312 NORMAL HISTOLOGY. 



The Dura Mater. Within the skull (dura mater encephali), the 

 membrane consists of an outer and an inner layer. The former replaces the 

 periosteum and is intimately attached to the bones to which it carries nutritive 

 blood-vessels (branches of the meningeal arteries). The inner layer forms 

 the incomplete fibrous partitions, as the falx and tentorium, which separate 

 and support the several subdivisions of the brain. Along the attachments 

 of these partitions, the inner layer splits to form the walls of the large venous 

 spaces, the dural sinuses, which are lined with endothelium and constitute 

 the channels into which the blood returned by the veins from the nervous 

 tissue is poured. 



Within the vertebral canal (dura mater spinalis)jhs. dura forms a loose 

 sac for the cord which corresponds to a prolongation of the inner cranial 

 layer. It loses its intimate relation to the bones at the foramen magnum and 

 lies within the vertebral canal often separated from the periosteum by con- 

 siderable tracts of areolar tissue. 



In structure, the dura consists of closely placed bundles of unusually 

 rigid fibrous tissue, intermingled with elastic fibres. Although the latter 

 exist in considerable numbers, especially in the inner layer of the brain-dura, 

 they are so overshadowed by the preponderance of the fibrous tissue that the 

 membrane as a whole is relatively inelastic. Within the outer layer in the 

 skull, the fibres pursue a general antero-lateral to postero-medial direction, 

 while those of the inner layer follow an opposite, antero-medial to postero- 

 lateral, course. Within the spinal sac, their disposition is chiefly longitudinal. 

 The connective tissue cells are represented by flattened plate-like elements 

 between the fibrous bundles and some plasma cells in the vicinity of the 

 blood-vessels. The inner surface of the dura, the outer .wall of the subdural 

 space, is covered by a continuous layer of endothelial cells. The existence 

 of isolated patches of endothelium on the outer surface of the membrane 

 within the skull, is regarded as evidence of the existence of limited epidural 

 spaces, 



The blood-vessels of the brain-dura, not taking into account the large 

 venous sinuses, are all branches from the various meningeal arteries. In 

 addition to supplying the dural tissue, their purpose is to provide nourish- 

 ment to the bones of the cranium, which, therefore, are the objective dis- 

 tribution for the larger part of the terminal vessels. The outer layer, being 

 virtually the periosteum, contains many more vessels than the inner, the larger 

 trunks showing as elevated ridges on the cranial surface. Meningeal veins 

 are also present, but, in many cases, do not accompany the arteries and pur- 

 sue an independent course. The spinal dura contains comparatively few 

 blood-vessels. 



The nerves within the dura are numerous and include two sets those 

 destined for the walls of the blood-vessels, the more plentiful and sympa- 

 thetic in character, and the less numerous nervi proprii which contain sen- 

 sory fibres derived from the cranial and spinal nerves. They end in free 

 filaments or in bulbus expansions. 



The Pia Mater. This membrane, the vascular tunic, lies in contact 

 with all parts of the cerebro-spinal axis and, since it contains the blood- 

 vessels supplying the nervous substance, accurately follows all the irregu- 

 larities of the surface of the brain, with its many convolutions and fissures, 

 and of the spinal cord. Additionally, in certain places where the wall of the 

 brain-tube is always very thin, the pia pushes before it the attenuated brain- 

 layer and seemingly gains entrance into the ventricles. Examples of such 

 invagination are afforded in the relations of the velum interpositum and the 



