750 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



The external face of the dura mater is very slightly adherent — especially above 

 — to the walls of the spinal canal ; and it is even separated from them, at the inter- 

 vertebral spaces, by a certain quantity of adipose tissue which is never absent, 

 though the animals be ever so emaciated. This face covers, inferiorly, the com- 

 mon superior ligament, and the veins we have described as spinal sinuses. 



(It does not form an endosteum for the vertebrae as it does for the cranial 

 bones.) 



The internal face gives attachment, between each pair of nerves, to the 

 festoons of the dentated membrane, a dependency of the pia mater. It is 

 rendered smooth and polished by the external layer of the arachnoid, to which it 

 is so firmly united that it is needless to attempt their separation. Here the 

 external layer of the arachnoid is reduced to a simple row of cells with flattened 

 nuclei. 



On each side, the substance of this meninge is completely traveled by a 

 double series of orifices for the passage of the spinal nerves, around which it sends 

 small special sheaths as far as the intervertebral foramina. 



Cranial or Encephalic Dura Mater (Dura Mater Cerebralis). — 

 This membrane forms a sac which is exactly moulded by its external face to 

 the cranial parietes, and by its internal face to the surface of the brain. The 

 latter, therefore, completely fills the cavity of the cranium, a circumstance that 

 explains why an accumulation of fluid is impossible in this region. 



External surface. — It adheres strongly, by cellulo-vascular bands, to the cranial 

 walls, the undulations on which it follows. This adhesion is not, however, equally 

 marked everywhere, for on the sides of the roof of the cerebral compartment it 

 is least intimate, and it is closest on the middle plane of this roof, on the crista 

 galli, around the parietal protuberance, on its crests, and towards the lateral faces 

 of the cerebellar compartment at the petrous bones, where the membrane is very 

 thin. 



This face gives rise to a number of prolonged sheaths, corresponding to the 

 nerves leaving the base of the cranium. The principal are found around the 

 ethmoidal filaments, the optic nerves, and the two thick branches furnished by 

 the Gasserian ganglion. 



Internal surface. — The internal surface of the cranial dura mater is covered 

 by the parietal layer of the arachnoid, which is firmly attached to it only in the 

 spinal region. It sends into the cranial cavity three prolongations, which are 

 distinguished as the falx cerebri {falx, " a sickle "), tentorium cerebelU {tentorium, 

 " a tent "), and the pituitary fold. These processes complete the partitioning of 

 the cranial cavity, isolate the various external bulgings of the encephalic mass, 

 and protect them from the compression they might exercise on each other. 



a. The Falx Cerebri is a vertical middle layer comprised between the two 

 cerebral hemispheres, and owes its name to its sickle-like form. 



Its antero-superior border is adherent and very convex, and corresponds to 

 the crista galli process, as well as to the median ridge on the inner face of the 

 frontal and parietal bones. This border is very thick, and hollowed internally 

 by a prismatic and triangular venous canal — the middle sinus. 



Towards its inferior border, which is free and concave, and rests on the 

 corpus callosum, the falciform process is extremely thin, and cribbled like 

 lacework. 



The posterior extremity, or base of the falx, rests on the parietal protu- 

 berance. 



