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THE NERVOUS SYSTEM. 



THE NERVOUS SYSTEM. 



The nervous system is divided into two subdivisions : the 

 cerebrospinal system and the sympathetic system. 



The cerebrospinal system includes the brain and spinal cord, 

 which together form the cerebrospinal axis, and the nerves which 

 come from them, namely, the cranial and spinal nerves. 



SPINAL CORD. 



The spinal cord is situated in the vertebral canal, and is 

 covered by three membranes the dura mater, arachnoid, and 

 pia mater. It is about 0.43 meter in length, and, in general, 

 is of a cylindrical shape ; it weighs 42.5 grams. It extends from 

 the medulla oblongata above to the first lumbar vertebra below, 

 where it ends in the filum terminate, although in fetal life it 

 extends to the bottom of the sacral canal. 



Enlargements of the Spinal Cord. Two enlargements 

 along the course of the spinal cord are noteworthy. The cervical 



FIG. 270. Different views of a portion of the spinal cord from the cervical 

 region, with the roots of the nerves. In A the anterior surface of the specimen is 

 shown, the anterior nerve-root of its right side being divided; in B a view of the 

 right side is given; in C the upper surface is shown ; in D the nerve-roots and gang- 

 lion are shown from below : 1, the anterior median fissure ; 2, posterior median 

 fissure ; 3, anterior lateral depression, over which the anterior nerve-roots are seen 

 to spread ; 4, posterior lateral groove, into which the posterior roots are seen to 

 sink ; 5, anterior roots passing the ganglion ; 5', in A, the anterior root divided ; 6, 

 the posterior roots, the fibers of which pass into the ganglion, 6 ; 7, the united or 

 compound nerve; 7', the posterior primary branch, seen in A and I) to be derived 

 in part from the anterior and in part from the posterior root (Allen Thomson). 



enlargement extends from the third cervical to the first or the 

 second dorsal vertebra, and the lumbar enlargement is at the 

 eleventh and twelfth dorsal vertebrae. From the cervical en- 

 largement go off the nerves which supply the upper, and from the 

 lumbar those which supply the lower, extremities. 



