362 BRAIN MEMBRANES. 



nicating branches passing between the ganglia, of others passing 

 between the ganglia and the cerebro-spinal axis, and of branches of 

 distribution which are remarkable for their frequent and plexiform 

 communications. The sympathetic nerves also differ from other 

 nerves in their colour, which is of a grayish pearly tint. Examined 

 with the microscope the sympathetic nerves are seen to be composed 

 of an admixture of gray and white fibres; the white fibres belong to 

 the cerebro-spinal system: the gray are much smaller than the 

 white, less transparent, and the neurilemma is less easily distinguish- 

 able from its contents: some of the nerves are composed of gray 

 fibres only, without any admixture of white. The sympathetic gan- 

 glia contain the globules observed in the gray substance of the 

 brain; they are firmer in structure and enclosed in a strong invest- 

 ing capsule. The fasciculi of fibres entering the ganglion become 

 divided and form a plexus around the globules; they then con- 

 verge to constitute another fasciculus, by which they quit the 

 ganglion. 



The nervous system may be divided for convenience of descrip- 

 tion into 1. The brain. 2. The spinal cord. 3. The cranial nerves. 

 4. The spinal nerves. 5. The sympathetic system. 



THE BRAIN. 



The brain is a collective term which signifies those parts of the 

 nervous system, exclusive of the nerves themselves, which are con- 

 tained within the cranium ; they are the cerebrum, cerebellum, and 

 medulla oblongata. These are invested and protected by the mem- 

 branes of the brain, and the whole together constitute the encephalon 

 (sv, xsyaXy, within the head). 



MEMBRANES OF THE ENCEPHALON. 



Dissection. To examine the encephalon with its membranes, the 

 upper part of the skull must be removed by sawing through the 

 external table, and breaking the internal table with the chisel and 

 hammer. After the calvarium has been loosened all round, it will 

 require a considerable degree of force to tear the bone away from 

 the dura mater. This adhesion is particularly firm at the sutures, 

 where the dura mater is continuous with a membranous layer inter- 

 posed between the edges of the bones ; in other situations, the con- 

 nexion results from numerous vessels which permeate the inner 

 table of the skull. The adhesion subsisting between the dura mater 

 and bone is greater in the young subject than in the adult. 



Upon being torn away, the internal table will present the deeply 

 grooved and ramified channels, corresponding with the branches of 

 the arteria meningea magna. Along the middle line will be seen a 

 groove corresponding with the superior longitudinal sinus, and on 

 either side may be frequently observed some depressed fosses, cor- 

 responding with the Pacchionian bodies. 



