1198 



HUMAN ANATOMY. 



ventricle (page noo). The paths by which the fluids collected within the brain- 

 membrane are carried off, thereby insuring under normal conditions the prevention 

 of excessive intracranial tension, will be considered with the description of the dura 

 and arachnoid, suffice it here to mention the sheaths contributed by these envelopes 

 along the nerve-trunks as they leave the cranium and the Pacchionian bodies as the 

 most important. 



The Dura Mater. This structure (dura mater encephali ) is a dense and inelas- 

 tic fibrous membrane, which lines the inner surface of the cranial cavity and sends 

 partitions between the divisions of the brain. In contrast to its relation within the 

 vertebral canal, where it is separated from the bony wall by a considerable space 

 (page 1022), within the brain-case the dura everywhere lies closely applied to the 

 bone a relation essential in fulfilling its function as a blood-carrying organ for the 

 nutrition of the cranium. Around the margins of the larger foramina, over the pro- 

 jecting inequalities of the fossae and along the lines of the more important sutures, the 

 attachment of the dura to the skull is particularly close, and at some of these points 



FIG. 1033. 



erebri 



Junction of falx 

 and tentorium 



Tentorium cerebelli 



Opening for brain-stem 



'iaphragma sellse 

 ree margins of tentorium 



Portion of skull removed, showing partitions of dura in place. 



the foramina and the ununited sutures the dura is continuous with the periosteum 

 covering the exterior of the skull. On separating the dura from the bone, as may be 

 readily done beneath the calvaria, except along the line of the sagittal suture, its 

 outer surface is marked with the conspicuous ridges produced by the meningeal 

 blood-vessels, which lie much nearer the outer than the inner surface of the mem- 

 brane and hence give rise to the corresponding furrows seen on the inner aspect 

 of the skull. In addition, the roughened surface of separation is beset with fine 

 fibrous processes, the larger of which contain minute blood-vessels, that have 

 been drawn out of the canals affording passage for the nutrient twigs. The inner 

 aspect of the dura, on the contrary, is smooth and shining and clothed with a layer 

 of endothelium which lines the outer wall of the subdural space. As the nerves enter 

 the foramina in their exit from the cranium, they receive a tubular prolongation of 

 the dura which accompanies the nerve-trunk for a short distance as the dural sheath, 

 separated from the nerve by the underlying subdural cleft, and finally becomes con- 

 tinuous with the epineurium, whilst the subdural space communicates with the 

 lymph-clefts within the connective tissue envelopes of the nerves. The dural sheath 



