////: CENTRAL .i.v/s <>r Tin: M-:it\-<>rs >T.<//:I/. 



accumulation is impossible for nearly tho whole extent of the cranial 

 region. 



The external face of tho dura mator is very slightly adherent, especially 

 abovo, to tho walls of tho spinal canal ; and it is oven separated from them, 

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

 never absent, though tho animals be over so emaciated. This face covers, 

 inforiorly, the common superior ligament, and tho veins wo have described 

 as spinal sinuses. 



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

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

 rendered smooth and polished by tho external layer of tho arachnoid, to 

 which it is so firmly united, that it is needless to attempt their separation. 

 Here tho external layer of tho arachnoid is reduced to a simple layer formed 

 by a row of cells with flattened nuclei. 



On each side, the substance of this meningo is completely traversed by a 

 double scries of orifices for the passage of the spinal nerves, around which 

 it sends small special sheaths as far as tho intervertcbral 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 tho encephalon. The latter, 

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

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



External surface. It adheres strongly, by ccllnlo-vascular bands, to tho 

 cranial walls, whoso undulations it follows ; this adhesion is not, however, 

 equally marked everywhere, for on tho sides of tho roof of tho cerebral 

 compartment it is least intimate, and it is closest on tho 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 tho 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 

 tho ethmoidal filaments, the optic nerves, and tho two thick branches 

 furnished by tho Gasscrian ganglion. 



Internal surface. Tho internal surface of tho cranial dura mater is 

 covered by the parietal layer of tho arachnoid, which is firmly attached to 

 it only in tho spinal region. It sends into tho cranial cavity three pro- 

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

 tentorium cerebelli (lentorium, a tent), and tho pituitary fold. These processes 

 complete the partitioning of tho cranial cavity, isolate the various external 

 bulgings of the encephalic mass, and protect them from tho compression 

 they might exorcise on each other. 



a. The falx cerebri is a vortical lamina comprised between tho two 

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



Its antero-suporior border is adherent and very convex, and corresponds 

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

 face of tho frontal and parietal bones. This border is very thick, and 

 hollowed internally by a prismatic and triangular venous canal, which con- 

 stitutes tho median sinus. 



Towards its inferior border, which is free and concave, and correspond^ 

 to the corpus callosum, tho falciform process is extremely thin, and cribblcd 

 like lace-work. 



Tho posterior extremity, or base of tho falx, rests on the parietal 

 protuberance. 



