structure and Relations. It is a very dense and inelastic 

 membrane composed of white fibrous and yellow elastic tissue 

 lined with flat endothelial cells, which constitute its internal 

 surface. In children it is closely adherent to the cranial bones of 

 which it forms the real periosteum; but it is attached chiefly at 

 the foramina and along the sutures in adults. The dura of the 

 brain is made up of two layers which are separable up to the 

 eighth or tenth year. The external layer constitutes the endos- 

 teum of the cranial bones. It is their nutrient membrane. 

 Through the cranial foramina and sutures it is continuous with 

 the external periosteum. In the adult the internal layer of the 

 dura separates from the outer layer only over the apex of the 

 petrous bone, to form Meckel's space for the semilunar ganglion 

 (Gasseri); at the foramina, to form sheaths for the nerves; and, 

 along the sinuses, to form their internal boundary and to produce 

 the great incomplete partitions, called processes, which project 

 centrally into the great fissures of the brain. 



Processes. (Processus dura matris). From the inner surface 

 of the dura the great processes are given off. The falx cerebri 



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