662 THE CENTS AL AXIS OF THE NERVOUS SYSTEM. 



accumulation is impossible for nearly the whole extent of the cranial 

 region. 



The external face of the dura inater is very slightly adherent, especially 

 above, to the walls of the spinal canal ; and it is even separated from them, 

 at the intervertebral spaces, by a certain quantity of adipose tissue which is 

 never absent, though the animals be ever so emaciated. This face covers, 

 inferiorly, the common superior ligament, and the veins we have described 

 as spinal sinuses. 



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 layer formed 

 by a row of cells with flattened nuclei. 



On each side, the substance of this meninge is completely traversed 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 OB ENCEPHALIC DURA MATER. This membrane forms a sac 

 which is exactly moulded by its external face to the cranial parietes, and by 

 its internal face to the superficial surface of the encephalon. 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, whose undulations 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 pro- 

 longations, which are distinguished as the falx cerebri (falx, a sickle), 

 tentorium cerebelli (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 lamina 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, which con- 

 stitutes the median sinus. 



Towards its inferior border, which is free and concave, and corresponds 

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

 like lace-work. 



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

 protuberance, 



