300 



PHYSIOLOGY 



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 cylindrical 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 cres- 

 centic 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 ma^er known as the anterior and posterior grey com- 

 missures. 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, 



FIG. 143. Nerve-cell from the spinal cord, 



stained by Nissl's method, 

 a, axis-cylinder process or axon ; b, proto- 

 plasm of cell, consisting of c, fibrillated 

 ground substance, and e, the granules of 

 Nissl ; d, nucleus. (LENHOSSEK.) 



FIG. 144. The point of origin 

 of the axon, the ' nerve- 

 hillock,' highly magnified, 

 to show absence of Nissl's 

 granules from the origin of 

 the process. (HELD.) 



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 sur- 

 rounding 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, 



