300 



PHYSIOLOGY 



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 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 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, 



Fio. 143. Nerve cell from the spinal cord, 



stained by Nissl's method, 

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

 plasm of cell, consisting of c, fibrillated 

 L'n. Mini substance, and e, the granules of 

 Nissl; d, nucleus. (LENHOSSEK.) 



FIG. 144. The point of oriiriti 

 of the axon, the 'nrru-- 

 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 mrthylene blue, or hardened by alcohol 



