MEMBRANES COVERING THE CENTRAL NERVOUS SYSTEM. 297 



one which in all respects resembles the dura mater spinalis, and 

 an outer one which lines the inner surface of the skull like a 

 periosteum. In the latter layer there are two sheaths of fibres 

 crossing one another. According to Luschka, the inner sur- 

 face of the dura mater cerebralis is covered with a double layer 

 of flattened epithelium. The dura mater is poor in blood- 

 vessels, with the exception of the outer layer of the dura mater 

 cerebralis, which contains numerous vessels and sends off many 

 to the bones of the skull. According to some authors, the 

 spaces which can be injected in the dura are to be considered 

 as lymph-channels in communication with the subdural space, 

 and lined partly with endothelium. 



The dura mater cerebralis contains many nerves which end 

 partly in the dura itself and partly in the vessels. Those 

 ending in the dura, (nervi proprii) break up into numerous 

 branches, which terminate between the endothelial cells on 

 the inner surface of the dura by means of bulbar thickenings. 



The arachnoidea is a thin membrane made up of connective- 

 tissue bundles joined together in a net-like structure. It con- 

 tains numerous elastic fibres, and is clothed on both sides by a 

 layer of endothelial cells. From its inner surface it sends off 

 many connective- tissue strands, which pass through the sub- 

 araclmoidal space to join with the pia mater. These also are 

 covered by endothelium. There are to be found in the arach- 

 noidea neither vessels nor nerves. In certain places (e. g., on 

 both sides of the sinus sagittalis superior) there are on the outer 

 surface of the arachnoidea non- vascular villus-like outgrowths, 

 which bulge out the thin inner lamella of the dura and project 

 into the venous sinuses. These are the so-called Pacchionian 

 bodies or granulationes arachnoidales (Pacchioni). 



The pia mater is a very thin membrane made up of fine con- 

 nective tissue. It covers the whole brain and spinal cord, and 

 follows all the sulci and down-dippings of the surface. In the 

 cord it forms the septum longitudinale ventrale. There are to 

 be distinguished in the pia mater of the spinal cord two layers : 

 the outer one resembling the arachnoidea in structure, and join- 

 ing with the connective-tissue strands passing across the sub- 



