MEMBRANES OF THE BRAIN. 



upon the posterior fossae of the base of the cranium. As the 

 brain is a double organ, each of these parts is symmetrical, or 

 consists in right and left halves perfectly alike. 



SECT. I. OF THE MEMBRANES OF THE BRAIN, OR ENGEPHALON. 



Of the Dura Mater. 



This membrane, the most exterior of the three belonging to 

 the encephalon, lines the whole internal face of the cavity of 

 the cranium, and is attached with great tenacity to it, particu- 

 larly in early life, from which cause it is also considered as 

 an internal periosteum. 



Its external surface has a rough and unequal appearance, and 

 adheres much more strongly where the sutures exist than on 

 the common surface of the bones, owing to its detaching many 

 large filaments, which penetrate into the sutures and reach to 

 the pericranium. Its adhesion to the surface generally of the 

 bones is accomplished by fine filaments of fibres, and by very 

 numerous and small blood vessels which become evident from 

 the dots of blood collected upon it, when the bones are torn up, 

 as in the usual manner of examining the head. To the base 

 of the cranium, its adhesion is still stronger, owing to the abun- . 

 dance of the foramina and fissures there; to the margin of each 

 one of the foramina it is fixed with extreme compactness, and 

 may be considered as continuous with the adjacent pericra- 

 nium. 



The external surface of the Dura Mater is marked by the 

 arteries and veins which creep and ramify through it, and 

 make, as mentioned elsewhere, corresponding furrows in the 

 bones. 



The Dura Mater consists of two laminae, one within the 

 other; they, however, are attached so closely in the greater 

 part of their extent, that it requires the knife, or strong artificial 

 force, to separate them. Sometimes, in tearing off the skull- 

 cap of a middle-aged person, the external lamina is brought 

 away with the bone. 



Several processes arise by a duplicature of the internal la- 

 mina of the dura mater, and extend from the circumference to- 



