640 THE NERVOUS SYSTEM. 



and contains the blood-vessels for their supply. At the margin of the foramen 

 magnum it becomes continuous with the periosteum lining the spinal canal. The 

 meriingeal or supporting layer is lined on its inner surface by a layer of nucleated 

 endotheliurn, similar to that found on serous membranes : these cells were formerly 

 regarded as belonging to the arachnoid membrane. By its reduplication the 

 meningeal layer forms the falx cerebri, the tentorium and falx cerebelli, and the 

 diaphragma sellse. The two layers are connected by fibres which intersect each 

 other obliquely. 



Its arteries are very numerous, but are chiefly distributed to the bones. Those 

 found in the anterior fossa are the anterior meningeal branches of the anterior and 

 posterior ethmoidal and internal carotid, and a branch from the middle meningeal. 

 In the middle fossa are the middle and small meningeal branches of the internal 

 maxillary, a branch from the ascending pharyngeal, which enters the skull through 

 the foramen lacerum medium basis cranii, branches from the internal carotid, and 

 a recurrent branch from the lachrymal. In the posterior fossa are meningeal 

 branches from the occipital, one of which enters the skull through the jugular 

 foramen, and the other through the mastoid foramen ; the posterior meningeal, 

 from the vertebral ; occasionally meningeal branches from the ascending pharyngeal, 

 which enter the skull, one at the jugular foramen, the other at the anterior 

 condyloid foramen, and a branch from the middle meningeal. 



The veins, which return the blood from the dura mater, and partly from the 

 bones, anastomose with the diploic veins. These vessels terminate in the various 

 sinuses, with the exception of two which accompany the middle meningeal artery, 

 and pass out of the skull at the foramen spinosum to join the internal maxillary 

 vein ; above they communicate with the superior longitudinal sinus. Many of 

 the meningeal veins do not open directly into the sinuses, but indirectly through 

 a series of ampullas termed venous lacunae. These are found on each side of the 

 superior longitudinal sinus, especially near its middle portion, and are often 

 invaginated by Pacchionian bodies ; they also exist near the lateral and straight 

 sinuses. They communicate with the underlying cerebral veins, and also with 

 the diploic and emissary veins. 



The nerves of the dura mater are filaments from the Gasserian ganglion, from 

 the ophthalmic, superior maxillary, inferior maxillary, vagus, and hypoglossal 

 nerves, and from the sympathetic. 



Processes of the Dura Mater. The processes of the dura mater, sent inward 

 into the cavity of the skull, are four in number : the falx cerebri, the tentorium 

 cerebelli, the falx cerebelli, and the diaphragma sellse. 



The falx cerebri, so named from its sickle-like form, is a strong arched process 

 of the dura mater, which descends vertically in the longitudinal fissure between 

 the two hemispheres of the brain. It is narrow in front, where it is attached to 

 the crista galli of the ethmoid bone, and broad behind, where it is connected with 

 the upper surface of the tentorium. Its upper margin is convex, and attached to 

 the inner surface of the skull, in the middle line, as far back as the internal occip- 

 ital protuberance ; it contains the superior longitudinal sinus. Its lower margin 

 is free, concave, and presents a sharp curved edge, which contains the inferior 

 longitudinal sinus. 



'The tentorium cerebelli is an arched lamina of dura mater, elevated in the 

 middle and inclining downward toward the circumference. It covers the upper 

 surface of the cerebellum, and supports the occipital lobes of the brain, and 

 prevents them pressing upon the cerebellum. It is attached, behind, by its convex 

 border to the transverse ridges upon the inner surface of the occipital bone, and 

 there encloses the lateral sinuses ; in front, to the superior margin of the petrous 

 portion of the temporal bone on either side, enclosing the superior petrosal 

 sinuses ; and at the apex of this bone the free or internal border and the attached 

 or external border meet, and, crossing one another, are continued forward, to 

 be attached to the anterior and posterior clinoid processes respectively. Along 

 the middle line of its upper surface the posterior border of the falx cerebri is 



