BRAIN. MEMBRANES. 405 



converge to constitute one or more fasciculi, by which they quit the 

 ganglion. 



The termination of nerves takes place by a separation of the fasci- 

 culi into their primitive nervous fibrils, and by the distribution of the 

 latter to the various tissues and surfaces of the body in the form of 

 loops. In the muscles these terminal loops surround the ultimate fasci- 

 culi, in the skin they enter into the composition of the papillae, and in 

 very thin membranes they are modified so as to constitute a net-work. 

 The nerves of special sense offer other modifications in the mode of 

 termination of the primitive fibres ; thus, in the olfactory nerve, the 

 termination is by loops ; in the auditory nerve, partly by loops, and 

 partly by free extremities ; and in the optic nerve by free rounded ex- 

 tremities alone. 



The capillary vessels of nerves are very minute. They run parallel 

 with the nervous fasciculi, and every here and there are connected by 

 transverse communications, so as to give rise to a net- work composed 

 of oblong meshes very similar to the capillary system of muscles. 



The nervous system may be divided for convenience of description 

 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 contained 

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

 oblongata. These are invested and protected by the membranes of the 

 brain, and the whole together constitute the encephalon (ev 

 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 ex- 

 ternal table and breaking the internal table with the chisel and ham- 

 mer. 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 interposed between 

 the edges of the bones ; in other situations, the connection 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 and in old persons than in the adult. Upon 

 being torn away, the internal table will present numerous deeply 

 grooved and ramified channels, corresponding with the branches of the 

 arteria meningea media. Along the middle line will be seen a groove 



