THE MEMBRANES OF THE BRAIN. 



I2OI 



thelial plates which constitute the immediate outer wall of the subdural lymph-space. 

 Patches of endothelium sometimes seen on the external aspect of the membrane are 

 regarded as indications of uncertain epidural lymph-spaces. The outer or periosteal 

 lamella is less compact and richer in cells than the inner layer and contains a wide- 

 meshed net-work of capillary blood-vessels. The larger bundles of fibrous tissue are 

 disposed with some order so that a definite radiation from the two ends of the falx 

 cerebri may often be recognized. Within the last-named fold, from the point where 

 the free border of the falx and that of the tentorium meet, the fibres radiate towards 

 the convex attached margin, some, therefore, arching far forward. From the same 

 point the fibres within the tentorium pass laterally. 



Minute calcareous concretions, also known as brain-sand or acervulus, are 

 not infrequently found in the otherwise normal dura, especially in subjects of 

 advanced years. They consist of aggregations of particles of calcium carbonate and 

 phosphate arranged in concentric layers and surrounded by a capsule of fibrous 

 tissue. They seldom exceed a diameter of .070-. 080 mm., but may be so numer- 

 ous that a distinctly gritty feel is imparted to the inner surface of the dura. 



The blood-vessels within the dura are the branches of the meningeal arteries, 

 and their accompanying veins, derived from various sources from the ophthalmic, 



FIG. 1035. 



Medullary branch 



Larger pial artery 



White matter 



Pia within fissure- 

 Portion of injected cerebral cortex, showing capillary supply of gray and white matter. X 18. 



internal maxillary, vertebral, ascending pharyngeal and occipital arteries. They are 

 destined, for the most part, for the nutrition of the skull, which they enter as minute 

 twigs through innumerable openings in the bone. Some few perforating arteries 

 traverse the bone and communicate with the pericranial vessels, whilst others are 

 distributed to the tissue of the dura itself. 



Definite lymphatics have not been demonstrated within the dura, the system 

 of absorbent vessels being represented within this membrane by numerous lymph- 

 spaces within the connective tissue stroma. These communicate indirectly with the 

 subdural lymph-space, the contained fluid escaping at the foramina chiefly into the 

 lymph-paths surrounding the cranial nerves, but to some extent also directly into 

 the venous sinuses around the Pacchionian bodies. 



The nerves of the dura include principally sympathetic fila/nents, distributed 

 to the blood-vessels and to the bone, and sensory fibres. The immediate sources 

 are the meningeal twigs contributed by the trigeminus, the vagus and the hypo- 

 glossal nerves. Those from the last source, apparently from the twelfth, are really 

 sensory fibres from the upper cervical spinal nerves and sympathetic filaments from 

 the cervical sympathetic cord ; in the other cases, the sensory fibres are probably 

 accompanied by sympathetic filaments, which secure this companionship by means of 



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