THE MEMBRANES OF THE CENTRAL NERVOUS SYSTEM. 437 



layer of the dura is continued some distance along the cerebrospinal 

 nerves. 



The dura mater of the spinal cord does not form the periosteum 

 for the bones forming the vertebral canal ; these possess their own 

 periosteum. The spinal dura mater is covered on its outer surface 

 by a layer of endothelial cells and is separated from the wall of the 

 vertebral canal by the epidural space, containing a venous plexus 

 imbedded in loose areolar connective tissue and adipose tissue. 



The dura consists chiefly of connective-tissue bundles having a 

 longitudinal direction along the spinal cord. Within the cranium, 

 however, the bundles of the inner and outer layers cross each other ; 

 those of the outer having a lateral direction anteriorly and a mesial 

 posteriorly ; those of the inner, a mesial direction anteriorly and a 

 lateral posteriorly. In the falx cerebri, tentorium, etc., the fibers are 

 arranged radially, extending from their origin toward their borders. 

 The shape and size of the connective-tissue cells vary greatly, and 

 their processes form a network around the bundles of connective 

 tissue. Few elastic fibers are present, and, according to K. Schultz, 

 these are entirely absent in the new-born ; they are somewhat more 

 numerous in the dura of the spinal cord. The dura is very rich in 

 blood-capillaries, and the presence of lymphatic channels in com- 

 munication with the subdural space may be demonstrated by means 

 of puncture-injections. The inner surface of the dura mater is cov- 

 ered by a layer of endothelial cells. 



The dura mater is quite richly supplied with nerves, especially 

 in certain regions. These are of two varieties : (i) Vasomotor fibers, 

 which form plexuses in the adventitial coat of the arteries, and 

 would seem to terminate in the muscular coat of the arteries ; (2) 

 medullated nerve-fibers, which either accompany the blood-vessels 

 in the form of larger or smaller bundles or have a course inde- 

 pendent of the vessels. After repeated division these medullated 

 nerve-fibers lose their medullary sheaths and terminate between the 

 connective-tissue bundles in fine varicose fibrils, which -may often 

 be traced for long distances (Huber, 99). 



The arachnoid is separated from the dura by a space which is 

 regarded as belonging to the lymphatic system the subdural space. 

 The outer boundary of the arachnoid consists, as does the inner lin- 

 ing of the dura, of a layer of flattened endothelial cells. The arach- 

 noid is made up of a feltwork of loosely arranged connective-tissue 

 trabeculse, which also penetrate into the lymph-space between it 

 and the pia the subarachnoid space. For a short distance from 

 their points of origin the cerebrospinal nerves are accompanied by 

 arachnoid tissue. In the brain the arachnoid covers the convolu- 

 tions and penetrates with its processes into the sulci. These pro- 

 cesses are especially well developed in the so-called cisterns ; in 

 the cisterna cerebellomedullaris, fossae Sylvii, etc. In the spinal 

 cord the subarachnoid space is separated by the ligamenta den- 

 ticulata into two large communicating spaces a dorsal and a ven- 



