NERVOUS CENTRES. (HUMAN ANATOMY. THE MENINGES.) 



C29 



The cranial dura mater is not a simple bag. 

 From its internal surface partition-like processes 

 pass inwards, which serve to separate certain 

 subdivisions of the encephalon. These are, 

 theJaLr cerebri, the tentorium cerebelli, and the 

 J'ulx cerebelli. 



The falx cerebri is a process of fibrous mem- 

 brane corresponding to the mesial plane and 

 lying in the great median fissure of the brain, 

 where it separates the lateral hemispheres from 

 each other. Its shape is falciform ; its superior 

 convex border corresponds to the frontal and 

 sagittal sutures, and encloses the great longitu- 

 dinal sinus ; its inferior border is concave and 

 much shorter than the superior, and corres- 

 ponds to the superior surface of the corpus 

 callosum, which connects the hemispheres of 

 the brain. In front the falx is very narrow and 

 almost pointed ; it embraces the crista galli of 

 the ethmoid bone, which appears to be enclosed 

 between its layers. As the falx proceeds back- 

 wards it increases considerably in depth ; its 

 superior edge may be traced back to the internal 

 occipital protuberance; its inferior edge termi- 

 nates at a point corresponding to the middle 

 line of the posterior margin of the corpus 

 callosum. The falx cerebri contains within it, 

 along its posterior border, a large vein, which 

 is called the inferior longitudinal sinus. 



The falx cerebri is continuous at its posterior 

 border on each side with the tentorium cere- 

 belli. This process is nearly horizontal in its 

 direction ; it forms a vaulted roof to a cavity 

 (the floor of which corresponds to the occipital 

 fossa) in which the cerebellum is lodged. Its 

 upper surface is convex on each side of the 

 attachment of the posterior extremity of the 

 falx cerebri : it supports the posterior lobes of 

 the brain. The inferior surface is adapted to 

 the upper convex surfaces of the cerebellar 

 hemispheres. Its posterior and outer edge 

 adheres to the occipital bone and to the pos- 

 terior border of the petrous portion of the 

 temporal bone, reaching as far inwards as the 

 posterior clinoid processes of the sella Turcica. 

 The occipital portion of this edge contains a 

 considerable part of the lateral sinus (Jig 362, e) 

 and that portion which adheres to the petrous 

 bone contains the superior petrosal sinus. The 

 anterior or inner margin of the tentorium is 

 concave and free in the greater part of its 

 extent; it forms the posterior and lateral boun- 

 dary of a large opening (which the sella Tur- 

 cica completes in front), through which the 

 crura cerebri and other parts connected with them 

 pass. This margin is attached by its anterior 

 extremities to the anterior clinoid processes, to 

 reach which it crosses the posterior border. 

 The crossing of these two edges at a point 

 external to the sella Turcica gives rise to the 

 formation of a little triangular space, the base 

 of which corresponds to the sella Turcica; its 

 outer angle is perforated for the transmission of 

 the third pair of nerves ; and its anterior one for 

 that of the fourth pair. 



From the inferior surface of the tentorium 

 cerebelli at its posterior edge, a short and thick 

 fold of very slight depth descends to die pos- 

 terior edge of the foramen magnum. This is 



ihej'alx cerebelli ; it corresponds to the median 

 notch between the hemispheres of the cerebellum . 

 Its anterior border is slightly concave. Two 

 veins called occipital sinuses are contained in it. 

 The internal surface of the cranial dura mater 

 presents the same smooth appearance as we 

 have noticed in the spinal membrane of the 

 same name. We observe, however, an excep- 

 tion to this on each side of the line along the 

 great longitudinal sinus. Here it is very com- 

 mon to find the membrane presenting a peculiar 

 cribriform appearance, which occupies a space 

 of from half an inch to two inches in length and 

 not more than a quarter of an inch in breadth, 

 but exhibiting great difference in various sub- 

 jects as to the number and deptli of the foramina 

 or depressions upon which the sieve-like struc- 

 ture depends. These depressions are caused 

 by the presence of little bodies which grow 

 from the layer of arachnoid that covers the 

 brain, glanc/ultf Pacchioni, which will be 

 described by-and-bye. The anatomist may 

 expect to find in a large proportion of adult 

 brains a greater or less degree of adhesion 

 between these parts of the dura mater and the 

 edges of the hemispheres of the brain. 



The dura mater is perforated by numerous 

 orifices for the transmission of the encephalic 

 nerves. It adheres firmly to the border of each 

 of the foramina in the cranial bones, and is 

 partly continued in the shape of neurileinma 

 over the nerve that escapes through it. In the 

 case of the optic nerve a strong fibrous sheath 

 is prolonged from the dura mater, and at the 

 same time that membrane appears to become 

 continuous with the periosteum of the orbit, as 

 if it had, opposite the optic foramen, split into 

 two layers, one of which formed the sheath of 

 the optic nerve, and the other applied itself to 

 the interior of the orbit, forming a periosteum 

 to the walls of that cavity. 



Of the arteries and veins of the dura mater. 

 The disposition of the bloodvessels of the 

 dura mater, both of the spine and of the cra- 

 nium, deserves a special description. The 

 former membrane derives its arteries from the 

 numerous vessels which take their rise close to 

 the spinal column in its various regions* These 

 are ramifications of the abdominal and thoracic 

 aorta or of their large primary branches. In the 

 neck the deep cervical, the occipital, and the 

 vertebral arteries send in numerous branches, in 

 the back the intercostal arteries, and in the loins 

 the lumbar arteries. These vessels pass in at 

 the vertebral foramina, and send branches to 

 the spinal membranes as well as to the bones 

 themselves. 



The blood which is returned from the spinal 

 cord and its membranes, as well as from the 

 vertebra, is poured into a very intricate plexus 

 of veins which surrounds the dura mater on 

 its lateral and posterior surfaces, ramifying 

 among the lobules of soft fat by which the 

 exterior of that membrane is invested. This 

 plexus is less intricate in the dorsal than in 

 the cervical or lumbar regions; it communi- 

 cates very freely with the plexus of veins which 

 lies on the exterior of the vertebral laminae and 

 processes (the dorsi-spinal veins of Dupuytren). 



