CHAPTER IX. THE NERVOUS SYSTEM. 



1. Central Nervous System. We must now consider 

 the arrangement of the nervous system in more detail. We 

 have already said that in the rabbit the central nervous 

 system consists of the brain and spinal cord : it is well to 

 insist at this stage that these are really one and not two. 

 The brain is simply the highly specialized anterior end of the 

 spinal cord. The central nervous system is, in the earliest 

 stages of its development, a simple tube of nerve-cells, and 

 though it undergoes great changes in form in its develop- 

 ment, this primitive tubular character is still shown by the 

 presence of a central canal in the spinal cord, which expands 

 into the cavities or ventricles of the brain. 



2. The Spinal Cord. The general structure of the 

 spinal cord and its related nerves will be best understood 

 from the diagrammatic transverse section of it (fig. 53). 

 The exact outline of such a section varies with the region of 

 the cord, but in all cases it is approximately elliptical, and 

 is nearly cleft in two by the dorsal and ventral fissures 

 (occupied by connective tissue). Two kinds of tissue are 

 seen the grey matter, H -shaped in section, with the central 

 canal in the middle of the cross-bar, and the white matter 

 around this. The former consists chiefly of nerve-cells and 

 non-medullated fibres, the latter of medullated fibres running 

 longitudinally. 



The spinal cord gives off a paired and metameric series of 

 spinal nerves, which alternate with the metameric vertebrae. 

 These nerves are numbered according to the vertebra in 

 front of them thus the nerve-pair between the third and 

 fourth lumbar vertebrae is called the third lumbar pair of 

 nerves. An exception is made for the cervical region, 

 because the first spinal nerve emerges between the skull 

 and the atlas ; thus there are eight pairs of cervical 

 nerves. 



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