620 THE NERVOUS SYSTEM 



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 vertebral cavity 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 lymph spaces or chambers, 

 lined by endothelium and collectively forming the epidural space. 



The dura mater is composed of interlacing bundles of fibrous tissue 

 containing few elastic fibers. The disposition of its fiber bundles varies 

 somewhat in its different portions. In its spinal portion, most of the 

 bundles are longitudinally disposed, comparatively few passing circularly 

 around the circumference of the vertebral canal; within the cranial 

 vault the bundles cross at acute angles; in the falces and in the ten- 

 torium cerebelli they are radially disposed. 



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 proper. It 

 is the inner layer only which is prolonged inward to form the falx 

 oerebri and the falx and tentorium cerebelli. The venous dural sinuses 

 of the cranium occupy clefts in the dura along the lines of attach- 

 ment. 



Although the dura mater is but poorly supplied with blood-vessels, 

 it is relatively rich in lymphatics, which open into the subdural and 

 epidural spaces and are continuous with the perivascular and perineural 

 lymphatics which leave the cerebrospinal cavities in company with 

 the cerebral and spinal nerves and the larger blood-vessels. In this 

 way the lymphatics of the dura mater and its adjacent spaces are in 

 communication with the lymphatic vessels of the eye, nose, ear, and 

 cervical lymph nodes. These communications are of special importance 

 as indicating the path followed by certain pathological processes which 

 involve the meninges. 



Where the outer surface of the dura is not attached to the surround- 

 ing bone or connective tissue, it is covered by a thin endothelioid coat, 

 the lining endothelium (mesenchymal epithelium) of the epidural spaces. 

 Its inner surface is lined by somewhat thicker endothelial cells, forming 

 the wall of the subdural space. The dura contains sympathetic fibers 

 for its blood-vessels, and also naked spinal and cerebral sensory 

 fibers. 



The arachnoid is a thin membranous sheet which is suspended be- 

 tween the dura and the pia mater. It is composed of a delicate areolar 



