THE NERVOUS SYSTEM OF VERTEBRATES 337 



paths ' which are a necessary condition of increased efficiency of 

 the reacting organ. Some time after the outgrowth of the axon a 

 medullary sheath is formed, apparently by the agency of the axon 

 itself, so that each group of axons leaving or entering the cord forms 

 a bundle of medullated nerve fibres. The long branches of the 

 posterior or dorsal roots running up towards the head form a mass of 

 fibres behind the tube of cells known as the posterior columns. Fibres 

 starting in the spinal cord itself run upwards and downwards to end 

 in other parts of the cord, or in the more anterior divisions of the 

 central nervous system forming the brain, and surround the neural 

 tube on its ventral and lateral aspects with a sheath of white 

 matter. To these white fibres are added others, which take origin 

 in the brain and pass all the way down the cord. Meanwhile the 

 cells themselves become separated by the ramifications between 

 them of the branches of axons entering the cord, as well as of 

 the dendrites of the cells themselves. Thus, in its adult form, 

 the spinal cord consists of a central mass of nerve-cells and fibres, 

 known as the grey matter, which is encased in a sheath of white 

 matter formed of medullated nerve fibres. The cord itself is cylin- 

 drical in shape, and is divided into two symmetrical halves by the 

 anterior and posterior fissures. In each half of the cord the grey 

 matter on cross-section is crescentic in shape, presenting an anterior 

 or ventral horn and a posterior or dorsal horn, and is connected 

 with the corresponding mass in the other half of the cord by grey 

 matter known as the anterior and posterior grey commissures. Between 

 the two grey commissures is the central canal, relatively very minute 

 when compared with the condition in the foetus and lined by a single 

 layer of columnar ciliated epithelium, the cells of which are directly 

 descended from the neural epithelium lining the medullary canal. 



THE STRUCTURE OF NERVE-CELLS 



In the adult animal a typical nerve-cell, such as those forming a 

 prominent feature in the anterior horn of the spinal cord, is a large 

 cell with many branches. It has a large vesicular nucleus with very 

 little chromatin, which may be collected into one or two nucleoli. 

 The body of the cell presents different appearances according to the 

 manner in which it has been treated for histological examination. 

 When separated from the surrounding tissues by means of dissociating 

 fluids it may present traces of striation, the individual striae running 

 from one process to another of the cell. When treated fresh with 

 methylene blue, or hardened by alcohol or corrosive sublimate and 

 stained with methylene blue or toluidine blue, the protoplasm is 

 seen to contain angular masses which are deeply coloured with the 

 dye (Fig. 144). These masses are known as the Nissl granules or 



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