374 PROCESSES OF THE DURA MATER. 



It covers the brain in general, enters double between all its 

 convolutions, and lines the different cavities called ventricles. 



It serves to conduct and support the vessels of the brain, and 

 allows them to divide into such minute parts, as to prevent the 

 blood from entering the tender substance of this viscus with too 

 great force. 



The arteries of the pia mater are the same with those of the 

 brain and are derived from the internal carotids and vertebrals. 



The veins differ in no respect from those of the other viscera, 

 excepting in this, that they do not accompany the arteries. 



It gets its nerves from the branches of the sympathetic 

 which attend the blood-vessels. Both it and the tunica arach- 

 noidea send cylindrical prolongations down the vertebral canal, 

 which surround the medulla spinalis. 



The pia mater is changed in character so as to become a 

 fibrous membrane, as it passes down over the crura cerebri, 

 the pons varolii and the spinal medulla, which latter it closely 

 embraces. 



Processes of the Dura Mater. 



From the dura mater certain membranous processes go off, 

 forming incomplete partitions, which partially divide the cavity 

 of the cranium ; and in the same partial manner, separate the 

 parts of the brain from each other : thus preventing them from 

 pressing upon each other, and keeping them steady. 



They are formed of the internal lamina or layer of the dura 

 mater, like a plait, and therefore each of them consists of a 

 double membrane. 



The most conspicuous of these is denominated the falx, 

 which extends from the anterior to the posterior part of the 

 cranium, and divides the upper part of the brain into two 

 hemispheres ; but it is not sufficiently deep to divide the whole 

 of the brain : for, between the under edge of it, and the base 

 of the cranium, there is a large space occupied by a portion of 

 the brain, which is undivided : and, therefore, common to both 

 hemispheres. 



The falx begins at the middle of the sphenoid bone, and, 

 continuing its origin from the crista galli of the ethmoid bone, 



